Surfactant/liquid-crystal chemistry

Although surfactant chain length may be used to tailor pore size within a defined range, e.g. 15-50 A, much more dramatic variation in pore size can be obtained by exploiting the concepts of surfactant/liquid-crystal chemistry. It is well established that a surfactant micellar array is influenced by a number of factors including temperature, pressure, pH and the presence of solvents/solutes in the reaction media. 

In THE PAST DECADE, IMPROVEMENTS IN infrared spectroscopic instrumentation have contributed to significant advances in the traditional analytical applications of the technique. Progress in the application of Fourier transform infrared spectroscopy to physiochemical studies of colloidal assemblies and interfaces has been more uneven, however. While much Fourier transform infrared spectroscopic work has been generated about the structure of lipid bilayers and vesicles, considerably less is available on the subjects of micelles, liquid crystals, or other structures adopted by synthetic surfactants in water. In the area of interfacial chemistry, much of the infrared spectroscopic work, both on the adsorption of polymers or proteins and on the adsorption of surfactants forming so called "self-assembled" mono- and multilayers, has transpired only in the last five years or so. 

Surfactant aggregates (microemulsions, micelles, monolayers, vesicles, and liquid crystals) are recently the subject of extensive basic and applied research, because of their inherently interesting chemistry, as well as their diverse technical applications in such fields as petroleum, agriculture, pharmaceuticals, and detergents. Some of the important systems which these aggregates may model are enzyme catalysis, membrane transport, and drug delivery. More practical uses for them are enhanced tertiary oil recovery, emulsion polymerization, and solubilization and detoxification of pesticides and other toxic organic chemicals. 

For amphotropic liquid crystals it is obvious that the so-called surfactants (including for example soaps) are the best investigated, as documented in two reviews [61, 62]. Therefore, an own chapter of this book is dedicated to that field of supramolecular chemistry (see Chapter VII of this volume). Since the lyotropic properties of this type of amphotropic liquid crystals are discussed there in detail, we focus now mainly on the thermotropic behavior of such compounds. 

The tendency of surfactants to adsorb at interfaces and self-assemble results in unique physical properties and behavior. Formation of micelles, liquid crystals, macroemulsions, microemulsions, and foams, as well as surface tension reduction and improving wettabiUty of aqueous solutions, are just a few phenomena exhibited by surfactants. This behavior is of both fundamental interest as a unique subset of physical chemistry as weU as leading to many practical applications. 

One particular asset of structured self-assemblies is their ability to create nano- to microsized domains, snch as cavities, that could be exploited for chemical synthesis and catalysis. Many kinds of organized self-assemblies have been proved to act as efficient nanoreactors, and several chapters of this book discnss some of them such as small discrete supramolecular vessels (Chapter Reactivity In Nanoscale Vessels, Supramolecular Reactivity), dendrimers (Chapter Supramolecular Dendrlmer Chemistry, Soft Matter), or protein cages and virus capsids (Chapter Viruses as Self-Assembled Templates, Self-Processes). In this chapter, we focus on larger and softer self-assembled structures such as micelles, vesicles, liquid crystals (LCs), or gels, which are made of surfactants, block copolymers, or amphiphilic peptides. In addition, only the systems that present a high kinetic lability (i.e., dynamic) of their aggregated building blocks are considered more static objects such as most of polymersomes and molecularly imprinted polymers are discussed elsewhere (Chapters Assembly of Block Copolymers and Molecularly Imprinted Polymers, Soft Matter, respectively). Finally, for each of these dynamic systems, we describe their functional properties with respect to their potential for the promotion and catalysis of molecular and biomolecu-lar transformations, polymerization, self-replication, metal colloid formation, and mineralization processes. 

Figure 4.49. Electrocatalytic activity of mesoporous PtRu as a function of CH3OH concentration. Catalyst film prepared by liquid crystal (surfactant Ci EOg) templated reduction using Zn. Electrode potential 0.38 Vvs. RHE, 333 K. Data taken after 900 s of polarization [247]. (Reprodueed from Journal of Electroanalytical Chemistry, 543(2), Jiang J, Kucemak A, Electrooxidation of small organic molecules on mesoporous precious metal catalysts II CO and methanol on platinum-ruthenium alloy, 187-99, 2003, with permission from Elsevier.)...

Araos, M. U. and Warr, G. G. (2005). Self-assembly of nonionic surfactants into lyotropic liquid crystals in ethylammonium nitrate, a room-temperature ionic liquid. Journal of Physical Chemistry B, 109, 30,14275-14277. 

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