Molecular Gels: Structure and Dynamics: Volume 25 - Rilegato

Rilegato

ISBN 10: 1788011112 ISBN 13: 9781788011112

Casa editrice: Royal Society of Chemistry, 2018

Riassunto

Over the past decade, research on molecular gels has expanded and progressed rapidly. This timely book presents the latest developments and understanding of the topic, covering the different classes of gels (ionogels, metallogels, hydrogels and organogels) from their molecular level interactions and formation to their properties as materials and applications.

The book starts with a general introduction to molecular gels including their definition and a comparison of the different types. Chapters then examine the physical chemistry of molecular gels, including the most recent theories, experimental techniques and computational approaches. Final chapters on the applications of molecular gels illustrate, with modern case studies, the principles developed in previous chapters.

Written by leading researchers, this is an indispensable resource on the different types of gels for postgraduate students and researchers across supramolecular chemistry, materials science, polymer chemistry, soft matter and chemical engineering.

Su questo libro

Informazioni sull?autore

Richard G. Weiss received an ScB degree from Brown University and MS and PhD degrees from the University of Connecticut under the mentorship of Eugene I. Snyder. He was an NIH Postdoctoral Fellow with George S. Hammond at California Institute of Technology for 2 years and a Visiting Assistant Professor and National Academy of Sciences Overseas Fellow at the Instituto de Quimica of the Universidade de São Paulo in Brazil for 3 years. He has been a member of the faculty of Georgetown University since 1974 and currently holds the rank of Professor.  He is also a member of the Institute for Soft Matter Synthesis and Metrology at Georgetown. He has been a visiting professor at universities and institutes in Brazil, China, Costa Rica, Germany, France, Italy, Japan, Slovakia, and Spain.  He is a Fellow of IUPAC and a member of the Brazilian Academy of Sciences, and received a doctorate honoris causa from Université de Bordeaux 1.  He was a senior editor of the ACS journal Langmuir for 10 years, is currently a member of the editorial advisory boards of the Journal of the Brazilian Chemical Society and Gels, and is an Associate Editor for Polimeros and a member of the Scientific Committee of Substantia. He has authored or coauthored more than 300 peer-reviewed publications and more than 25 book chapters. His research interests include investigations of photochemical, photophysical and thermal reactions of molecules in anisotropic environments, and the development and application of ionic liquids, ionic liquid crystals, and new molecular and polymer gels for various purposes, including chemical spill remediation and the conservation of objects of cultural heritage.

Dalla quarta di copertina

Over the past decade, research on molecular gels has expanded and progressed rapidly. This timely book presents the latest developments and understanding of the topic, covering the different classes of gels (ionogels, metallogels, hydrogels and organogels) from their molecular level interactions and formation to their properties as materials and applications.

The book starts with a general introduction to molecular gels including their definition and a comparison of the different types. Chapters then examine the physical chemistry of molecular gels, including the most recent theories, experimental techniques and computational approaches. Final chapters on the applications of molecular gels illustrate, with modern case studies, the principles developed in previous chapters.

Written by leading researchers, this is an indispensable resource on the different types of gels for postgraduate students and researchers across supramolecular chemistry, materials science, polymer chemistry, soft matter and chemical engineering.

Dal risvolto di copertina interno

Over the past decade, research on molecular gels has expanded and progressed rapidly. This timely book presents the latest developments and understanding of the topic, covering the different classes of gels (ionogels, metallogels, hydrogels and organogels) from their molecular level interactions and formation to their properties as materials and applications.

The book starts with a general introduction to molecular gels including their definition and a comparison of the different types. Chapters then examine the physical chemistry of molecular gels, including the most recent theories, experimental techniques and computational approaches. Final chapters on the applications of molecular gels illustrate, with modern case studies, the principles developed in previous chapters.

Written by leading researchers, this is an indispensable resource on the different types of gels for postgraduate students and researchers across supramolecular chemistry, materials science, polymer chemistry, soft matter and chemical engineering.

Estratto. © Ristampato con autorizzazione. Tutti i diritti riservati.

Molecular Gels

Structure and Dynamics

By Richard G. Weiss

The Royal Society of Chemistry

Copyright © 2018 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-78801-111-2

