Supramolecular Amphiphiles: Volume 23 (Monographs in Supramolecular Chemistry) - Hardcover

Hardcover

ISBN 10: 1782625429 ISBN 13: 9781782625421

Publisher: Royal Society of Chemistry, 2017

Synopsis

This book provides a detailed and systematic overview of the history, development and current scientific knowledge and applications of supramolecular amphiphiles.

About this title

About the Author

Prof. Xi Zhang, received his B.S. degree (1986) and Ph.D (1992) from the Department of Chemistry, Jilin University. As a joint-training Ph.D student, he worked with Prof. Helmut Ringsdorf, University of Mainz, Germany, between 1991 and 1992. He became a full professor of Jilin University in 1994. Since 2003, he has been a full professor of the Department of Chemistry, Tsinghua University. Currently, he is also the Department Chair.

From the Back Cover

An amphiphile is a molecule that contains a hydrophilic part and a hydrophobic part, linked by covalent bonding. Supramolecular amphiphiles (supra-amphiphiles) are amphiphiles linked by non-covalent interactions. As they employ non-covalent interactions, these species demonstrate adaptability and reversibility in conformational transformation, making them one of the most important emerging species in supramolecular chemistry. They have proven important in bridging the gap between molecular architecture and functional assembly.
This book is written and edited by the current leaders in the topic and contains a foreword from Professor Jean-Marie Lehn, a father of the supramolecular chemistry field. Bringing together supramolecular chemistry and colloidal and interfacial science, the book provides a detailed and systematic introduction to supramolecular amphiphiles. Chapters explain how to employ non-covalent interactions to fabricate supra-amphiphiles. The book opens with an introduction to the history and development of the field, followed by chapters focussing on each type of interaction, including host-guest interaction, electrostatic interaction, charge-transfer interaction, hydrogen bonding and dynamic covalent bonds.
This book will be a valuable resource for students new to this field and experienced researchers wanting to explore the wider context of their work.

Excerpt. © Reprinted by permission. All rights reserved.

Supramolecular Amphiphiles

By Xi Zhang

The Royal Society of Chemistry

Copyright © 2017 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-78262-542-1

Contents

Chapter 1 Evolution of Supra-Amphiphiles from Amphiphiles Yuetong Kang and Xi Zhang, 
Chapter 2 Supra-Amphiphiles Based on Host–Guest Interactions Guangtong Wang, Jiang-Fei Xu and Xi Zhang, 
Chapter 3 Supramolecular Amphiphiles Based on Multiple Hydrogen Bonds N. Kimizuka, 
Chapter 4 Electrostatic Supra-Amphiphiles O. A. Bell, K. Watanabe and C. F. J. Faul, 
Chapter 5 Supra-Amphiphiles Based on Charge Transfer Interactions Amrita Sikder and Suhrit Ghosh, 
Chapter 6 Supra-Amphiphiles Based on Coordination Bonds Chao Wang, Cheng-Hui Li and Xi Zhang, 
Chapter 7 Dynamic Covalent Surfactants and Amphiphiles David A. Fulton, 
Subject Index, 

CHAPTER 1

Evolution of Supra-Amphiphiles from Amphiphiles

YUETONG KANG AND XI ZHANG

1.1 Molecular Amphiphiles

Molecular amphiphiles are molecules that have both a hydrophilic part and a hydrophobic part linked by covalent bonds. With their ability to significantly decrease surface tension, amphiphiles can self-assemble at air–liquid or liquid–liquid interfaces, leading to the formation of organized molecular assemblies such as Langmuir monolayers, micelles, vesicles and emulsions. Amphiphiles are therefore used extensively as detergents, emulsifiers, solubilizers and foaming agents, and are essential in a number of modern industries, from mining and manufacturing to textiles and medicine. Amphiphiles such as phospholipids are crucial components of living organisms because they are the primary constituents of biological membranes, which are essential in the structure of cells. Membranes provide a compartmentalized microenvironment, ensuring that biochemical processes occur in a systematic rather than a chaotic manner, enabling the existence of life itself.

