Mesoporous Silica: Anionic Amphiphilic Molecular Templates 9783110555318, 9783110554205

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Shunai Che Mesoporous Silica

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Shunai Che

Mesoporous Silica

by Anionic Amphiphilic Molecular Templates

Author Prof. Shunai Che School of Chemistry and Chemical Engineering State Key Laboratory of Metal Matrix Composites Shanghai Jiao Tong University 800 Dongchuan Road, Shanghai, 200240, P. R. China. [email protected]

ISBN 978-3-11-055420-5 e-ISBN (PDF) 978-3-11-055531-8 e-ISBN (EPUB) 978-3-11-055485-4 Library of Congress Control Number: 2020950020 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2021 Walter de Gruyter GmbH, Berlin/Boston Cover image: Iaremenko/iStock/Getty Images Plus Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Preface Mesoporous silicas were first discovered in the early 1990s by templating of surfactant micelles or liquid crystalline phases. These have attracted intense interest from worldwide researchers because mesoporous silicas extend the pore size of molecular sieves to 2−50 nm, which endow these materials in various applications, for example, catalysis, drug delivery, and bio- and chemical absorptions. Mesoporous silica syntheses rely on surfactant as templates for the assembly and subsequent and/or simultaneous condensation of inorganic precursors. Among the four types of surfactants (cationic, anionic, nonionic, and amphoteric), the anionic surfactants are widely produced and consumed in vast quantities because of their high detergency and low cost. However, these anionic surfactants have not been used in the synthesis of mesoporous silica until 2003 when we published the first paper in Nat. Mater. by introducing a costructure-directing agent (CSDA) into the synthesis system. Although various excellent specialized books for mesoporous materials have been published, yet there are not books for anionic surfactant-templated mesoporous silicas (AMSs) in detail. The CSDA route differs from the previous cationic and nonionic templating pathways, and the AMS materials also gave rise to many properties that cannot be given by other templating routes. Structurally, CSDA contains two parts: an alkoxysilane site that is capable of being cocondensed with a silica source, and an organic site that is capable of forming electrostatic, covalent, hydrogen bonding or π−π interactions, and so on, with the head groups of the surfactant. Therefore, CSDA bridges the organic and inorganic species to favor their selforganization into ordered assemblies, which (i) makes it possible for successful preparation of mesoporous materials in systems that are difficult or impossible to achieve by conventional strategies; (ii) the interaction between the surfactant and the CSDA produces a uniform distribution of organic groups, and a regular array of the organic groups will be formed on the surface of the mesopores following the arrangement of the surfactant. This book discusses the nature and the determinant factors of the costructuredirecting effect in preparing this kind of organically functional mesoporous silicas. A brief review is presented on the interactions in costructure-directing synthesis, their effect on the structural and porous properties of the mesoporous silicas, and strategies to achieve different morphologies. The author also shows the recent studies on the arrangement of the functional groups on the surface of mesopores, revealing the homogeneous and regular array of the functional groups obtained by the costructure-directing route. This book also sheds a light on the potential applications of this kind of materials and describes the trends of the research on this subject from our understanding. The author thanks T. Tatsumi, O. Terasaki, K. Sakamoto, H. Kunieda (late), A.E. Garcia-Bennett, Z. Liu, T. Ohsuna, and Y. Yao for excellent collaborative https://doi.org/10.1515/9783110555318-202

VI

Preface

discussion, providing samples and experimental characterizations, and also thanks the students X. Wu, H. Jin, Q. Chen, C. Gao, H. Qiu, L. Han, H. Zheng, J. Xie, L. Xing, B. Liu, D. Xu, Z. Huang, Y. Cao, Y. Duan, C. Jin, Y. Yu, R. Gong, C. Ma, Y. Zhang, Y. Wang, and X. Zhang for their excellent works. December 2018, Shunai Che

Contents Preface

V

Abbreviated terms 1 1.1 1.1.1 1.1.2 1.2 1.2.1 1.2.2 1.3 1.3.1 1.3.2 1.4 1.4.1 1.4.2 1.4.3

XI

Introduction: review on porous materials 1 Porous silica 1 Microporous silica 1 Mesoporous silica 2 Chemistry of surfactant/silicate aqueous solution 3 Surfactant: surfactant packing parameter g and mesophase structures 3 Silicate: hydrolysis and condensation of silicon alkoxide 4 Synthesis mechanism of mesoporous silica 5 Cooperative self-assembly of surfactant and silica source 7 “True” liquid crystal templating 9 Synthesis routes 9 Cationic surfactant templating route 11 Nonionic surfactant templating route 13 Anionic surfactant templating route 13 References 14

2

Costructure-directing route for synthesizing AMSs References 22

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11

Synthesis of AMSs with different anionic surfactant AMS-1 24 AMS-2 25 AMS-3 26 AMS-4 26 AMS-5 27 AMS-6 28 AMS-7 28 AMS-8 30 AMS-9 30 AMS-10 30 Chiral mesoporous silica (CMS) 33 References 36

18

23

VIII

4 4.1 4.1.1 4.1.2 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.3 4.3.1 4.3.2 4.3.3 4.4 4.5

5 5.1 5.2 5.3 5.4 5.5

6 6.1 6.2 6.3 6.3.1 6.4 6.4.1 6.4.2

Contents

Structural control of AMSs 38 Control of mesostructure by ionization degree of anionic surfactant 38 TMAPS/anionic surfactant system 39 APS/anionic surfactant system 40 Formation of different cage-type mesophases 43 Diverse mesostructures in the synthesis field diagram 43 Effect of the synthesis composition on the mesostructure 45 Mesophase formation dominated by the organic/inorganic interface curvature 45 Mesocage/mesocage electrostatic interactions 45 Control of mesostructures by properties of surfactant and CSDA 46 Effect of geometry of surfactant on mesostructure 47 The effect of the chain length of surfactant on the mesophase formation 48 The effect of the geometry of the CSDA on the mesophase formation 48 Kinetics phase transformation on the AMSs 50 Evolution of g parameters in the structural transitions of AMSs 52 References 58 Pore size and wall thickness controls of AMSs 60 Control of pore size by hydrophobic chain length of surfactant 60 Control of pore size by the ionization degree of anionic surfactant 60 Control of pore size by CSDA/surfactant ratio 60 Control of pore size by surfactant molecule design 61 Control of wall thickness by surfactant molecule design References 66

63

Morphological control of AMSs 67 Mesoporous silica hollow sphere (MSHS) 67 Mesoporous silica nanotubes (MSNTs) 69 Monodispersed mesoporous silica nanoparticles (MMSN) 74 Effect of Brij-56 concentration on the morphology of monodispersed mesoporous silica nanospheres 74 Control of morphology and helicity of CMS 79 Steric and temperature control of enantiopurity of CMS 79 Stirring effect on controlling morphology and helicity of CMS 84

Contents

6.4.3 6.4.4 6.4.5 6.5 6.5.1 6.5.2

7 7.1 7.1.1 7.1.2 7.1.3 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.3 7.4 7.5

8 8.1 8.2

9 9.1 9.1.1 9.1.2 9.1.3 9.2

Temperature dependence on the formation of various CMS 86 Controlling the pitch length of CMSs 89 Formation mechanism of achiral amphiphile-templated helical mesoporous silicas 92 Silica nanotube with chiral mesoporous wall structure 96 Chiral mesoporous silica nanotube with addition of (R)-(+)-APP 97 Time course of tube formation 97 References 100 Nonsurfactant self-assembly by costructure-directing route 101 DNA-directed mesoporous silicas 101 DSC with rare 2D-square p4mm symmetry 101 DNA–silica complex with helical architectures 106 DNA-templated chiral silica films 108 Peptides directed for mesoporous silicas 116 Peptide-templated CMSs 118 Hydrophilicity in mesostructural control 120 Roles of hydrophobic tails on mesostructure 121 Polypeptide–silica complex with 2D-square p4mm symmetry 124 Porphyrin-directed CMS 128 Folate-templated mesoporous silica 132 Mesostructured fluorescent silica hybrids (MFSHs) 134 References 140 Cationic surfactant-templated mesoporous silica achieved with anionic CSDA 141 Synthesis of mesoporous silicas with various structures 141 Monodispersed mesoporous silica nanoparticles (MMSNs) 144 References 146 Organo-group functionalized mesoporous silicas 147 Template removal strategy for organo-group functionalized AMSs 147 Formation of N+-AMS by extraction of AMS synthesized with TMAPS as the CSDA 148 Formation of NH2-AMS or NH3+-AMS by extraction of AMS synthesized using APS as the CSDA 149 Loadings of the functional groups on the mesopore surfaces of AMS 150 Amphoteric amino acid functional mesoporous silicas 151

IX

X

9.3

Contents

Regular arrangement of functional groups on pore surface of CMSs 157 References 161

10 Applications of AMSs 162 10.1 Application of AMSs on controlled drug delivery 162 10.1.1 Coordination bonding-based pH-responsive drug delivery systems 162 10.1.1.1 pH-responsive release of anticancer drugs bearing binding sites 163 10.1.1.2 Vector-aided pH-responsive release of drugs without binding capabilities 166 10.1.1.3 Coating outer surface of MMSNs for pH-responsive release of drugs without binding capabilities 167 10.1.2 Controlled release of hydrophobic drugs using AMS 168 10.2 Metal and metal oxides casting in AMSs 171 10.2.1 Chirality of isotropic metal NPs in CMS 171 10.2.2 Chirality of anisotropic metal nanowires with a distinct multihelix 174 10.3 Gold nanorod@CMS core–shell NPs with unique enantioselectivity 177 10.4 Application of AMS on catalysis 181 10.4.1 Soai reaction on CMS 181 10.4.2 An amphoterically functionalized mesoporous silica catalyst in aldol reaction 183 10.5 Application of AMS on adsorption 184 10.5.1 Adsorption of Pb2+ and Cu2+ on amino-functionalized AMS 184 186 10.5.2 CO2 adsorption on amino-functionalized AMSs 10.5.3 Enantioselective adsorption on CMS 186 10.6 Application of CMS on enantiomeric separation 190 References 193 Index

