Crystalline surfactants

The question may then be raised as to whether insoluble monolayers may really be treated in terms of equilibrium thermodynamics. In general, this problem has been approached by considering (i) the equilibrium spreading pressure of the monolayer in the presence of the bulk crystalline surfactant, and (ii) the stability of the monolayer film as spread from solution. These quantities are obtained experimentally and are necessary in any consideration of film thermodynamic properties. In both cases, time is clearly a practical variable. 

IPEC or hydrogen-bonded complexes may form not only between mutually interacting polymer blocks but also between a polymer block and low-MW molecules. Complexes between surfactants and block copolymers have been investigated for the formation of micelles. As illustrated by the work of Ikkala and coworkers [313], one of the major interests of these systems is that they combine two different-length scales of supramolecular organizations, i.e., the nanometer-scale organization of the (liquid) crystalline surfactant molecules and the ten-nanometer scale relative to block copolymers. This gives rise to the so-called hierarchical systems. The field of (block)... 

Analogous to the MBG-method, Boy and Voss introduced the enzyme catalysis in liquid crystalline surfactant phases [114,115]. The enzymes, e.g. alcohold-ehydrogenase, were entrapped in a liquid crystalline surfactant rich phase, and this phase was rinsed with an organic phase containing the substrate. In this way, they can perform the reaction continuously using a packed reactor module without apparent loss of activity. 

Apart from anomalous situations where surfactant interacts with the organic phase, the stability of HIPEs is linked to the interfacial tension of the system. Ruckenstein and coworkers [109] showed that the maximum volume of hydrocarbon which could be incorporated in an o/w HIPE increased with increasing surfactant concentration, presumably due to a concomitant decrease in the interfacial tension. Solans et al. [9] claimed that the interfacial tension between the aqueous phase and the liquid-crystalline surfactant layer in their highly... 

In two-component systems of association of colloid and water the sequence of phases, as the water content decreases, is micellar solution - hexagonally packed polar rods complex phases with rod-shaped aggregates lamellar mesophase D - crystalline surfactant. Some of these steps may be absent, depending, for example, on the temperature. 

A temperature-composition phase diagram for a surfactant solution is a characteristic phase diagrarr that delineates the conditions under which crystalline surfactant, monomers, or micelles will exist. On the phase diagram shown in Figure 12.5 (Smirnova, 1995), L represents the liquid phase, S the solid phase, and )(the surfactant mole fraction. The critical micellar temperature, CMT, is deLned as the line between the crystalline and micellar phases. Micelle formation occurs at temperatures greater than the CMT. The critical micellar concentration, CMC, line separates the micellar and... 

Dr. Alan S. Tracey s research career has concentrated on two major research areas, liquid crystalline surfactant materials and the aqueous chemistry of vanadium(V), with emphasis on biochemical applications. He is the author of 150 scientific publications. He obtained his undergraduate degree in honors chemistry from the University of British Columbia and his doctorate from Simon Fraser University. After postdoctoral fellowships in Brazil, Switzerland, and Australia, he returned to Simon Fraser University. He has recently taken early retirement. 

Engels, T. and von Rybinski, W. (1998) Liquid crystalline surfactant phases in chemical applications. /. Mater. Chem, 8(6), 1313-20. 

Forster G, Meister A, Blume A (2001) Chain packing modes in crystalline surfactant and Hpid bilayers. Curr Opin Coll Interface Sci 6 294-302... 

Krishnaswamy R., Remita H., Imperor-Clerc M., Even C., Davidson P., Pansu B., Synthesis of singlecrystalline Platinum nanorods within a "soft" crystalline surfactant-Pt(ll) complex, Chem. Phys. Chem. 2006,7,1510-1513. 

Vapor sorption measurements yield equilibrium composition and fugacity or chemical potential the isopiestic version (19) is used to determine the uptake of a pure vapor by a nonvolatile material. This technique determines equilibrium composition of a phase which cannot be separated quantitatively from the liquid phase in equilibrium with it. In our application, a nonvolatile crystalline surfactant specimen S is equilibrated with vapor of V, which is, in turn, at equilibrium with a system of S and V consisting of two phases, one rich in S, and one rich in V. At equilibrium, the Gibbs-Duhem relation guarantees. that the initial specimen of S takes up enough V from the vapor phase that the chemical potential of S, as well as of V, is the same as in the biphasic system, and so the composition of the phase formed by vapor sorption is the same as that of the S-rich phase. This composition is easily determined by weight measurement. If the temperature were a triple point, i.e. three phases at... 