Molecular amphiphiles can be of different topologies (Figure 1.1) in terms of the different motifs acting as the linkage between the hydrophilic and hydrophobic parts. A hydrophilic headgroup can be covalently linked to single, double or triple hydrophobic alkyl chains. The use of synthetic molecular amphiphiles to form vesicle-like structures dates back to the 1977 paper by Kunitake et al. Two hydrophilic headgroups can be covalently linked by one or two hydrophobic alkyl chains, resulting in the so-called bolaform amphiphiles. Two molecular amphiphiles covalently linked at their charged headgroups are classified as gemini amphiphiles. The varied molecular amphiphiles have different properties, leading to different applications. For example, the critical micelle concentration of gemini amphiphiles is typically lower than that of conventional single-tailed amphiphiles, resulting in more stable micelles. The phase transition temperature of bola-form amphiphiles can be higher than that of conventional molecular amphiphiles. In other words, if a biological membrane contains a high percentage of bola-form amphiphiles, it remains stable at relatively high temperatures, which is essential for organisms that survive in harsh environments such as hot springs and submarine volcanoes.

Polymeric amphiphiles can self-assemble into structures with enhanced stability and structural diversity relative to the small molecular amphiphiles. The problem with polymeric amphiphiles is the two-dimensional (2D) orientation of the three-dimensional polymer coil. Ringsdorf et al. introduced the concept of a flexible spacer into polymeric amphiphiles to overcome this problem. Figure 1.2 shows that there are three ways to introduce flexible spacers. Homopolymers with side-group spacers (Figure 1.2A) have been synthesized from lipids containing a hydrophilic spacer group between the membrane-forming amphiphilic part and the polymerizable unit. Hydrophilic comonomer amphiphilic copolymers with main-chain spacers (Figure 1.2B) can be prepared by the copolymerization of simple lipids. The combination of both spacer types (Figure 1.2C) can be realized by the copolymerization of spacer-containing lipids with hydrophilic comonomers.

Amphiphilic block copolymers are another kind of polymeric amphiphile. As reported by Eisenberg et al., amphiphilic block copolymers can selfassemble to form vesicle-like structures. By fine tuning the chain length, monomeric unit and structural topology of both the hydrophilic and hydrophobic segments of amphiphilic block copolymers, diverse organized structures can be achieved, such as spherical micelles, cylindrical micelles, bicontinuous rods, lamellae and vesicles.

1.2 Molecular Amphiphiles for Self-Assembly

Self-assembly is a spontaneous process of forming ordered structures from a disordered initial state. Because the hydrophobic group carried by every amphiphile strongly avoids contact with water while the hydrophilic group favors contact with water, molecular amphiphiles tend to aggregate, although this does not usually result in macroscopic-scale phase separation. The unfavorable interaction between the hydrophobic group and water can be suppressed when amphiphiles self-assemble into micelles or onto low-energy surfaces, in which case the hydrophobic group aggregates in the core and the hydrophilic group is distributed on the interface with the bulk water. In this way, the net free energy of the whole system is reduced.

Amphiphiles can self-assemble to form well-defined organized structures, such as micelles and vesicles. The driving force behind the formation of such organized structures is the hydrophobic effect. The molecular basis for self-assembly is amphiphilicity. This means that controllable self-assembly can be realized by tuning the amphiphilicity of the building blocks. For example, molecular amphiphiles can form micelles by self-assembly. By relying on supramolecular chemistry to make the amphiphiles more hydrophilic, we can destroy the molecular basis for self-assembly, leading to the disassembly of micelles. When the amphiphilicity is recovered due to the noncovalent nature of supramolecular chemistry, the destroyed micelles can reform.

1.2.1 One-Dimensional Assemblies from Molecular Amphiphiles

Micellar structures with a defined size and shape can be used in applications such as templates for making nanostructured materials and mimicking biomineralization processes. Engineering robustness is required to make these applications practical. However, micellar structures are inherently dynamic and fluid. The most straightforward method of stabilizing micellar structures is based on a covalent approach, mainly using polymerizable amphiphiles.

There is a supramolecular approach to stabilizing micellar structures by noncovalent interactions. To this end, we need to use an appropriate molecular design to enhance the intermolecular interactions between the amphiphiles. As an example, we designed and synthesized a bola-form amphiphile bearing a biphenyl mesogenic group and two hydrophilic pyridinium heads (BP-10). In situ atomic force microscopy (AFM) observations (Figure 1.3) showed that the bola-form amphiphile BP-10 formed cylindrical micelles with a spaghetti-like morphology at the mica–liquid interface. The average width of the structures was around 40 nm and some reached as long as several micrometers in length. The ex situ AFM image shows the same structure as the in situ AFM image, suggesting that BP-10 can retain the structure formed in solution. Therefore enhancing the noncovalent interactions between amphiphiles stabilizes the micellar structure against drying.