Contents

Chapter 1 Introduction: An Overview of the "What" and "Why" of Molecular Gels Richard G. Weiss, 
Chapter 2 Viscoelastic Properties: The Rheology of Soft Solids Daniel L. Blair, 
Chapter 3 Thermodynamic Aspects of Molecular Gels J. F. Toro-Vazquez and J. D. Perez-Martinez, 
Chapter 4 Effects of Kinetics on Structures of Aggregates Leading to Fibrillar Networks Jing-Liang Li, Zhi-Sen Zhang and Xiang-Yang Liu, 
Chapter 5 Exploring Gelator Efficiency Jenifer Rubio-Magnieto and Beatriu Escuder, 
Chapter 6 Interfacial Considerations — Fibers and Liquids Michael A. Rogers and Maria G. Corradini, 
Chapter 7 Stimuli-responsive Supramolecular Gels Rakesh K. Mishra, Satyajit Das, Balaraman Vedhanarayanan, Gourab Das and Vakayil K. Praveen and Ayyappanpillai Ajayaghosh, 
Chapter 8 Structural Techniques at Different Length Scales Jean-Michel Guenet, Philippe Mesini and Marc Schmutz, 
Chapter 9 Applications of Supramolecular Gels David K. Smith, 
Subject Index, 

CHAPTER 1

Introduction: An Overview of the "What" and "Why" of Molecular Gels

RICHARD G. WEISS

Department of Chemistry and Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, DC 20057-1227, USA Email: weissr@georgetown.edu

1.1 Why Molecular Gels?

As will be discussed throughout this book, molecular gels have properties that differ from those of polymer gels, microgels, sol gels, and related forms of soft matter. One of the most important differences is that the three-dimensional (3D) networks that permeate the gels and are responsible for the immobilization of the liquid components are made up of molecules which associate through relatively weak, non-covalent, physical interactions. As a result, many of these gels can be converted reversibly and repeatedly, by heating or other simple perturbations, into their solution or sol phases; one can cycle these soft materials between viscoelastic and Newtonian fluids much more easily than one can polymer gels, in which the immobilizing networks are held together by covalent (chemical) bonds. Thus, several applications are possible for molecular gels that cannot be imagined for other forms of 'soft matter' (see Chapter 9).

Recognition by the scientific community of these possibilities is indicated clearly by the tremendous growth in interest in molecular gels during the last three decades: from nine publications registered by the Web of Science in 1991 to 3878 in 2017; a narrower classification, using 'molecular hydrogels' and molecular organogels', gives 26 articles in 1991 and 997 in 2017 (Figure 1.1). The two bar graphs show that the frequency of citations has increased even more rapidly: from 3 in 1991 to 39 544 in 2017! The interest can be attributed as well to the challenges that remain in understanding the structures and rheological properties of these gels and, especially, the dynamic processes associated with their formation. As one question concerning these materials is answered, two more arise. The study of molecular gels has been ongoing for more than a century and there is every indication that it will remain a vibrant area of research for another century.

1.2 Before Gels–Other Self-Assembled Soft Materials

The discussion of molecular gels cannot begin without a brief introduction to the general forms of self-assembly of 'small' molecules. Although this book focuses on molecular gels, implying viscoelastic as well as structural attributes, it is also about a still not completely understood phenomenon: How and why do solutions and sols of some molecules aggregate and organize into objects with very large aspect ratios (i.e., 1D objects)? The leap from 1D objects to 3D networks (that are necessary for gel formation) is a fascinating process without which the applications discussed in Chapter 9 would not be possible.

For many years, scientists have studied the self-assembly of large polymers and small molecules, and nature has shown us the importance of aggregating materials over different distance scales so that they can act synergistically and with interesting viscoelastic properties, even as living organisms. An enormous amount of effort has been expended, especially for systems in aqueous media, to understand the fundamental forces responsible for why separated and disorganized molecules (and, thus, higher in overall entropy) aggregate and organize into systems of overall lower entropy. The classic work in which Israelachvili devised an elegant (and simple) framework for our understanding of the relationships among the structures of surfactants, their concentrations, and their forms of assembly is a hallmark in this regard. Figure 1.2 summarizes in simplistic terms the elegant relationships among these parameters for lipids with 'melted' chains in water: the phase packing parameter (P) can be predicted on the basis of a few structural parameters of the molecules — the head group cross-sectional area at the critical micellar concentration (a0), the alkyl chain length (lc), and the hydrophobic chain volume (v). Those concepts have been expanded beyond water by considering how the polarity and type of solvent affect the modes of aggregation.

One of the most impressive examples of how incorporation of solvent can be lead to new insights is shown in Figure 1.3 where seemingly similar objects, tubules, are formed by very different routes and with very different packing arrangements depending on whether they are grown by aggregation of the lipid, L-dodecanoylserine, in water or in toluene. In fact, a distinction is usually made between gels in which the liquid component is aqueous (i.e., hydrogels) and in which it is organic (i.e., organogels), although the same principles govern both. As discussed in Chapter 6, the true distinction between hydrogels and organogels is the magnitudes of the factors that govern the aggregation of the gelator molecules in the two types of media. The importance of another factor, molecular chirality, is evidenced by the absence of a twist in the ribbons made from the racemic modification of the dodecanoylserine; the ribbons from the L-enantiomer are twisted and have a regular pitch whose magnitude depends on additional factors related to the aggregation and growth steps (as well as the properties of the solvent).