195

Abbreviated terms Amino-functionalized mesoporous silica 3-Aminopropyltriethoxysilane 3-Aminopropyltrimethoxysilane Anionic surfactant-templated mesoporous silica Barrett–Joyner–Halenda Brunauer–Emmett–Teller Carboxyethylsilanetriol sodium salt Cetyl trimethyl ammonium bromide Chiral AIEgen-silica hybrid hollow nanotube Chiral DNA–silica films Chiral mesoporous silica Circular dichroism Costructure-directing agent Critical micellar concentration Diffuse-reflectance circular dichroism Dimethylaminopropyltrimethoxysilane DNA–silica complex Evaporation-induced self-assembly Fourier diffraction Fourier transform infrared High-resolution transmission electron microscopy Impeller-Like helical DNA-silica complex Induced circular dichroism Liquid crystal Liquid crystal templating Lyotropic liquid crystal Mesoporous silica hollow sphere Mesoporous silica nanotube Mesostructured fluorescent silica hybrid Methylaminopropyltrimethoxysilane Monodispersed mesoporous silica nanoparticles Multiply Twinned Particle Nuclear Magnetic Resonance N-Trimethoxysilylpropyl-N,N,N-trimethylammonium chloride Optical activity Oriented chiral DNA–silica film Peptide-templated chiral mesostructured silicas Polyethylene Oxide Polypeptide–silica complex Poly styrene Scanning electron microscopy Silica hollow sphere Structure directing agent Tetraethoxysilane Tetramethoxysilane

https://doi.org/10.1515/9783110555318-204

AFMS APES APS AMS BJH BET CES CTAB CASN CDSF CMS CD CSDA cmc DRCD DMAPS DSC EISA FD FTIR HRTEM IHDSC ICD LC LCT LLC MSHS MSNT MFSH MAPS MMSN MTP NMR TMAPS OA OCDSF PCMS PEO PSC PS SEM SHS SDA TEOS TMOS

XII

Abbreviated terms

Transmission electron microscopy Trimethylbenzene X-Ray diffraction

TEM TMB XRD

1 Introduction: review on porous materials Porous solids have unique chemical and physical properties and are widely used technically as adsorbents, catalysts, catalyst supports, electronic, and optical devices owing to their high surface areas and highly ordered tailorable structures. According to the International Union of Pure and Applied Chemistry definition [1], porous materials are divided into three classes: microporous (50 nm). The porous materials can be amply found in nature. For example, microporous zeolite and biological structures, such as diatoms, marine sponges, corals, wood, and vegetal products. These natural porous materials generally exhibit many particular physical properties and inspire today’s researchers to invent a series of smart designs of high-performance structures. In artificial porous materials, well-known member of the microporous class is the zeolites, which provide excellent catalytic properties by virtue of their crystalline aluminosilicate network and adsorption properties due to their high surface areas and uniform pore sizes [2, 3]. Ordered mesoporous oxides [4, 5], metal–organic frameworks [6, 7], and covalent organic frameworks [8] have attracted increasing attention due to their range of applications in storage, sensing, separation, and transformation of small molecules. These also have very broad mesopore-size distributions, as well as additional micropores. Silica is most commonly found in lithosphere as natural sand, quartz, and in various living organisms [9, 10]. Silica is one of the most complex and most abundant families of materials, existing as a compound of several minerals and as synthetic product. Notable examples include fused quartz, fumed silica, silica gel, and aerogels. It is used in structural materials, microelectronics, and as components in the food and pharmaceutical industries. In this book, the subject will be limited to ordered mesoporous silica-based materials.

1.1 Porous silica 1.1.1 Microporous silica Zeolite is the most well-known microporous silica material, which has enormous impact as catalyst and adsorbent in the chemical and petroleum industries [11]. The classical example is the fluidized catalytic cracking of hydrocarbons to gasoline by faujasite zeolite and more specifically the siliceous USY variants [12]. However, expanding the pore sizes of zeolite from the micropore region to mesopore region is demanded for both industrial applications and fundamental researches. For example, https://doi.org/10.1515/9783110555318-001

2

1 Introduction: review on porous materials

the large pores are needed for treating heavy feeds, separating and synthesizing large molecules, improving transmission and diffusion of organic molecules with large dimensions, introducing nanometer particles into zeolites, and for electronic and optical applications [13–15]. Numerous zeotype materials with pore diameters larger than those of the traditional zeolites have been discovered, such as AlPO4-8 [16] containing 14-membered rings, VPI-5 [17], cloverite [18], JDF-20 [19], ITQ-43 [20], and ITQ-37 [21]. However, these materials have not been used significantly because of either their inherently poor stability or their weak acidity. Briefly, applications of microporous materials are limited by the relatively small pore size; therefore, pore enlargement has been one of the main challenges in zeolite chemistry.

1.1.2 Mesoporous silica In 1992, M41S family of silicate/aluminosilicate, highly ordered mesoporous molecular sieves with large pore structure [22], has been discovered through lyotropic liquid crystal (LLC) templating route. Three different mesophases, that is, MCM-50 (a stabilized lamellar phase), MCM-41 (Two-dimensional (2D)-hexagonal p6mm phase), and MCM-48 (three-dimensional (3D)-cubic Ia-3d phase), were synthesized with alkyltrimethylammonium surfactants as templates under basic conditions. These mesophases possess highly ordered pore structure with diameters in the range of 15–100 Å depending on the templates size, the addition amount of auxiliary organic compounds, and the reaction conditions. Since the discovery of MCM-41, the mesoporous materials have attracted the attention of chemists and materials scientists. The research interest can be largely classified into (i) synthesis, (ii) characterization, and (iii) application of various mesoporous materials. Synthesis of mesoporous silicas focused on the following main subjects: (i) the challenges of their synthesis routes and structural and morphological control; (ii) the mechanism of formation; (iii) the synthesis of new composition of inorganic and organic materials based on the M41S family synthesis concept. Characterizations of mesoporous materials were processed on the X-ray diffraction, combination of scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), and adsorption analysis. The technical applications of various mesoporous materials were carried out on (i) catalysis, (ii) separation, (iii) drug delivery, (iv) storage, and (v) sensing. As mentioned earlier, the “protagonists” in the synthesis of mesoporous silicas can be considered to be “surfactant” and “silica source.” Therefore, this chapter introduce the chemistry of surfactant and silicate at first for better understanding following mesoporous silica synthesis rotes and formation mechanism.

1.2 Chemistry of surfactant/silicate aqueous solution

3

1.2 Chemistry of surfactant/silicate aqueous solution Reaction gel chemistry is considered to play an important role in zeolite synthesis. Therefore, naturally, in the mesoporous materials synthesis system, much effort has been made to elucidate the gel chemistry related to the formation mechanisms and to the resultant products. In this sense, knowledge of the chemistry of surfactant/silica solution is a prerequisite for understanding the synthesis and mechanisms responsible for the formation of mesoporous silicas from its precursors.

1.2.1 Surfactant: surfactant packing parameter g and mesophase structures In a binary water–surfactant system, with increasing concentration of surfactant molecules, the existence status of which is changed from energetically favorable monomolecules, self-assembled isotropic micelles throughcritical micelle concentration (cmc), and to LLC mesophases [23]. The structure of LLC mesophase can be divided into micellar, cylindrical, bicontinuous, and lamellar. It is known that the structure of mesophases depends on the concentrations, the nature of itself (length of the hydrophobic chain and hydrophilic head group), and the environmental parameters (pH, temperature, ionic strength, and other additives) [24]. Generally, the cmc is decreased with increasing chain length of a surfactant, the variance of the counterions, and the ion strength in solution. In the charged surfactant–water mixture, the micellar structure or packing of the surfactant is determined by a balance between three types of free energy contributions: (i) the tendency of the hydrophobic chains to minimize their contact with water and maximize their interorganic interactions; (ii) the coulombic and dipolar interactions among the charged head groups; (iii) the solvation energies arise from the presence of water or organic molecules in the hydrophilic, intermediary hydrophobic–hydrophilic “palisade,” and hydrophobic regions. The continuum surface model has been useful in understanding structures and transformations of mesophases in which surfactants play a dominating role in determining the overall structural symmetry, which can be applied equally well to inorganic mesostructured composites. A key question is how to relate these mesophases to surfactant molecular structure. Several investigators have shown that it is possible to mathematically relate molecular size, charge, and shape to the more global surface curvature, bending energies, and morphology [25, 26]. The classical molecular description of surfactant assembly in LLC arrays has been described in terms of the surfactant packing parameter, g = V/aol, where V is the total volume of the surfactant chain with any cosolvent molecules in the hydrophobic chains, ao is the effective head group area at the organic–water interface, and l is the kinetic surfactant chain length or the curvature elastic energy [27].