Combining various intermolecular interactions, including hydrogen bonding and p–p interactions, allows the fabrication of highly stable organized structures. For example, we designed and synthesized a bola-form amphiphile (DPP-11A) with two carboxylic acid headgroups and a diketopyrrolopyrrole (DPP) chromophore in the central part. Figure 1.4 shows that DPP-11A can self-assemble to form supramolecular nanofibers. We tested the thermal stability of these fibers by hot-stage AFM. When the temperature was increased from 28 to 130 °C, no obvious change in the contours of the fibers was observed. When the temperature was increased to 140 °C, several parts of the nanofibers melted together. Therefore these supramolecular nanofibers have good thermostability up to 130 °C.

1.2.2 Two-Dimensional Assemblies from Molecular Amphiphiles

Two-dimensional planar structures possess a low curvature in addition to a smaller thickness than their planar dimensions. Bola-form amphiphiles are usually able to self-assemble to form zero- or one-dimensional (1D) assemblies. However, we demonstrated that a cationic bola-form amphiphile (DPP-11-Ts), bearing DPP as the central chromophore, can form 2D planar aggregates. Different counter anions induced different properties in these cationic bolaamphiphiles. To keep the same structural skeleton, the bola-amphiphiles form 1D assemblies when the counter ion is bromide, whereas they form 2D aggregates when the counter ion is tosylate. Therefore the counter ion can induce a transition from 1D to exclusively 2D planar structures (Figure 1.5).

To understand why the tosylate counter ion can induce a transition from 1D to 2D structures, we propose a simple model (Figure 1.6). First, the counter ion significantly affects the average interfacial area occupied by each headgroup. If the counter ion, such as the bromide ion, prefers to dissolve as free ions in solution, then the electrostatic repulsion between adjacent headgroups gives rise to a relatively large average interfacial area occupied by each headgroup. As a result, the curvature of its interface is high, resulting in bent interfaces (Figure 1.6a). The bent interface leads to the formation of vesicles, spherical micelles or fibers. If, however, the counter ion, such as tosylate, exhibits strong binding with a cationic headgroup and prefers the insertion of the organic substructure into the hydrophobic interior of the assemblies, the average interfacial area occupied by each headgroup is small enough to generate planar interfaces of low or zero curvature (Figure 1.6b). Following these arguments, we suggest that these interfaces possess low curvature, which is responsible for the formation of 2D planar structures.

It is anticipated that crystal structure data will support this assumption. However, single crystals of DPP-11-Ts cannot be prepared due to its long alkyl chains containing 11 carbon atoms. Fortunately, by shortening the chain length to seven carbon atoms, single crystals of DPP-7-Ts can be obtained. This crystal is crystallized directly from a concentrated aqueous solution and is therefore likely to reflect the molecular arrangement of its aggregates in solution. Figure 1.7 shows that the crystal is made up of numerous 2D planar monolayers (only one monolayer is shown here). In each monolayer, the DPP chromophores exhibit extended orientational stacking along the 2D plane. A favorable insertion of hydrotropic anions such as tosylate at the aggregate–water interface can be clearly observed from the crystal structure. Figure 1.6b shows that both sides of the 2D planar monolayers are hydrophilic surfaces consisting of cationic pyridinium groups and tosylate ions. The tosylate ion inserts its hydrophobic segment into the hydrophobic cavity of the aggregates, leaving its polar head pointing toward the water phase. This energy-favorable orientation means that tosylate prefers to stay at the aggregate–water interface. Compared with the example of the freely dissolved bromide ion, the inserted tosylate anion weakens the repulsive forces between the adjacent cationic head groups, favoring interfaces of low curvature and thus leading to the formation of 2D planar structures.

In addition to the tosylate counter ion, we used different hydrotropic anions to reveal how the size and substituent of the organic segments in hydrotropic anions determine their ability to induce the formation of 2D planar aggregates. We have demonstrated that the ability of a hydrotropic counter ion to induce the formation of 2D planar aggregates depends weakly on its polar head, but is strongly correlated with the size and substitution pattern of its organic portion. In addition, the shapes of the obtained 2D planar aggregates can be triangular, quadrangular or hexagonal and they can be modulated by both hydrotropic counter anions and the embedded conjugated moieties. Using this knowledge, it is possible to control the self-assembly of bola-amphiphiles by easily exchanging the counter ions, which provides a simple and feasible way of approaching functional supramolecular materials.

