Surfactants liquid crystal formation

There are a number o-f processes which have not been discussed (because o-f space) in which mixture o-f sur-f actants are important. Among these are foaming, emulsion formation, liquid crystal formation, microemulsion formation, adsorption as 1iquid—1iquid interfaces, and phase partitioning of surfactants between immiscible liquid phases. These areas will also see increased interest in the use of surfactant mixtures. 

Vartuli, J. C., Schmitt, K. D., Kresge, C. T., Roth, W. J., Leonowicz, M. E., McCullen, S. B. Hellring, S. D., Beck, J. S., Schlenker, J. L., Olson, D. H. and Sheppard, E. W. Effect of surfactant silica molar ratios on the formation of mesoporous molecular-sieves-inorganic mimicry of surfactant liquid-crystal phases and mechanistic implications,... 

The formation mechanism of this family of materials is determined by two features [45], The first is the dynamics of surfactant molecules to shape molecular assemblies, which leads to micelle, and, ultimately, liquid crystal formation. The second is the capability of the inorganic oxide to undergo condensation reactions to form extended, thermally stable structures. 

For the 1 M NaCl system the solubility region was further reduced. Fig. 13, and the water solubilization maximum found at even higher surfactant/cosurfactant ratio. The series with the lower ratios of surfactant to cosurfactant showed an uptake of the aqueous solution somewhat similar to the series in the system with 0.5 M NaCl. The series with the surfactant/(cosurfactant + surfactant) ratio equal to 0.4 gave an initial liquid crystal formation lasting for 2-3 days folllowed by a middle phase lasting a longer time. The liquid crystalline and the middle phase layer were both more pronounced for the sample with initial salt concentration equal in the water and in the microemulsion. Fig. 14A, than for the sample with all the salt in the water. Fig. 14B. 

The research on aggregation of surfactants in nonaqueous, polar solvent systems can be motivated, mainly, with two different arguments. First, are the basic considerations of amphiphile aggregation involving a description of the hydrophobic interaction leading to, for example, micelle and liquid crystal formation. What can be learned from comparing water with other polar solvents Much work has been performed to elucidate those properties of the solvent that are essential in order to obtain a hydrophobic (or solvophobic ) interaction. Comparisons of critical micelle concentrations in different solvents with parameters characterizing the solvent are numerous in the literature [1,2],... 

Hydrotropy When there are strong chain-chain and head-head interactions between surfactant molecules (due to long, straight chains and close-packed heads), either insoluble crystal formation (low Krafft point, p. 214) or liquid-crystal formation (Chapter 3, Section IIC) may occur. Since there is much less space available for solubilization in rigid liquid-crystal structures than in the more flexible types of micelles, the onset of crystal formation usually limits the solubilization capacity of the solution. The tendency to form crystalline structures can be reduced by the addition of certain nonsurfactant organic additives called hydrotropes. 

W/o-droplet microemulsions with non-ionic surfactants containing rapeseed oil methyl ester have been successfully used for in situ extraction of polycyclic aromatic hydrocarbons [40]. However, enhancement of oil content and solubilisation capacity failed with these systems. The use of co-surfactants and co-solvents for suppression of liquid crystal formation was considered to be critical for in situ application. 

Due to their importance for life sciences, for example the formation of biological membranes, and their technical importance as surfactants and soaps, potential lyotropic properties of amphiphilic compounds are much better investigated than other ones of different molecular types the broad field of surfactant liquid crystals is discussed in Chapter VII of this volume. 

There is a vast body of data concerning the influence of third components on surfactant liquid crystals. Because of the possible great complexity of the inherent mesophase behavior this array of data can appear to be enormously difficult to rationalize. However, if we can consider the simple concepts described above (micelle formation, micelle... 

Amphoterics can also be used to stabilize emulsions by immobilizing the oil droplets in a network to retard or prevent coalescences (109). Chun (110), in a conference paper presented in 1978, proposed using an amphoteric surfactant (see Figure 15.29) in combination with a fatty amphophile, such as cetyl alcohol, to form a gel network (similar to that of liquid crystal formation) to further improve emulsion stability. A number of commercial skin preparations have used this technique to stabilize treatment lotions. 

Origins of the Formation of Surfactant Liquid Crystals - Reversed Phases. . . ... 