4

1 Introduction: review on porous materials

1.2.2 Silicate: hydrolysis and condensation of silicon alkoxide Tetramethoxysilane and tetraethoxysilane are the most commonly used metal alkoxide precursors in the synthesis of mesoporous silicas. At the functional group level, three bimolecular nucleophilic reactions are used to describe the sol–gel process [28]: the hydrolysis reaction (eq. (1.1)) replaces alkoxide groups with hydroxyl groups, the silanol groups produce siloxane bonds, and the by-products alcohol (eq. (1.2)) or water and their reverse reaction (eq. (1.3)). Usually, hydrolysis and condensation reactions are concurrent: ≡ Si – OR + H2O → ≡ Si – OH + ROH

(1.1)

≡ Si – OH + RO – Si ≡ ⇄ ≡ Si – O – Si ≡ + ROH

(1.2)

≡ Si – OH + HO – Si ≡ ⇄ ≡ Si – O – Si ≡ + H2O

(1.3)

The roles of acid or base catalysts are shown schematically in Figure 1.1 [29]. The hydrolysis reaction shows minimum rate at pH 7 [30–32]. The most strongly hydrolyzed form of silica detectable in aqueous solution is orthosilicic acid, Si(OH)4 [33]. Below about pH 2, acid catalysts protonate the alkoxide group (eq. (1.4)), making a better leaving group (ROH) and avoiding the requirement for proton transfer in the base catalysts dissociate water, producing a stronger nucleophile (OH-) [34]. At above pH 7, further hydrolysis involves the deprotonation of a silanol group to form an anionic species (eq. (1.4)) [33]. Hydrolysis is enhanced under acidic conditions but is retarded under basic conditions. The condensation reaction depends on the acidity of the silicate reactants.

Condensation

Dissolution

1/gel time

Log10 rate

Hydrolysis

RLCA

MCA ripening aggregation 0

2

4

6 PH

8

10

Figure 1.1: Illustration of the pH dependence of the hydrolysis, condensation, and dissolution rates of silicate. Condensation rates are judged by the reciprocal of gel times.

1.3 Synthesis mechanism of mesoporous silica

5

≡ Si – OH + H+ → ≡ Si – OH2+

(1.4)

Si(OH)4 (aq) → Si(OH)3 O– + H+

(1.5)

Three stages of polymerization were recognized in silicate condensation. Polymerization of monomers to form particles, growth of particles, and linking of particles into branched chains, networks, and final gels [9]. The polymerization process are divided into three approximate pH domains: pH 7. A pH of 2 is a boundary, the point of zero charge, which is the isoelectric point, where the electrical mobility of the silica particles is zero [9]. At below about pH 2, weakly acidic silanols or ethoxides are protonated, so good leaving groups (H2O or ROH) are created and the requirement of charge transfer in the transition state is avoided (eq. (1.6)). At between pH 2 and 6, polymerization occurs by a bimolecular nucleophilic condensation mechanism involving the attack of hydrolyzed, anionic species on neutral species [34]. The condensation occurs preferentially between more condensed species and less condensed, neutral species (eq. (1.7)): ≡ Si – OH2+ + Si – OH2+ → ≡ Si – O – Si ≡ + H3O+

(1.6)

≡ SiO– + HO – Si ≡ → ≡ Si – O – Si ≡ + OH

(1.7)

At above pH 7, particle surface is significantly charged negatively, so, particle aggregation is unfavorable. The greater solubility of silica and the greater size dependence of solubility above pH 7 leading to the growth of primary particles continues by Ostwald ripening, in which smaller, more soluble particles dissolve and reprecipitate on large, less soluble particles. Growth ceases when the difference in solubility between the largest and smallest particles is negligible [9]. The solubility of silica is also important. At pH > 7, the dissolution of silica is more favored, nucleation and growth are predominant. Dissolution ensures a constant supply of monomers with high mobility, which coupled with condensation being favored at more highly condensed sites, leads to the generation of highly crosslinked, large particles stabilized by electrostatic repulsions. 29Si Nuclear Magnetic Resonance (NMR) spectroscopy shows that under basic conditions fully condensed or Q4 [Si(OSi)4] species and monomers are the predominant species present. Under low pH conditions, no Q0 [Si(OH)4]or Q1 [SiOSi(OH)3]and a distribution of Q2[Si(OSi)2(OH)2], Q3[Si(OSi)3OH], and Q4 species are present [35].

1.3 Synthesis mechanism of mesoporous silica Mesoporous silica can be synthesized by self-assembling of surfactant and silicate. As mentioned earlier, the micelles can be formed with above cmc of surfactant; the curvature of micelles can be decreased with increasing surfactant concentration and further, LLCs can be formed with further increasing surfactant concentration.

Liquid solution

Cooperative aggregation and phase separation

Transformation of precursors to aimed materials

Liquid crystal formation with molecular inorganics

Template elimination

Elimination Mesoporous framework of final product

Template

Further polymerization and condensation of inorganics

Mixture of solution and precipitation

Figure 1.2: Two synthetic strategies of mesoporous materials: (a) cooperative self-assembly and (b) “true” liquid crystal templating process [5].

Incorporation of inorganics’ precursor

Cooperative nucleation

Inorganic species

Surfactant

Liquid crystal formation

(B)

(A)

6 1 Introduction: review on porous materials

1.3 Synthesis mechanism of mesoporous silica

7

The subsequent research found that the mesoporous silica can be synthesized with different surfactant concentration from lower than cmc and the surfactant concentration that can form LLCs. Therefore, the synthesis mechanism of mesoporous silicas can be divided into two synthesis pathways: cooperative self-assembly and “true” liquid crystal (LC) templating processes (Figure 1.2) based on the differences of surfactant concentration, at low and high concentration [5].

1.3.1 Cooperative self-assembly of surfactant and silica source The mesoporous silicas can be synthesized at low surfactant concentration (that cannot form LLCs), by self-assembly of a silica source, surfactant, base or acid, and large amount of water. After the synthesis mixtures were aged at elevated temperatures (≧100 C) for desired times, the organic–inorganic mesostructured product was filtered, washed, and dried. The product was calcined at 500–650 °C under a flowing gas to remove surfactant, or was extracted using alcohol–acid solutions, to produce the mesoporous silicas. Firstly, the Mobil researchers proposed a “liquid crystal templating” mechanism based on the LLC assemblies of surfactant [22] as shown in Figure 1.3. Taking MCM-41 as example, two mechanistic pathways were postulated by Mobil researchers: (i) the aluminosilicate precursor species occupied the space between a preexisting hexagonal LLC phases and deposited on the micellar rods of the LC phase; (ii) the inorganic mediated, in some manner, the ordering of the surfactants into the hexagonal arrangement. It is now known that pathway I did not take place because the surfactant concentrations used were far below the cmc required for hexagonal LC formation. The second mechanistic pathway of LCT was vaguely postulated as a cooperative self-assembly of the ammonium surfactant and the silicate precursor species below the cmc. Several mechanistic models have been advanced which share the basic idea that the silicate species promoted LC phase formation below the cmc.

Hexagonal array Surfactant micelle

Micellar rod Calcination

Silicate 1

MCM-41 Silicate 2

Figure 1.3: Two possible pathways for the LCT mechanism [22b].

8

1 Introduction: review on porous materials

The success of the cooperative templating model [36], referred to as the generalized LCT mechanism (Figure 1.4), was illustrated by the diverse compositions of organic–inorganic mesostructures found possible. When oppositely or identically charged silicates and surfactant (or mediated by counterion) mixed under alkaline or acidic conditions (e.g., when positively charged quaternary ammonium surfactants are combined with basic silicate oligomers), the formation has been shown to proceed through a cooperative self-assembly process. Ion exchange of poly-charged silicate oligomers for the monovalent counterions of the surfactants is driven by electrostatic considerations. This ion exchange reduces the repulsion between surfactant micelles and allows the system to cooperatively self-organize. At the same

Micelles

Surfactant Molecules

Inorganic molecular species A

Cooperative

Nucleation

C

B

Liquid crystal formation with molecular inorganics

D Inorganic polymerization and condensation

Figure 1.4: Cooperative templating of the generalized LCT mechanism [36]. (a) Cooperative nucleation, (b) LC formation with molecular inorganic compounds, (d) inorganic polymerization and condensation.

1.4 Synthesis routes

9

time, condensation of the silicate oligomers provides a route to kinetic trapping that locks the structure in place [36]. The interplay between kinetic and energetic (electrostatic) factors determines the final structure of these materials during synthesis.

1.3.2 “True” liquid crystal templating Attard and coworkers synthesized mesoporous silicas using nonionic surfactants at high concentrations that can form LLC [37]. The inorganic precursors are introduced into the hydrophilic and solvent part of the LLC leading to the formation of inorganic frameworks. The solvent evaporation-induced self-assembly strategy can also be assigned to this pathway in which the concentration of surfactant is gradually increased with continuous evaporation [38]. The evaporation of volatile polar solvents can improve the organic–inorganic assembly and form organic–inorganic frameworks. Inorganic frameworks can be formed with the further hydrolyzing and crosslinking inorganic precursors during solvent evaporation. Mesoporous materials with diverse compositions and various morphologies have been produced.