Supersaturated solutions of the surfactant can be prepared in the usual way by heating a biphasic suspension to a temperature at which all of the surfactant dissolves, and then cooling to the original temperature. They are also generated by mixing saturated or nearly saturated surfactant solution in water with aqueous NaCl solution so that the surfactant solubility is reduced — and the solubility is very sensitive indeed to salinity. In both cases the resulting solution is at first visually clear and transparent and at room temperature remains so for hours or days or, in the most extreme cases, weeks. It may be that nucleation of the partially ordered, liquid crystalline, surfactant-rich phase is slow. 

Micelles can only form when the surfactant solubility is equal to or greater than the CMC. In general this occurs only above a particular temperature known as the Krafft point (temperature). Below this temperature surfactant solubility increases slowly with increasing temperature because the surfactant dissolves as monomers. The limit to monomer solubility occurs when the chemical potential of the monomers is equal to that of the pure (usually crystalline) surfactant. Above this temperature the solubility increases very rapidly because the surfactant dissolves as micelles the contribution of each micelle to the surfactant chemical potential being the same as that of a monomer. 

Whilst Lp phases have been accepted for years recently there has been debate about whether the state really does exist as a true thermodynamic equilibrium phase, based on very reasonable criticism of deficiencies in their location on properly determined phase diagrams [65]. However, in at least one case (the nonionic surfactant trioxyeth-ylene hexadecyl ether [66]) the Lp phase in water melts at a higher temperature than the crystalline surfactant. It forms on mixing water and the liquid surfactant just above the crystalline surfactant melting point, clear proof that it is the equilibrium state. 

Ethanol solubilizes the hexagonal phase, shifting the boundary from =40% to =45% although it initially stabilizes it. There is almost no increase in maximum solubility of crystalline surfactant. Where me-sophases occur over a wider concentration range (say 10-40%) then hydrotropes can sharply increase the isotropic solution range by removing the mesophase. 

One may expect that the local motions, and hence the S value, should be similar in the liquid crystalline surfactant aggregates and in the micelles in the dilute solution phase. This is also found to be the case. As mentioned above, the S value obtained from the fit to the relaxation dispersion (Fig. 26) was found to be S = 0.186. Avery similar value, 0.192, was measured in the hexagonal phase at higher concentrations [60]. 

Within the gel phase, the bilayers have rigid, mostly a -trans alkyl chains, as shown by a sharp, wide-angle X-ray spacing of about 4.2 A and a large transition heat on melting, typically 25-75% of the crystalline surfactant melting transition. This indicates restricted chain motions, mostly limited to rotation about the long axis only. In contrast, the water (polar medium) is in a liquid-like state, with fast rotational and translational mobility. (Since the structure contains both crystalline and liquid domains it is a true LIQUID CRYSTAL ). 

As water or other solvent is added to a crystalline surfactant, the structure of the system will undergo a transition from the highly ordered crystalline state to one of greater disorder usually referred to as a liquid crystalline or mesophase. Such phases are characterized by having some physical properties of both crystalline and fluid structures. These phases will have at least one dimension that is highly ordered and, as a result, will exhibit relatively sharp X-ray diffraction patterns and optical birefringence. In other dimensions, the phases will behave in a manner more similar to nonstructured fluids. 

The key aspect of the LCT mechanism is that the liquid crystalline mesophases or micelles act as templates rather than individual single molecules or ions. Accordingly, the final product is an inorganic (e.g., silicate) skeleton that contains voids, mimicking the shape of the surfactant mesophases. This mechanism, first proposed by researchers at the Mobil Corporation for the synthesis of M41S materials, is based on similarities between liquid-crystalline surfactant assemblies and the resulting mesoporous solid product. The whole process can be described by two possible mechanistic pathways, which are represented schematically in Fig. 4. 

Addition of soluble inorganic salts can also induce the precipitation from aqueous solutions of crystalline or amorphous crystalline surfactant precipitates. Consider then, for example, the increase in the Krafft temperature of SDS caused by the addition of a common ion, Na" [32-34]. This follows from simple mass action because the degree of micelle dissociation < 1 (i.e., the number of bound Na+ counterions is less than the micelle aggregation number) [35]. Peck [25] has shown, in measurements at 20°C, that the foamability of SDS declines markedly in the presence of added salt at concentrations > 0.3 M. The Krafft temperature of SDS under these conditions is >25°C [34], which means that crystalline SDS particles should be present as indicated by the onset of turbidity. Filtration to remove the turbidity partially restores the foamability [25] to a significant extent, which implies that the crystalline SDS particles exhibit some antifoam behavior. A combination of slow transport of surfactant to air-water surfaces and antifoam action by the crystalline surfactant would account for the almost total loss of foamability in the case of 0.01 M SDS in the presence of >0.3 M NaCl solution [25]. Antifoam action by crystalline particles... 

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