Thus, chirality at the molecular level can be transmitted in some (but not all) cases to much larger objects. However, ribbons made from the association of achiral molecules can be twisted as well if such changes reduce interfacial energies with the solvent. The parameters associated with how and when the chirality of single molecules is manifested in their assemblies has been investigated in depth by Shimizu, Masuda, and their coworkers in the closure of ribbons into nanotubes and theoretically and experimentally by Selinger et al. In that regard, Oda et al. have examined in detail the parameters controlling the degree and sense of twist of ribbons comprised of nonchiral cationic gemini surfactants, such as one with the formula C2H4-1,2-((CH3)2N+ C16H33) (16-2-16), and both the chiral enantiomeric and meso forms of tartrate as the counterion. A graphic presentation of how the nature of the tartrate (and other conditions described in the references cited) can influence the aggregates is shown in Figure 1.4. Formation of the objects in the figure occurs when the enantiomeric excess (ee) of the tartrate is as low as 80%. Below ee values of 60%, only twisted ribbons can be formed; they do not close to form tubules. Also, as expected, the pitch of the ribbons approaches infinity (i.e., flat ribbons) as the ee approaches zero. For enantiomerically pure tartrate, the objects form with decreasing temperature or increasing concentration in the temporal sequence: twisted ribbons->helical ribbons->tubules.

In another study, Terech et al. have followed the evolution aqueous solutions of salts of lithocholic acid, leading eventually to nanotubules and gels when the ionic strength is increased. Some of the interplay between concentration and time can be seen in the micrographs shown in Figure 1.5.

As mentioned, the formation of twisted ribbons and tubules depends on edge and surface interfacial energies and the direction of twist is controlled by the chirality of the constituent molecules. However, interesting examples of twisted ribbons that convert to nanotubules in which none of the participating molecules are chiral have been reported. In theory (and in practice), the number of tubules of opposite chirality is equal. One example employs dodecyltrimethylammonium bromide, and 1-phenylazo-2-naphthol-6,8-disulfonic acid disodium salt (Orange G) in aqueous solution. Another, in which the progression from vesicles of 10,12-pentacosadiynoic acid and geraniol, a short-chained alcohol, to helical ribbons and to tubules has been followed in time by light microscopy, provides insights into how tubules with relatively large diameters are formed. Perhaps the most impressive example requires the assembly of only one molecule, an anthraquinone tethered through a decyl chain to a carboxylic acid group (AQU), in water.

1.3Gels are a Subclass of 'Soft Matter'

1.3.1A Brief Description of Gels

For many decades, scientists in different fields have tried to formulate a definition that fits all gels. In the opinion of this author, they have failed thus far, and are not likely to succeed in the future because it has become increasingly apparent that the differences among some gel types are greater than their similarities. The problem is reminiscent of the parable, 'The Blind Monks and the Elephant', from the Buddhist text, the Udana, which has been known for more than two millennia. In it, each of the monks holds a different part of an elephant and is certain that his description of the total beast is correct (Figure 1.6). Although the debate about 'What is a gel?' has not been ongoing for so long, it has at times taken on some of the flavor of the parable. Some of the definitions of a gel that have been offered are enigmatic, such as the rather famous (and prophetic) pronouncement by Dorothy Jordon Lloyd, ' ... the colloid condition, the gel, is easier to recognize than to define'. A more complete list of the attempts to define gels and, more specifically, molecular gels, has appeared in several reviews, and will not be repeated here.

Specifically excluded from discussion in this book are 'sol gels', materials with much more rigid and permanent structures than those found in soft matter; in fact, they are better placed within the realm of 'hard matter'. We will also exclude polymeric gels (i.e., gels in which the networks pervading the liquid components are held together by species with molecular weights much larger than the arbitrary 3000 Dalton limit set for molecular gelators; see below); they are referred to frequently as chemical gels because some of the important interactions among the components of the network are based on covalent bonds that make true polymers. The polymeric gelators may take the form of 1D, 2D, or 3D objects, depending on the degree and type of chain branching, even before their aggregation and gelation, because covalent bonds hold the monomeric units together. A common feature of polymeric and molecular gels is that their networks percolate continuously throughout the liquid component. Even so, some polymeric gels are reversible upon heating or cooling and others may not require crosslinks between the polymeric chains. In fact, how polymeric 1D objects, especially those made up of unbranched chains (i.e., analogous to 1D objects of molecular gelators in which the intermolecular interactions are very strong) aggregate and convert to 2D and 3D objects has been studied much more extensively than the 0D->1D transformations involving molecular gelators; experimental observations become much easier as objects increase in size.