1.4 Synthesis routes Following the chemistry of surfactant and silicate, two synthesis pathways of mesoporous silicas was described earlier. Whether the cooperative assembly or “true” LC templating pathway, the synthesis system should be satisfied: (i) the LCs should be formed, (ii) the silicate should be hydrolyzed and polymerized, and (iii) the head group of surfactants should be properly interacted with silicate. Furthermore, the synthesis systems should be certainly controlled thermodynamically and kinetically. The interaction types between surfactant and silicate can be classified based on the surfactant types. For the surfactant/inorganic species interfaces, Monnier and Huo et al. [36, 39] proposed a formula of the free energy in the whole process: ΔG = ΔGinter + ΔGwall + ΔGintra + ΔGsol in which ΔGinter is the energy associated with the interaction between inorganic walls and surfactant micelles, ΔGwall is the structural free energy for the inorganic frameworks, ΔGintra is the van der Waals force and conformational energy of the surfactant, and ΔGsol is the chemical potential associated with the species in solution phase. For the surfactant-templating assembly of mesoporous silicates, ΔGsol can be regarded as a constant in a given solution system. Therefore, the key factor is the interaction between surfactant and inorganic species. The more negative the ΔGinter is, the more easily the assembly process can proceed. The inorganic–organic

10

1 Introduction: review on porous materials

interactions types can be divided based on the surfactant types: cationic, nonionic, and anionic. Table 1.1 lists the main synthesis routes classified based on the surfactant types. Table 1.1: Various types of inorganic-surfactant interactions in the mesoporous silica synthesis routes. Surfactant Conditions Silicate Route Cationic (S+)

Nonionic (S°)

Anionic (S–)

Interactions

Classical products

I–

S+I–

Electrostatic Coulomb force

SBA-, SBA-, FDU-, MCM-, FSM-, FSM-, SBA-, MCM-, IBN-, MCM-, FDU-, FDU-

I–

S+---I–

Covalent bond

Mesoporous silica

Acidic pH < ~ (HnXn–)

I+

S+X–I+

Electrostatic Coulomb force

Neutral

I–

S°I° (N°I°)

H bond

Acidic pH < ~ (HnXn–)

I+

S°H+X–I+ Electrostatic Coulomb force, double layer H bond

SBA-, SBA-, SBA-, SAB-, FDU-, FDU-, FDU-, KIT-, CMI-, KIT-, KIT-

pH = – I–

S–N+---I-- Electrostatic Coulomb force

AMS-n, NFM-

Basic

SBA-, SBA-, SBA-, SBA- HMS, MSU, TUD-, disordered worm-like mesoporous silicates

S is surfactant, S+, S°, and S– are cationic, nonionic, and anionic surfactant, respectively; I is silicate species, I+, I°, and I– are positively charged, nonionic, and negatively charged silicate, respectively; N° is neutral amines; N+ is quaternary ammonium group; X‒ is anionic ion.

In the energetic self-organization, it is thought that the packing of the organic surfactant and the charge density matching between the surfactant and the inorganic precursor are essential for the formation of the ordered mesostructure. The surfactant packing depends on the molecular geometry of the surfactant species, such as the number of carbon atoms in the hydrophobic chain, the degree of chain saturation, and the size or charge of the polar head group. The packing of the surfactant molecules can be quantified through the packing parameter, g. Since g = V/aol, the value of g increases as a0 decreases, V increases, and l decreases. These transitions reflect a decrease in surface curvature from 3D spherical micellar structure through 2D rod assembling, 3D bi, tricontinuous to lamellar. For surfactants to associate in a spherical structure, the surface area occupied by the surfactants polar head groups should be large. If on the other hand the head groups are permitted to pack tightly, the aggregation number will increase,

1.4 Synthesis routes

11

and rod or lamellar packing will be favored. The values of g (between 1/2 and 2/3) for the 3D bi- and tricontinuous phase depend upon the volume fraction of surfactant chains [40]. In the synthesis of mesoporous materials, it has been considered that the kinetic matching of inorganic and organic ordering during the assembly and silica polymerization is critical to the morphological, structural, and property design of mesophase materials. For example, as shown in Figure 1.5 [41], the packing type of micelle was changed via the condensation of silicate around that under basic condition. Silica condensation causes the negative (or positive) charge density of the silicate network to decrease. To maintain charge matching in the interface, the organic surfactants pack to form a high surface curvature to increase the effective head group area and lower the packing parameter so that the high curvature mesophases with lower g parameters is favored.

(a)

+

+

+

+

+

+

+

+

+ +

+ High charge density

+

Low charge density

(b) Si O–

Si O–

Si

Si O–

OH High charge density

Condensation

Si Si Si O O– O– Lower charge density Si

Figure 1.5: The change in micelle packing order that occur in silicate condensation [41].

1.4.1 Cationic surfactant templating route The cationic surfactants with quaternary ammonium head groups are usually used in the synthesis of mesoporous silicas. Cationic surfactants are permanently positively charged regardless of basic or acidic conditions. The cationic surfactant templating route can be divided into three types based on the different organic–inorganic interaction patterns: (i) under basic conditions, negatively charged silicate anions (I-) interact with positively charged surfactant head group (S+) through Coulomb forces (S+I–), and covalent connection between the cationic surfactant head group and silicate (S+---I–); (ii) under acidic conditions (pH < 2), positively charged silicate anions (I+) interact with positively charged surfactant head group (S+) through Coulomb forces balanced by negatively charged anions X (Cl–, Br–, I–, SO42–, NO–, etc.).

12

1 Introduction: review on porous materials

Under basic conditions (pH = 9.5–12.5), the reversible hydrolysis and condensation of silicate can be proceeded with various base such as NaOH, KOH, NH4OH, tetramethylammonium hydroxide, and tetraethylammonium hydroxide. Various highly ordered mesoporous silicas were synthesized with different surfactants under basic conditions. SBA-2 [40], SBA-6 [42], and FDU-2 [43] with g < 1/3 of P63/mmc, Pm-3n, and Fd-3m have been synthesized using Gemini surfactants CnH2n+1N+(CH3)2(CH2)sN+(CH3)3, bolaform surfactant C18H37-OC6H4O-(C4H8)-N(CH3)2-C3H6-N(CH3)3Br2, and triquaternary ammonium salt surfactants [CmH2m+1N+ (CH3)2CH2CH2N+(CH3)2CH2CH2CH2N+(CH3)33Br-] (Cm-2-3-1, m = 14, 16, 18) as a template, respectively. MCM-41 [22] (p6mm), FSM-16 [44] (p6mm), KSW-2 [45] (rectangle arrangement of square- or rhombus-shaped pores), and SBA-8 [43] (cmm) with g = 1/2 have been synthesized with cetyltrimethylammounium biromide/chloride (CTAB/C) and double heads surfactant (CH3)3N(CH2)n-O-C6H4-C6H4O-(CH3)n-N(CH3)3, n = 4, 6, 8, 10, 12. MCM-48 [22] and IBN-9 [46] with g = 1/2–1/3 space groups of Ia-3d and 3D hexagonal mesoporous silica with tricontinuous pore structure have been synthesized by using small head surfactant CTAB, and a designed cationic surfactant, (S)-(1-tetradecylcarbamoyl-2-phenylethyl)-dimethyl-ethylammonium bromide as template, respectively. The large hydrophilic head group and large hydrophobic phenyl groups would give proper g parameter to form the palisade-like micelles, thus facilitating a unique mesostructure with low surface curvature. MCM-50 with lamellar (g = 1) structure was synthesized with CTAB and large hydrophobic chain as template. Acidic conditions (pH < 2) can be easily achieved by various acids such as HCl, HBr, HI, H2SO4, and HNO3. SBA-7 [40] with space group of P63/mmc, SBA-1 [36] with Pm-3n (g < 1/3), and SBA-3 [40] with p6mm (g = 1/2) have been synthesized using large head surfactant Gemini surfactant, C6H13N(C2H5)3Br and small head surfactant C6H13N(CH3)3Br, respectively. Under acidic conditions, it has been found that the presence of various counteranions in the interfacial region of the silicate–surfactant mesophase gives rise to strategy for fabrication of the phase structure. Well-ordered 3D hexagonal P63/mmc, cubic Pm-3n, 2D hexagonal p6mm, and cubic Ia-3d mesoporous materials have been synthesized with the same surfactant (C6H13N(C2H5)3Br), depending on the different kinds of acids [47]. The counteranions of acidic media have resulted in increasing surfactant packing parameter g in the order SO42–N-acylglycinate≈carboxylate. The mesostructure of AMS is determined by the geometry of the surfactant including head-group area and chain length. Due to the large head-group, TMAPS is effective in forming the higher-curvature mesophases. Ordered mesostructures can also be obtained when

https://doi.org/10.1515/9783110555318-003

24

3 Synthesis of AMSs with different anionic surfactant

using sulfuric and phosphoric acids and their salt anionic surfactants as templates, and either APS or TMAPS as CSDA. Disordered mesoporous silicas were achieved when combining sodium salts of anionic surfactants with APS, or amino acids with TMAPS. Table 3.2: Synthesis of AMS-1 to 5 by various surfactants [2]. Surfactanta Acid CGluA, CGluA CGluA, CAlaA, CAlaA, CGlyA, CGlyA, CGlyA, CAA, CAA CAlaA C-CAlaA, C-CGlyA, C-CAA Sodium salt CGluS, CGluS CAlaS, CGlyS, CAS CGluS, CGluS, CGlyS, CGlyS, CAlaS, CAlaS, CAS, CAS

Gel compositionb

pH value

Mesophase

APS/Sur

Si/Sur

– –

– .–

.–. .–.

AMS- AMS-

.– 

–. –

.–. ~.

AMS- AMS-

TMAPS/Sur

Si/Sur

 

 .

.–. .–.

AMS- AMS-



.

.–.

AMS-

a CnXY Y-A, free acid; Y-S, sodium salt. X-Glu, L-glutamic acid; X-Gly, glycine; X-Ala, L-alanine. Cn-CnH2n-1O (C12H23O, N-lauroyl; C14H27O, N-myristoyl; C16H31O, N-palmitoryl; C18H35O, N-stearoyl) CnAX AX-AA, free acid; AX-AS, sodium salt. Cn-C12H25, lauric; Cn-C14H29, myristic; Cn-C16H33, palmitic. b The well-ordered AMS-n silicas have been synthesized with 1.0 wt.% surfactants at 60 °C for 1 day.