Lowering the dimensionality of intermolecular interactions is another effective strategy for fabricating 2D structures from amphiphiles. Figure 1.8a shows that a 3D structure is formed from melamine protonated by various kinds of acid. In the direction perpendicular to the figure, p–p interactions drive the stacking of protonated triazine rings. In one of the directions parallel to the figure, counter ion bridging (including counter ion bridged multiple hydrogen bonds as well as electrostatic interactions) drives the assembly. In the other direction, adjacent protonated melamines are connected by multiple hydrogen bonds. As the interaction of multiple hydrogen bonds can be easily blocked by the introduction of alkyl chains, dialkylated melamine derivatives with different length alkyl chains (Mela-n) are synthesized as building blocks (Figure 1.8b), where n refers to the carbon number of the alkyl chain. In the assemblies of protonated melamine derivatives, alkyl chains are used to eliminate the interaction of multiple hydrogen bonds, while the interactions of counter ion bridging and p–p stacking can be preserved, thus generating 2D structures.

Self-assembly of the protonated melamine derivative (Mela-12·HCl) leads to the formation of 2D microsheets. As indicated by transmission electron microscopy (TEM) (Figure 1.9a), the 2D assemblies are planar sheets with elegant rectangular shapes, extending in size over the micrometer scale. The AFM image of Mela-12·HCl in Figure 1.9b shows microsheets with highly flat surfaces over a length scale of up to several micrometers and with stepped edges of height intervals on the nanometer scale, which suggests that the assemblies are 2D microsheets of lamellar structures. A similar strategy also works to further lower the dimensionality of the self-assemblies of melamine-derived amphiphiles from 2D to 1D structures.

1.2.3 Stimuli-Responsive Molecular Amphiphiles

Stimuli-responsiveness in an amphiphile system means that the physicochemical properties of the system can vary under specific external stimuli. Various factors can be designed and applied as external stimuli for stimuli-responsive amphiphiles, such as pH, redox conditions, heat and specific chemical substances. It is not possible to cover every aspect of this vastly developed area in this chapter, so we will briefly introduce amphiphile systems responsive to carbon dioxide (CO2), enzymes and light.

1.2.3.1 CO2-Responsive Amphiphiles

As a benign gas that is easily added to, and separated from, a system, CO2 is a good stimulant to modulate the physicochemical properties of a surfactant system. Jessop et al. designed an elegant CO2responsive amphiphile containing an amidine group as the polar head and a long alkyl chain as the hydrophobic part (Figure 1.10). This amphiphile is a poor surfactant before exposure to CO2, but becomes an effective surfactant when exposed to CO2 because the pristine amidine group can react with CO2 to yield the amidinium bicarbonate salt, which is of higher polarity. The hexadecane–water mixture containing this amphiphile can be shaken to generate an emulsion. If previously treated with CO2, the emulsion is significantly stabilized, with no evidence of phase separation within 3 h, whereas the untreated emulsion clearly separates into two layers within 5 min of the end of shaking. The ability of the CO2-treated amphiphile to form an emulsion can be decreased by bubbling argon or air into the solution; the ability to form an emulsion is recovered when CO2 is bubbled through again. This process can be reversibly and facilely repeated, which is highly favorable in practical applications, such as enhanced oil recovery.

The same amidine moiety can be adopted in the design of functional molecular amphiphiles. For example, we synthesized a CO2-responsive dispersant (PyAH) bearing a pyrene and an amidine moiety, which became an effective surfactant after exposure to CO2. Figure 1.11 shows that, through strong p–p interactions between the pyrene moiety and single-walled carbon nanotubes (SWCNTs), we demonstrated that PyAH can be modified onto SWCNT surfaces to promote the dispersion of SWCNTs in water.Taking advantage of the gas-triggered interconversions between the amidinium cation and amidine, reversible control of the solubility of SWCNTs has been achieved simply through the alternate bubbling of CO2 and argon. This work demonstrates a new method for the controlled dispersion and aggregation of SWCNTs.

The amidine moiety can also be attached to the side-chains of block polymers for the development of CO2-responsive polymeric amphiphiles. Figure 1.12 shows that amidine-containing block copolymers can spontaneously form vesicles in aqueous media on the basis of their amphiphilicity. CO2 can tune the size of these vesicles over a wide range by controlling the degree of protonation of the amidine moieties. Alternating treatment with CO2 and argon realizes a smart expansion and contraction cycle of these vesicles, which can be considered as "breathing" nanocapsules.

(Continues...)

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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2017
• Language: English
• ISBN 10: 1782625429
• ISBN 13: 9781782625421
• Binding: Hardcover
• Number of pages: 180
• Editor: Xi Zhang