Only in the middle of the twentieth century did scientists begin seriously to consider and classify different types of gels. Even then, the type of gels to be covered in this book were not given much attention. For example, Flory included gels made by the reversible assembly of small molecules as an afterthought, naming them 'particulate, disordered structures'. Since that time, important strides have been made to answer a narrower question, 'What is a molecular gel?' However, many aspects of that question and others, related to how molecular gels form, remain unanswered. Many of the aspects already answered can be attributed to instrumental advances that have made possible the interrogation of the structure and dynamics of molecular gels at different distance and time scales; many of the unanswered parts can be traced to the need for additional instrumental techniques to probe aspects of these materials.

1.3.2A Brief Description of Molecular Gels

The word 'gel' has widely different meanings to scientists working in different fields. It encompasses a very broad part of condensed matter, ranging from inorganic sols to some polymers. Here, it refers to a subclass of soft matter, materials that consist of at least two components — one being a 'small molecule', a 'molecular gelator', that can be viewed as zero-dimensional (0D) on the sub-micrometre scale, that self-assembles and disassembles reversibly at specified temperatures in the other component(s), a liquid (or liquids) — and exhibits viscoelastic properties that are discussed in detail in Chapter 2. In contrast to the chemical gels mentioned above and which contain polymeric gelators, those comprised of molecular gelators are frequently referred to as physical gels because their networks are held together by non-covalent, weaker interactions. Although the liquids are highly mobile and diffuse rapidly on the nanoscale, the gels themselves do not flow macroscopically over long periods. For that reason, microgels, materials defined by IUPAC as 'particles of a gel of any shape with an equivalent diameter of approximately 0.1 to 100 µm,' are also not discussed here because they do not consist of a network that is continuous throughout a sample; they are droplets phase-separated within a liquid. The distinction is clear when one compares the IUPAC definition of a 'microgel' to that of a 'gel': the latter is a 'non-fluid colloidal network or polymer network that is expanded throughout its whole volume by a fluid.' Unfortunately, both of these definitions are limited to the structural properties of the systems. Neither considers viscoelastic aspects of the systems, as they must for the purposes of truly characterizing a gel and understanding what it is.

1.3.3 Molecular Gelators — Starting from 0D Objects.

As mentioned, molecular gelators are topologically 0D objects at sub-micrometre scales. Arbitrarily, their molecular weight (MW) is usually capped at 3000 Daltons (although with some 'poetic license'). The many examples of molecular gelators in the literature demonstrate the diversity of their shapes, flexibilities, and functionalities. The smallest molecular gelator for organic liquids of which we are aware is N,N'--dimethylurea (MW=88).

Gelator 'efficiency' can be defined according to several criteria which are discussed in depth in Chapter 5. Here, we prefer not to specify a definition, but refer simply to the range of liquids gelated and the concentration of gelator needed to make the gels. Gels of one of the most efficient and widely used gelators of organic liquids today, 1,3:2,4-di-O-benzylideneD-sorbitol (DBS), were reported first more than 120 years ago. Although some details about its structure remain unknown today, it is clear that the spatial disposition of the functional groups, allowing strong p-stacking of the phenyl rings and H-bonding of the hydroxyl groups, are the most important contributors to stabilization of its 3D networks, and those networks are acutely sensitive to solvent polarity. D-sorbitol itself, lacking the possibility of p-stacking and being much more labile conformationally than DBS, is a much less efficient gelator.

The gelators with the simplest molecular structures are long n-alkanes. In fact, long n-alkanes can gel short n-alkanes and several other liquids by forming networks comprised of interconnected platelets. Elaboration of the n-alkane structures, by incorporating different functional groups along the alkane backbones, has led to a myriad of gelators (Figure 1.7). This approach, starting from very simple molecules and moving incrementally to ones with increasingly greater complexity, has been employed to make homologous and structurally similar analogues of gelators and to explore the relations between molecular structures and the properties and network structures of their gels.

It has been employed as well to design many steroidal gelators, including those starting from the very basic steroids, cholesterol and dihydrocholesterol. Recently, it has been exploited beautifully by Bag and collaboratorsto investigate the effects of changing primarily the number and disposition of fused rings derived from naturally occurring triterpenes. Starting from the acyclic and very flexible molecule all-trans squalene, they have shown how the overall molecular dimensions as well as the gelating properties of the triterpenes are influenced by increasing the degree of folding (and, thereby, decreasing conformational lability by making fused rings) and adding substituents incrementally and selectively to the basic structures (Figure 1.8). Their research demonstrates that judicious changes in the number and position(s) of the substituent(s) can lead to a variety of aggregated materials in addition to gels.

(Continues...)

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Dettagli dell?edizione

• Editore: Royal Society of Chemistry
• Data di pubblicazione: 2018
• Lingua: Inglese
• ISBN 10: 1788011112
• ISBN 13: 9781788011112
• Rilegatura: Copertina rigida
• Numero di pagine: 376
• Redattore: Weiss Richard G.
• Contatto del produttore: non disponibile
• Persona responsabile: non disponibile