3.1 AMS-1 AMS-1 was synthesized by using CnGluS (n = 12 and 14) and TMAPS as CSDA (Figure 3.1). The X-ray diffraction (XRD) pattern shows intensities in 2θ region of 1–5°, which can be indexed to a hexagonal unit cell of a = 54.4 Å and c = 88.2 Å with a c/a ratio of 1.629. Transmission electron microscope (TEM) images recorded with incidence beam parallel to the [001]h orientation of calcined AMS-1 shows that this sample is composed of spherical/cylindrical particles of sizes ranging from 10 to 100 nm. The corresponding Fourier diffractions (FDs) show streaking of diffraction spots,

3.2 AMS-2

25

Figure 3.1: XRD pattern and HRTEM images of calcined AMS-1 [2].

which indicate that the particle contains large amounts of structural defects. A disordered one-dimensional channel shell with a thickness about 2 to 10 nm surrounds each particle [2, 3].

3.2 AMS-2 XRD pattern of calcined AMS-2 synthesized with C12GluA and APS shows two wellresolved sharp reflections in the region of 2θ = 1.5–3.0° (Figure 3.2). The ratio of d spacing of the two peaks is close to (5/2)1/2; which can be indexed as the 200 and 210 reflections (a = 96.1 Å) of a cubic lattice. Large ordered regions with wellordered mesostructure can be observed from high-resolution transmission electron microscope (HRTEM) images taken along the [100] and [210] zone axis. The FDs can be indexed to a three-dimensional (3D) cubic Pm-3n space group. However, it is clear that neighboring unit cells are displaced by half an antiphase boundary, which is represented by electron density modulations in the FDs. No extra reflections are observed on the XRD pattern of calcined AMS-2. On this basis, another cubic structure with lattice constant a2 can be assigned. The modulations are present throughout the whole crystal and not in isolated domains [2, 3].

Figure 3.2: XRD pattern and HRTEM images of calcined AMS-2 [2].

26

3 Synthesis of AMSs with different anionic surfactant

3.3 AMS-3 The XRD pattern of calcined AMS-3 synthesized by C16AS shows three well-resolved sharp reflections (Figure 3.3), which can be indexed as 10, 11, and 20, of twodimensional (2D)-hexagonal p6mm mesostructures. TEM images taken along pores parallel and perpendicular to the incident electron beam confirm that AMS-3 is indeed analogous to hexagonal MCM-41. Unit cells calculated by HRTEM (a = 93.6 Å) agree well with XRD data (a = 92.4 Å), and no defects or structural modulations were observed in AMS-3 [2, 3].

Figure 3.3: XRD pattern and HRTEM images of calcined AMS-3 [3].

3.4 AMS-4 A 3D-cubic phase (AMS-4) resulted with an APS/C12AlaA molar ratio of 1.0. XRD pattern of AMS-4 shows sharp peaks in the region of 2θ = 1.5–3.0° (Figure 3.4). HRTEM image shows an image recorded with incidence direction [011] based on a cubic

Figure 3.4: XRD pattern and HRTEM images of calcined AMS-4 [3].

3.5 AMS-5

27

face-centered structure with a = 183.9 Å, which agrees well with the TEM-derived unit cell parameter [3]. All AMS-1, 2, 3, and 4 samples show type-IV N2 adsorption–desorption isotherm with sharp capillary condensation steps, and consequently, narrow mesoporous size distributions (Figure 3.5). The structural properties of the mesoporous silicas are listed in Table 3.1. The wall thickness and pore diameter are in the ranges of 2.4–3.1 nm and 2.0–6.2 nm, respectively [2]. 600 C14GluS–ASM–1 C12GluA–ASM–2 C16AS–ASM–3

Volume adsorbed (cm3/g at STP)

500

C12AlaA–ASM–4

400

300

200

100 2 0

0

0.2

0.4

3 4 5 6 7 8 Pore diameter (nm)

0.6

0.8

1

Relative pressure (P/Po) Figure 3.5: N2 adsorption–desorption isotherms and BJH pore size distributions of AMS-n mesoporous silicas shown in Figures 3.1–3.4 [2].

3.5 AMS-5 AMS-5 with a lamellar mesophase was synthesized by using C12–C14AlaA, C12– C16GlyA, and C12–C16AA surfactants at low Si/surfactant molar ratios and high surfactant concentrations [2].

28

3 Synthesis of AMSs with different anionic surfactant

3.6 AMS-6 Figure 3.6 shows XRD pattern and HRTEM images recorded on calcined AMS-6. HRTEM images show contrast patterns similar to 3D-bicontinuous cubic (Ia-3d) mesoporous materials MCM-48, and can be indexed to a cubic I lattice with a = 107.7 Å. FDs could be indexed to a body-centered cubic mesostructure of unit cell with a = 104.3 Å by the incident beam parallel to the [111] and [110] zone axis [3, 4].

Figure 3.6: XRD pattern and HRTEM images of AMS-6 [3].

3.7 AMS-7 Figure 3.7 shows XRD pattern of AMS-7 synthesized with C14-GluSA anionic surfactant, and APS as CSDA with addition of tetrapropylammonium bromide. XRD peaks centered in the range of 2–5° (2θ) can be indexed on the basis of a cubic P with unit cell a = 100.0 Å. HRTEM images, however, reveal clear modulations in the structure, resembling those seen previously in AMS-2. HRTEM image taken along the [100] direction clearly shows short-range periodicity. Diffuse diffraction rings in the FD shows that modulations in AMS-7 are more abundant than in AMS-2 [3, 4].

Figure 3.7: XRD pattern and HRTEM image of calcined AMS-7 [4].

1

Amount adsorbed (cm3/g STP)

0

50

100

150

200

250

300

0

2

5

6.5 5.6 4.2 3.6 2.8

0.4

0.6

220 222 400 440 626

ADS DES

0.8

Relative pressure (p/p˚)

0.2

1.4 1.6 2.1 2.5 3.2

2Θ d(nm) hkl

SBET = 408.2 m2/g–1

3 4 2Θ (deg)

AMS–8

1

6

[112]

[121]

50 nm

222

50 nm

222

404 622

[011]

400

444

50 nm

50 nm

044

622

3.7 AMS-7

Figure 3.8: XRD pattern (a), N2 adsorption–desorption isotherm (b) and HRTEM images (c–f) of calcined AMS-8 [5].

29

30

3 Synthesis of AMSs with different anionic surfactant

3.8 AMS-8 Figure 3.8 shows the XRD pattern of calcined AMS-8 synthesized with C12GlyS as template. The peaks in the range of 2θ = 1.5–5° can be indexed on the basis of a cubic structure. HRTEM images and FDs taken along the [121], [233], and [112] directions show a high degree of order. No other stacking faults or evidence were found in AMS-8 suggesting that it is the only cage-type 3D mesophase exist in the bulk sample. The XRD pattern can be indexed on the basis of a cubic F with unit cell a = 183.4 Å, which is consistent with the result obtained from TEM images [4, 5]. A threshold value of the potential density was set to differentiate between the silica walls and the cages. A plot of the relation between pore volume fractions versus electron density of MAS-8 is shown in Figure 3.9a. The nonlinear relationship between pore volume fraction and cage dimensions can be observed, while the connectivity of cages is increased considerably with increasing pore volume. A 2D image of the 3Delectrostatic potential density map of AMS-8 was computed from the inverse FD of the structure factors (Figure 3.9b). Figure 3.9c and d shows similar images, corresponding to pore volume fractions of 44% and 50%, respectively, based on threshold values 189 and 199. The thickness of each slice corresponds to 1.834 Å. Figure 3.10 shows reconstructed AMS-8 viewed along the [100], [110], and [111] directions for pore volume fractions of 44% and 50%. The different connectivity between two threshold values can be easily observed.

3.9 AMS-9 AMS-9 with tetragonal P42/mnm was synthesized by using C12GlutA as template and aminopropyltriethoxysilane as CSDA. HRTEM image taken along the [110] orientation of AMS-9 (Figure 3.11a) shows periodic stacking faults characterized by unit cell shifts of one-half. The faults are arranged regularly to form a 2D antiphase boundary. A superlattice unit cell is determined from this HRTEM image and the corresponding diffraction pattern as shown in Figure 3.11a (inset). As determined from a series of HRTEM images and diffraction patterns, the superstructure of AMS-9 is tetragonal, with unit cells of a = 167.9 and c = 84.0. SAED patterns taken along the [001], [110], [101], and [111] directions (Figure 3.11b–e) and the electrostatic potential density maps reveal that the cages in AMS-9 are arranged with space group P42/mnm [6].

3.10 AMS-10 AMS-10, a cubic Pn-3m with bicontinuous double diamond symmetry was synthesized using C14GluA and TMAPS as template and CSDA, respectively, by controlling the neutralization degree of anionic surfactant. Different mesophases from cage-type

3.10 AMS-10

(a)

(b)

(c)

31

(d)

% Pore volume fraction

Y 100 120 100

80

80 60

189

40 20 0

60 Small cage

40 0

50 100 150 200 250 300 Threshold value

Small cage

20

Large cage 0

0

20

40

60

80

X

Large cage

100

Figure 3.9: Plot of pore volume fraction versus potential density of the wall for meso-caged AMS-8 [5].

Figure 3.10: Electron density 3D reconstruction of AMS-8 viewed in perspective along the [100] (left), [110] (middle), and [111] (right) directions for threshold values of 189 (top) and 199 (bottom) [5].

Figure 3.11: (a) HRTEM images of AMS-9 viewed along [001], where white insets show the periodic zigzag arrangement of cages and the superlattice. (b–e) SAED patterns of calcined AMS-9 recorded along [001] (b), [110] (c), [101] (d), and [111], and (e) directions [6].

32

3 Synthesis of AMSs with different anionic surfactant

tetragonal P42/mnm (AMS-9), cage-type cubic Fd-3m (AMS-8), to 2D-hexagonal cylindrical p6mm (AMS-3), and cubic bicontinuous double diamond Pn-3m (AMS-10) were obtained by decreasing the addition amount of NaOH (Figure 3.12a).