The latter are limited to hydrocarbon, perfluorocar-bon and polydimethylsiloxane chains. While the formation of micelles is well known, surfactants also form a wide variety of liquid crystalline phases in water which are much less familiar. Almost all surfactants that form micelles also form liquid crystals, while many do not form micelles but do form liquid crystals. Thus, liquid crystal formation by surfactants is more widespread than micelle formation. Indeed, an understanding and knowledge of liquid crystals can provide a comprehensive guide to the application of surfactants. This is because the size and shape of the surfactant molecules determine the structure of the self-assembled aggregates, which in turn, controls the liquid crystal... 

ORIGINS OF THE FORMATION OF SURFACTANT LIQUID CRYSTALS - WATER-CONTINUOUS PHASES... 

There is a vast body of data concerning the influence of third components on surfactant liquid crystals. Because of the potentially great complexity of the inherent mesophase behaviour, this array of data can appear to be enormously difficult to rationalize. However, if we consider the simple concepts described above (micelle formation, micelle shape/packing constraints, volume fractions and the nature of intermicel-lar interactions), then a reasonably simplified picture emerges, at least for the water-continuous phases. This present section does not attempt to be comprehensive - it simply reports selected examples of behaviour to illustrate the general concepts. The simplest way to show the changes in mesophase behaviour is to employ ternary phase diagrams. The reader should recall that the important factors are (i) the behaviour as a function of surfactant/additive ratio, and (ii) the volume... 

Equation 6.87 predicts that the time tp until liquid crystal formation begins is proportional to the square of the initial drop radius and inversely proportional to the bulk surfactant concentration. These predictions were in agreement with experiments for systems containing pure nonionic surfactants, n-hexadecane, oleyl alcohol, and water (Lim and Miller, 1991a). Moreover, for a hydrocar-bon alcohol ratio of 3 1 by weight and for solutions of at 30°C, the phase diagram was determined and K calculated as 0.52. When the data were htted to Equation 6.87, D2 was found to be 1.3 x 10" ° m /sec. The Stokes-Einstein equation was then used to estimate micelle radius r. 

However, anisometry of the micellar aggregates is a necessary (yet not sufficient) prerequisite of liquid crystal formation. Indeed, at higher concentrations many perfluoro surfactants are known to form nematic, lyotropic, liquid, and crystalline phases [109]. One such system is the tetramethylammoniumperfluorononanoate (TMAPFN) which exhibits a nematic phase over a relatively large concentration range but only in a small temperature interval [91]. This system has been studied in detail by SANS [92]. The experiments showed that... 

Due to the wide spectrum of behavior exhibited by this class of compounds, the discussion is broken up into the major areas of surfactant behavior, namely, thermotropic liquid crystal formation, microemulsions, lyotropic liquid crystalline behavior, reverse micellar systems, and dilute solution. This spectrum of available modes of self-assembly facilitates their use in an increasing range of applications. 

In a study by Kahn et al. [9] on a polydisperse octadecyl amide with nine oxyethylene units, the pure surfactant is found to be a viscous liquid at room temperature. Upon solubilization in water, a clear isotropic solution phase is observed for all concentrations between the cloud point and the solidification temperature. No liquid crystal formation is observed. 

First, it is clear that much of the complexity in surfactant phase behavior related to the existence of liquid-crystal phases, which are dominant features in water, does not exist in nonaqueous solvents [96]. The fundamental requirement for liquid-crystal formation is that one of the molecular fragments in amphiphilic molecules be highly incompatible with the solvent, and the other be highly compatible. If this severe exclusion of one part or the other is compromised in any way, the existence of liquid-crystal states (and the other characteristic features of aqueous surfactant phase behavior) is jeopardized and they may vanish altogether. This is apparent from data which show that introducing even weakly dipolar functional groups into lipophilic chains may destroy the surfactant-like aqueous phase behavior of the parent surfactant [97]. 

The model on which the above derivations are based is by no means unequivocal. There is no proof that micelles diffuse to the surface and adsorb, or, indeed, that hemi-micelles as depicted in Fig. 7.7 form, although Somasundaran et al. [32] have previously postulated their existence. The transfer of solute molecules to the micelle at the surface probably involves complex interactions between surfactant, fatty acid and water perhaps with liquid crystal formation as an intermediate stage following penetration of surfactant molecules. As the earlier steps in the process are not rate limiting their formulation is perhaps less important. Diffusion of the solubilizate4aden micelle is a process which must occur. 

The utilization of surfactants in electrochemical baths can result in micelle or even liquid crystal formation at high enough surfactant concentrations (>30 wt%). Subsequent electrodeposition results in materials with pores created by these micelles or liquid crystals. 

Liquid Crystal Formation of Small-Molecule Surfactants... 

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