B 211 220 221

111 200

110

A

400

10

350 d

20

311 222 400 331

11

300 250 N2 adsorbed (cm3 g–1) 200

220

c

311 002

a

321,410

400

150 b

dV/dD

100 50

1

2

3 4 2θ/°

5

6

0 0.0

2 4 6 8 10 12 14 Pore size/nm

0.2

0.4 0.6 P/Po

0.8

1.0

Figure 3.12: XRD patterns (a–d), N2 adsorption–desorption isotherm and pore size distribution (b) of calcined AMS-10 [7].

The XRD pattern (Figure 3.12a–d) of AMS-10 shows two well-resolved sharp peaks centered between 1° and 2°(2θ) with a d spacing ratio of about 3½/2½. The two peaks can be indexed to 110 and 111 reflections, or to 200 and 211 reflections, and so on. N2 adsorption and desorption isotherms of AMS-10 (Figure 3.12b) show a typical type IV isotherm with an evident H2 hysteresis loop in the range 0.45P42/mnm with increasing surface curvatures [1].

Figure 4.5: Schematic illustration of the pore size variation with changing mesophase curvature. (a) Larger surface curvature formed with larger head group area of surfactant, which leads to more windings of the tails resulting in a smaller pore size; while (b) smaller surface curvature result in an enlarged pore size [1].

4.2 Formation of different cage-type mesophases The key factors of the AMSs formation have been investigated by preparing a fullscaled synthesis field diagram, in which C14GluA, TMAPS, and NaOH, the compositions have been changed while keeping the concentration of surfactant, the TEOS/C14GluA ratio, and temperature constant.

4.2.1 Diverse mesostructures in the synthesis field diagram The synthesis conditions of the mesophases are summarized in Table 4.4. As shown in Figure 4.6, composition points in zone α gave rise to a coexistence of Fd-3m,

44

4 Structural control of AMSs

Pm-3n, P42/mnm, and Fm-3m; in zone β gave a mixture of cubic and 2D-hexagonal phase; and a mixed phase with bicontinuous cubic Pn-3m and 2D-hexagonal p6mm have been found in zone γ. However, no products have been formed in the zones δ, ε or ζ because of the too high mole fraction of NaOH, C14GluA, or TMAPS. The diverse mesostructures synthesized in the same system made it possible for the investigation on the structural formation mechanism.

Table 4.4: Synthesis conditions of the mesophases in the C14GluA/NaOH/TMAPS system [2]. Mesophase

Mesopore type

pHa

Pn-m pmm Fd-m Undefined Pm-n P/mnm Fm-m

Bicontinuous Cylindrical Micellar cubic – Micellar cubic Micellar tetragonal Micellar cubic

.–. .–. .–. .–. . . .–.

NaOH/CGluA

TMAPS/CGluA

.–. .–. .–. .–. . . .–.

.–. .–. .–. .–. . . .–.

a

The pH of the reaction was measured at 70 °C before the addition of TMAPS and TEOS.

Figure 4.6: Synthesis field diagram (mole fraction) of the C14GluA/NaOH/TMAPS synthesis system [2].

4.2 Formation of different cage-type mesophases

45

4.2.2 Effect of the synthesis composition on the mesostructure The mesostructure synthesized with the composition on the line starting at the point C14GluA/NaOH/TMAPS = 0:0:1 with the same ratio of NaOH/C14GluA with decreasing ratios of TMAPS/NaOH and TMAPS/C14GluA indicate that the determining factor is the ionization degree of the surfactant (NaOH/C14GluA); the amount of TMAPS is not prominent. In the line in direction c with keeping TMAPS fraction constant and increasing the NaOH/C14GluA ratio, the mesostructure was changed from bicontinuous cubic Pn-3m to 2D-hexagonal p6mm, cubic Fd-3m, an undefined cagetype phase and cubic Fm-3m; from 2D-hexagonal p6mm to cubic Pm-3n and tetragonal P42/mnm, indicating a dramatic mesopore geometry change from bicontinuous to cylindrical and further cage type, with increasing organic/inorganic interface curvature [2].

4.2.3 Mesophase formation dominated by the organic/inorganic interface curvature As a conclusion, the bicontinuous cubic phase with a low organic/inorganic interface curvature (1/2600 m2 g–1) and pore volumes (~1 cm3 g–1) [1]. Table 6.1: Porous properties of MSHSs synthesized with different EtOH/oleic acid molar ratios [1]. EtOH/oleic acid

Surface area (m g–)

Pore diameter (nm)

Pore volume (cm g–)

    

– . . . .

. . . . .

    

(a)

(b)

100 nm

(c)

100 nm

(d)

100 nm

100 nm

Figure 6.3: SEM and TEM images of calcined MSHSs synthesized with different APES/oleic acid molar ratios of 0.60 (a), 0.80 (b), 1.0 (c) and 1.2 (d) [1].

6.2 Mesoporous silica nanotubes (MSNTs)

69

In this synthesis system, APES plays an important role acting first as a CSDA to form mesopores and second as a base to transform electrostatically anionic surfactant into salt to emulsify the oil phase. Therefore, the APES/surfactant ratio profoundly affects the properties of the resultant MSHSs (Figure 6.3). The particle size of SHS was increased dramatically almost linearly with increasing APES/oleic acid ratio, while the thickness of the shell keeps almost the same value of 20 nm. Similar to the sample without cosolvent alcohol, the structural and morphological transformation from large particles of several microns size has been observed. A schematic illustration of formation of SHSs is shown in Figure 6.4. Initially, the system comprised oil and water phases. The surfactant ionized by APES acts as an emulsifier, and the nonionized surfactant and TEOS act as the oil phase, which gave rise to oil-in-water emulsion by mechanical stirring. The surfactant and TEOS diffused out from the oil droplets interact with APES and cocondensed with TEOS to form mesostructured silica, which is then deposited on the surface of the oil droplets to form MSHS. Without EtOH, due to a low hydrophile–lipophile balance number of the anionic surfactants neutralized by APES, mesostructures with low organic/inorganic interface curvature or large packing parameter g would be formed. As a result, a lamellar structured mesoporous silica is formed in the shell, and consequently to give nonporous SHS after calcination. By adding specific amount of EtOH, the effective head group area of surfactant would be enlarged and packing parameter g would be small, so that the mesostructure has higher curvature. As a result, MSHSs with high porosity shell can be obtained, giving access to the hollow sphere. The particle size is mainly controlled by the size of the oil droplets. Obviously, with increasing APES/oleic acid ratio, the concentration of emulsifier is increased; therefore, the size of the oil droplets is normally decreased, which is opposite to the result of this system. In this system, oleic acid salt formed by APES acts as an emulsifier, while oleic acid and TEOS act as the oil phase. Therefore, the large amount of APES promotes the emulsification of oil and facilitates the diffusion of oleic acid out of the oil droplet, resulting in the formation of large cavity MSHSs, and vice versa with low concentration of APES.

6.2 Mesoporous silica nanotubes (MSNTs) Mesoporous silica nanotubes (MSNTs) have been synthesized by partially neutralized anionic carboxylate surfactant C14-L-AlaS as template, N-trimethoxysilylpropyl-N,N, N-trimethylammonium chloride (TMAPS) as CSDA, and TEOS as silica source. The tube diameter was decreased by increasing the neutralization degree of surfactant and was

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6 Morphological control of AMSs

Figure 6.4: A formation mechanism of MSHSs by emulsion/micelle dual-templating route. The lamellar structured (a) and mesostructured (b) MSHSs were formed without and with adding cosolvent alcohol, respectively [1].

6.2 Mesoporous silica nanotubes (MSNTs)

(a)

50 nm

(b)

50 nm

(c)

50 nm

71

(d)

50 nm

Figure 6.5: TEM images of the MSNTs synthesized with different TMAPS/C14-L-AlaS molar ratios of 0.1 (a), 0.2 (b), 0.3 (c), and x = 0.4 (d) [3].

(a)

50 nm

(c)

(b)

50 nm

50 nm

Figure 6.6: TEM images of the MSNTs synthesized with different HCl/C14-L-AlaS molar ratios of 0.45 (a), 0.50 (b), and y = 0.55 (c) [3].

independent on the CSDA/surfactant molar ratios; the wall thickness was increased by increasing both the neutralization degree of surfactant and CSDA/surfactant molar ratios (Figures 6.5 and 6.6). All samples gave type IV isotherms with two sharp adsorption–desorption steps in the relative pressure range of 0.4–1.0, indicating the existence of two types of narrow mesopore size distributions and tube-like macropores. The morphological and structural evolution of the MSNT was investigated as a function of the reaction time. The insoluble carboxylic acid was immediately formed upon addition of HCl. The gel state was not changed before and after the addition of TMAPS and TEOS, indicating that the reaction occurred without dissolution of gels. The lipid nanotubes were composed of a hollow inner core surrounded by a wall of 10 nm thickness (Figure 6.7), which was considered to be formed by three to four interdigitated lipid bilayers, according to the d spacing (2.8 nm) of the lamellae determined from the XRD pattern. The sample synthesized at 12 h reveals the morphology with long hollow channels, which became the aggregate composed of several to dozens of some long cylinders (24 h) gradually, and then was divided into single tubes (48 h). The calcined product sampled after 12 h differs from the as-synthesized one, indicating that the long organic rods were removed. The wall formation can be observed from the

H-lipid nanotubes

50 nm

100 nm

12 h-cal

12 h-as

100 nm

200 nm

24 h-cal

24 h-as

200 nm

200 nm

48 h-cal

48 h-as

Figure 6.7: TEM images of lipid nanotubes (low- and high-magnified), as-synthesized (denoted as xx h-as), and calcined (denoted as xx h-cal) samples [3].

200 nm

1 μm

L-lipid nanotubes

72 6 Morphological control of AMSs

73

6.2 Mesoporous silica nanotubes (MSNTs)

0.6

16 14 12 10 8 6 4 2 0 –2

HCI/C14-l-AlaS=0.5

Absorbance (a.u.)

Elipticity (mdeg)

calcined product at 24 h, indicating that the composite of surfactant and inorganics was formed and then the MSNTs were generated after a longer reaction time of 48 h. It has been found that almost all chiral surfactants can produce MSNTs but achiral surfactants fail, suggesting that the helical assembly of the surfactant would be essential for this synthesis. The strong positive circular dichroism (CD) peak around 220 nm of the solution with HCl/C14-L-AlaS = 0.5, when compared to weak negative Cotton effect observed around 235 nm and the stronger positive Cotton effect at C16-L-Ile > C16-L-Val > C16-L-Ala. The absolute ee value of the CMS formed with C16-L-Pro was much smaller than others. The racemic and achiral N-acylamino acids gave rise to racemic CMSs. It was also found that the effect of NaOH/surfactant ratio was much smaller than temperature. It is unquestionable that the ratio of the handedness of CMSs (L- and R-CMS) is controlled either by kinetics or by thermodynamics in one of the most critical stages. Thus, the ee can be considered to be determined by the relative rate constant for L- and R-CMS formation (kl/kr) or by the relative stability of L- and R-CMS (Kl/Kr), which is equivalent to the relative abundance of L- and R-CMS, that is (100 + ee)/ (100 – ee), in which ee = 100% × [(l – r)/(l + r)]. The differential free energy change of the critical stage is ΔΔG = − RTlnðl=rÞ

(6:1)

81

6.4 Control of morphology and helicity of CMS

100

3.5

80

3.0

C16-L-Ala C16-L-Val C16-L-IIe C16-L-Met

60 40 20

In(I/r)

ee (%)

C16-L-Phe

2.5

0

2.0 1.5

–20 –40 –60 –80

C16-L-Ala C16-L-Val C16-L-IIe C16-L-Met C16-L-Phe C16-L-Pro C16-D-Phe

–100 280 290 300 310 320 330 340 Temperature (K)

1.0 0.5 0.0 0.0030

0.0032 1/T (K–1)

0.0034

Figure 6.17: Temperature dependence of the ee’s and ln(l/r) value of the CMSs synthesized with different N-acylamino acids [5].

where R and T are the gas constant and temperature. The ΔΔG is related to the differential enthalpy ΔΔH and entropy changes ΔΔS: ΔΔG = ΔΔH − TΔΔS

(6:2)

Then the following equation was obtained: lnðl=rÞ = − ΔΔH=RT + ΔΔS=R

(6:3)

According to eq. (6.3), the ln(l/r) values were plotted as a function of the reciprocal temperature (Figure 6.17b) base on five representative N-acylamino acid synthesis results. The plots show linear correlations, indicating both the ΔΔH and ΔΔS values are constant over the temperature range. The ΔΔH and ΔΔS values were calculated from the slope and intercept of each plot (Table 6.5). The absolute ΔΔH and ΔΔS values were decreased in the order of C16-L-Met>C16-L-Phe>C16-L-Ile>C16-L-Val>C16-L-Ala. As illustrated in Figure 6.18, the relation between ΔΔH values and ΔΔS values gave an excellent straight lines with almost negligible intercepts, which led to the empirical equation: ΔΔH = βΔΔS

(6:4)

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6 Morphological control of AMSs

Table 6.5: Differential enthalpy (ΔΔH) and entropy (ΔΔS) changes in the CMS synthesis system [5]. N-acylamino acid C-L-Ala C-L-Val C-L-Ile C-L-Met C-L-Phe a b

ΔΔHa

ΔΔSb

– – – – –

– – – – –

Unit in kJ mol–1. Unit in J mol–1 K–1.

Figure 6.18: Compensation plot of the differential enthalpy (ΔΔH) against the differential entropy change (ΔΔS) upon CMS synthesis system [5].

β-Value can be obtained from the slope of the compensation plot shown in Figure 6.34. Merging eq. (6.4) into the Gibbs–Helmholtz equation (eq. (6.2)) gives equation: ΔΔG = ð1 − T=βÞ ΔΔH

(6:5)

When the temperature is equal to β, any change in ΔΔH never affects the ΔΔG and the product selectivity throughout the alteration in internal or external conditions, such as surfactant, substituent, and solvent. From the slope of the straight line (Figure 6.18), β value, the equipodal temperature (T0) was calculated as 305 K. From the effects of the molecular structure of template and the synthesis temperature on the ee of CMS, it has been considered that the dynamic structure of template is essential for the formation mechanism of CMSs. The amphiphilic molecules prefer helical propeller-like packing due to the molecular chirality to form cylinder-like

6.4 Control of morphology and helicity of CMS

83

Figure 6.19: The molecular origin of the antipodal CMS derived from the helical propeller-like packing of chiral amphiphilic molecules [5].

micelles, handedness of which is dominated by the chirality of the amphiphilic molecule (Figure 6.19). The handedness of CMSs would be determined by the handedness of helical arrangement of amphiphilic molecule. CMSs with twisted larger helical assemblies would be achieved by closest packing of the propeller-like rod micelles. The chiral sense of packing controls the enantiopurity of CMSs. The effect of temperature on the ee interpreted thermodynamic equilibrium between two antipodal helical packing triggered by the temperature-driven conformational changes of amphiphilic molecules. By rotating the C–N single bond of the amino acid head, a new conformer diastereomeric to the original one can be formed. These rotational isomers (rotamers) are diastereomeric to each other relative to the chiral center, which form the antipodal helical structures independently. In L-form N-acylamino acids, the conformer of the lowest energy led to left-handed CMS, while the less stable conformer of smaller proportion gave right-handed CMS. The proportion of the conformer with higher energy is increased at higher temperature, which led to decreasing of ee. The absolute ΔΔH and ΔΔS values were decreased in the substituent size decreasing order of CH2CH2SMe, benzyl, sec-Bu, iso-Pr, and methyl, indicating that the ability (ΔΔH) of the chiral amphiphilic molecule to form CMS with high ee, and its

84

6 Morphological control of AMSs

sensitivity (ΔΔS) of the reaction temperature becomes weaker as the substituent size decreases. The conformational freedoms of the N-acylamino acids aggregation are determined by steric hindrance of the substituent connected to the chiral center. The bulky substituent should reduce the conformational freedoms leading to the formation of CMS of higher ee. At the same time, the smaller the substituent the more sensitive to the reaction temperature.

6.4.2 Stirring effect on controlling morphology and helicity of CMS The experimental observations were reported that the helicity and the morphology of the CMS are determined by the stirring rate. The synthesis mixtures of CMS were stirred for 10 min and then allowed to react under static condition. As shown in Figure 6.20, twisted ribbon-like structures and various twisted rod-like structures with different chiral pitches were formed with the stirring rate lower than 300 rpm, while highly ordered CMSs were formed with stirring rate increased to 400 rpm. The length and the diameter of the hexagonal rod were, respectively, decreased and increased with increasing stirring rate. The outer diameter of rod increased with increasing stirring rate, and the pitch length increased with an increase in the rod diameter with constant pitch/rod diameter ratio of ~15.5 (Figure 6.21). Obviously, the helical morphology of mesoporous silica is induced by stirring, and its size and pitch length are controlled by the stirring rate [6]. The XRD patterns and N2 adsorption–desorption isotherms exhibit that CMSs synthesized with different stirring rate have the 2d-hexagonal p6mm structure with similar a = 6.5–6.7 nm and the similar Barrett–Joyner–Halenda (BJH) pore diameter of ~3.6 nm, indicating that the same pore size and wall thickness have been formed regardless of the stirring rate. The ratio of left-/right-handedness is proved to be approximately 7.5/2.5 regardless of the difference in stirring rate or direction. The noneffect of stirring direction on the formation of CMS has also been confirmed from formation of racemic CMS by achiral surfactant as template, indicating that the handedness is dependent on the chirality of the surfactant. It has been considered that the vortices occurred upon stirring affect the aggregation of surfactant, CSDA, and inorganic source. At lower stirring rates, the supramolecular assemblies were less ordered in diameter and length growing in the longitudinal direction, while at higher stirring rates, the supramolecular assembling is more uniform in shape and facilitates the transversal growth of mesoporous rods of different helical pitches. The rod may be broken up easily by increasing torque created by stirring. The pitch of the helical channel is decreased with increasing the amount of channels. The helical pitch/rod diameter ratio has been found to be dependent on the surfactant structure.

Figure 6.20: TEM images of CMSs synthesized with different stirring rates of 200 (a), 400 (b), 600 (c), and 800 rpm (d) [6].

6.4 Control of morphology and helicity of CMS

85

86

6 Morphological control of AMSs

7

Pitch length (μm)

6 5 4 3 2 1 0 100

200 300 Rod diameter (nm)

400

Figure 6.21: Pitch length of CMS with different rod diameter [6].

6.4.3 Temperature dependence on the formation of various CMS The temperature is an important factor for the formation of CMS due to both the thermodynamics and the kinetics of the lipid molecules and the hydrolysis of TEOS followed by condensation/polymerization. Different mesoporous silicas were synthesized by using C14-L-AlaS as a template and APES as a CSDA in a temperature range of 0–20 °C (Figure 6.22). The exclusively right-handed helical ribbon (HR) was synthesized at 0 °C, in which two parallel porous systems do not penetrate through the wall. No resolved peaks were observed from the XRD patterns because of the presence of the disordered pore in the thin wall. The irregular hollow spherical particles were formed at 10 °C, in which the disordered pores penetrate the wall from inner to outside as confirmed by a broad peak of the XRD pattern in 2θ range of 1–2°. Disk-shaped crystals were synthesized at 15 °C, in which the pores are not straight at the edge with a 2d-hexagonal structure consistent with the XRD pattern. CMS was formed with increasing temperature of 20 °C [7]. The structural evolution of the mesophases was investigated during synthesis as a function of the reaction time (Figure 6.23). The lipid freeze-dried organogel showed an interdigitated layer structure, indicating that hydrophilic carboxyl head groups are present on both the inner and outer surfaces of the organic tube. At 0 °C, with the addition of APES and TEOS, the morphologies changed from flat-tape (1 min), to loosely coiled HR (30 min) and finally to tightly coiled HR or paper-roll-like tubules (2 h), which indicates that the HR was formed by anisotropic tape-to-helicity transformation from lamellar structure to disordered mesoporous structure with both shortening of the helical pitch and widening of the tape simultaneously. At 0 °C, chiral C14-L-AlaS self-assembles into flat-tape with lamellar structure. Chirality of HR demands the helical structure of bilayer strands to strengthen hydrogen bonding between

50 nm

1 μm

50 nm

(b)

(b)

50 nm

100 nm (c)

(c)

50 nm

500 nm (d)

(d)

50 nm

500 nm

Figure 6.22: SEM and TEM images of extracted mesoporous silicas synthesized at different temperatures: 0 °C (a), 10 °C (b), 15 °C (c), and 20 °C (d) [7].

(a)

(a)

6.4 Control of morphology and helicity of CMS

87

88

6 Morphological control of AMSs

0 °C - organic gel

0 °C –1 min

500 nm 10 °C –5 min

0 °C –30 min

500 nm 10 °C –20 min

500 nm 15 °C –5 min

500 nm

100 nm

100 nm

500 nm

100 nm 15 °C –2 h

100 nm 20 °C –15 min

500 nm

500 nm 10 °C –2 h

15 °C –30 min

20 °C –6 min

100 nm

500 nm 10 °C –35 min

15 °C –15 min

20 °C –20 s

0 °C –2 h

100 nm 20 °C –25 min

500 nm

500 nm

Figure 6.23: SEM images of the freeze-dried lipid organic gel and the as-synthesized products sampled at different reaction times and different reaction temperatures [7].

amide groups. With addition of APES and TEOS, the arrangement of C14-L-AlaS twist from neighbor to neighbor led to the whole ribbon to coil into tube and simultaneously into a mesoporous one through reassembly by penetrating APES and TEOS into the ribbon from its surface and convert the lipid wall. At 10, 15, and 20 °C, all of the starting materials are flat-tapes with lamellar structure, then the tapes melted into irregular monoliths and then the mixtures to form hollow spheres, disk-like, and helical hexagonal rod, respectively. The effect of temperature has been explained in terms of surfactant packing (Figure 6.24). The packing of the C14-L-AlaS is dominantly effective for the formation of the ordered mesostructure. With increasing temperature, the stereorepulsion between the head groups increases, which leads to an increase in the effective head-group area resulting in decreasing g parameter of micelle with a transition

6.4 Control of morphology and helicity of CMS

89

Figure 6.24: Schematic illustration of the mesoporous HR formation process.

from bilayered to cylindrical micelles. It is reasonable that the HR with bilayer lamellar structure was formed under lower temperature, and other highly ordered mesoporous silicas with high curvature were formed at higher temperature, respectively [7].

6.4.4 Controlling the pitch length of CMSs It has been found that CMSs can also be synthesized by using anionic achiral amphiphile SDS as the directing agent, TMAPS as the CSDA, and TEOS as the silica source. The effects of synthetic parameters, such as the TMAPS/SDS molar ratio, temperature, and pH value upon the morphology and mesostructure of the CMSs have been carefully examined [8]. Figure 6.25 shows that the rod diameter increased with increasing TMAPS/SDS molar ratios, and pitch lengths increased with increasing rod diameters as well as with increasing TMAPS/SDS molar ratios. The pitch length–rod diameter plots maintained a linear relationship.

90

6 Morphological control of AMSs

(a)

(b) 7,000

TMAPS/SDS 0.34 0.38 0.42 0.46 0.50

4,000

5,000

300 250

Pitch length (nm)

Pitch length (nm)

5,000

350

3,000

4,000 200 3,000 150

Diameter (nm)

6,000

6,000

2,000 100

2,000

TMAPS/SDS vs pitch length

1,000

1,000 0 0

100 200 300 Diameter (nm)

0 0.3

400

TMAPS/SDS vs diameter

0.35

0.4 0.45 0.5 TMAPS/SDS

50 0 0.55

Figure 6.25: Pitch length versus rod diameter (a), and rod diameter and TMAPS/SDS molar ratios of the CMSs synthesized as a function of TMAPS/SDS molar ratios of 0.34, 0.38, 0.42, 0.46, and 0.50 (b) [8].

Figure 6.26 shows that the rod diameter increased with increasing synthesis temperature; the pitch length increased almost linearly with increasing rod diameter; however, the slope of such linear relationship increased with increasing reaction temperature from 0 to 30 °C. The pitch length to rod diameter relationship at different pH values was plotted in Figure 6.27. Similar to the effect of the TMAPS/SDS molar ratio and reaction temperature, the rod diameter increased with increasing basicity. Besides, the pitch length increased with increasing rod diameter and almost kept a linear relationship across the pH range studied. It was hypothesized that the helical mesostructure would be originated from the helical propeller-like micelle of amphiphilic molecules due to their asymmetric shape. The pitch length of the HMS depends on the propeller-like micelle modulated by the twisting power. A quantitative description of the dependence of the pitch length P on the rod diameter D and the moment M0 of the propeller-like micelle was given as P∝

D M0

91

6.4 Control of morphology and helicity of CMS

(a)

(b) 7,000

4,500 0 °C 10 °C 20 °C 30 °C

3,500 Pitch length (nm)

5,000

200

4,000

4,000 3,000 2,000

150

3,000 2,500 2,000

100

1,500 1,000

50 Temperature vs pitch length

1,000 0

Diameter (nm)

6,000

Pitch length (nm)

250

5,000

500

Temperature vs diameter

0 0

100 150 200 Diameter (nm)

0

250

10 20 Temperature (°C)

0 30

Figure 6.26: Pitch length versus rod diameter of the CMSs synthesized at different reaction temperatures (a). The average pitch length and average diameter as a function of different initial aging temperature (b) [8].

(a)

(b) 5,000

6,000

4,500

10.3 10.6 10.8 10.9

4,000

200

3,500

3,000

2,000

3,000 2,500 2,000

100

1,500 1,000

1,000

150

Diameter (nm)

4,000

pH

Pitch length (nm)

Pitch length (nm)

5,000

250

Pitch length vs pH value

50

Diameter vs pH value

500 0 100

150

200 250 Diameter (nm)

300

0 10.2

0 10.4

10.6 10.8 pH Value

11

Figure 6.27: Pitch length versus rod diameter of the CMSs synthesized with initial pH values (a). The average pitch length and average diameter as a function of initial pH values of 10.3, 10.6, 10.8, and 10.9 (b) [8].

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6 Morphological control of AMSs

It can be considered that the shorter pitch length of the micelle would give rise to a strong moment for the hexagonally stacked rods in the same micelle diameter, and vice versa with a longer pitch length, as illustrated in Figure 6.28. The pitch length of the propeller-like micelle increased with increasing temperature leading to a smaller moment M0, resulting in the linear relationship with larger slope of the pitch length to rod diameter (Figure 6.25a).

Figure 6.28: Dependence of the pitch length of the HMSs on the helical propeller-like micellar packing of the amphiphiles [8].

6.4.5 Formation mechanism of achiral amphiphile-templated helical mesoporous silicas As mentioned earlier, the CMS (ee > 0) and racemic HMS (ee = 0) have been synthesized with chiral and achiral amphiphiles, respectively, indicating that chiral and achiral amphiphiles similarly induce the formation of helical superstructures, while the chirality determines the handedness supramolecular chirality [9]. The helical mesostructure would be originated from the asymmetric molecular shape of both chiral and achiral amphiphiles through their helical propeller-like micellar packing. Nine achiral amphiphiles were used to elucidate the asymmetric molecular shape and its effectiveness to generate the HMS structure, as shown in Figure 6.29. The XRD patterns and SEM and TEM images (not shown)of the samples showed the racemic twisted rod-like crystals with hexagonally ordered channels [10]. It has been considered that, similar to the chiral amphiphiles, the helical propeller-like micellar structure can also be formed by achiral amphiphiles due to their instantaneous asymmetric shape. For example, in the case of C16-PyrBr, the C–C and C–N bonds can rotate to give its asymmetric nature relative to the x- and/or y-axes in the micelle formation (Figure 6.30). Mirror-imaged conformations (I and II) can be generated in equal proportion, which give rise to an opposite-handed helical packing

6.4 Control of morphology and helicity of CMS

93

Figure 6.29: Molecular structures of the achiral amphiphiles used in this work [10].

sense with equal quantities. For other achiral amphiphiles, different nonsphere-like entities can be imaged. Different nonspherical molecular conformations lead to a helical propeller-like packing that is driving force for the formation of the HMS. For the chiral amphiphiles, mirror-imaged conformations cannot be generated, which gave rise enantiomeric chiral mesoporous materials. Figure 6.31 shows the correlation between the pitch length and diameter of the HMS templated by C16-2-AIBA, SDS, and C14-GlyNa. The pitch length increased linearly with increasing diameter. The slope of C14-GlyNa is smaller than C16-2-AIBA and SDS; and the pitch length was increased in sequence of C16-2-AIBA