Surfactant/water mixtures

Much is known about the surface tensions between surfactant/ water mixtures and air at 1 atm. However (except for thermodynamic equations), hardly anything is known about tensions between aqueous solutions saturated with CO2 at 10 MPa and their conjugate C02 rich phases. Although interfacial tension measurements at such pressures are very uncommon, values of the capillary number and their dependence on surfactant and hydrocarbon structures cannot be determined without such data. 

The regions within the local/global "phase diagram" for which these hyperbolic bilayer structures can be realised within a surfactant-water mixture are plotted in Fig. 4.8. 

We persist with the homogeneity approximation for h3q>erbolic surfaces, to calculate the local/global relation. The local constraint for a binar surfactant/water mixture is given by eq. 43. 

Figure 4.11 Plot of the approximate compositions for which surfactant/water mixtures can form monolayers versus the surfactant parameter of the surfactant. This plot is for chain lengths of 14A, which corresponds to hydrocarbons made up of about 12 carbon atoms. The notation for various mesophases is as follows Vi, V2 are bicontinuous cubic phases (the former containing two interpenetrating hydrophobic diain networks in a polar continuum, the latter polar networks in a hydrophobic continuum). Hi and H2 denote normal and reversed hexagonal phases. La denotes the lamellar phase, and Li and L2 denote isotropic micellar and reversed micellar phases (made up of spherical micelles).

In most cases, the complex array of interactions at work within an actual surfactant-water mixture leads to variations of the surfactant parameter with surfactant dilution and temperature. In general then, the phase behaviour of a binary surfactant-water mixture follows a curved trajectory through the local/global domain plotted in Fig. 4.11. If these variations in molecular conformation are small, the phase progression with water dilution is expected to follow a nearly-vertical line in the plot if the molecular architecture is sensitive to these external parameters, die succession of phases with water dilution is more nearly horizontal. 

An intermediate phase of tetragonal syiiunetry - the T phase - has also been detected in a number of systems. A rod structure related to a square mesh surface was foimd to agree well with X-ray and NMR data on a perfluorinated surfactant-water mixture forming the T phase [22], [34]. These examples demonstrate that surfactant or lipid monolayers lining mesh surfaces as well and bilayers wrapped onto three-periodic minimal surfaces (IPMS) are indeed found in these self-assembled systems. 

S.T. Hyde, Interfacial Architecture in Surfactant Water Mixtures - Beyond Sphers, Cylinders and Planes. Pure Appl. Chem., 1992, 64, 1617-1622. 

The closed loop is not the only characteristic of the nonionic surfactant-water binary phase diagram. Like the ionic surfactant-water mixture, nonionic surfactants, at higher concentration in water, exhibit lyotropic mesophases. Figure 3.14 shows a typical binary phase diagram exhibiting the full lyotropic mesophase sequence II, cubic isotropic phase HI, direct hexagonal phase (middle phase) VI, special cubic ( viscous phase) La, lamellar phase (neat phase). Note the presence of the two-phase domains surrounding each mesophase, the critical point on top of each, and the zero-variant three-phase feature. 

FIGURE 10.9 Highly schematic examples of some association colloids, (a) Micelles and bilayers. (From E. Dickinson, D. J. McClements. Advances in Food Colloids. Blackie, 1995.) (b) Crystal, lamellar, and gel structures of simple surfactant water mixtures T is temperature, T Kraft temperature. (Modified from a figure by N. J. Krog.)... 

Table I. Specific absorbance of surfactant-water mixtures at several wavelengths. A is total absorbance, Aabs fchat due to absorption, A-Aa S that due to scattering l is pathlength and c is concentration...

If a hydrophobic wax pattern is dipped in a surfactant-water mixture uniform wetting of the surface can be observed. However, if the pattern is rinsed in water, the surfactant is rinsed away and the surface again becomes... 

The complexity of the equilibrium phases and nonequilibrium phenomena exhibited by multicomponent oil-water-surfactant systems is amply demonstrated in numerous contributions in this volume. Therefore, the need for theoretical (and computational) methods that make the interpretation of experimental observations easier and serve as predictive tools is readily apparent. Excellent treatments of the current status of theoretical advances in dealing with microemulsions are available in recent monographs and compendia (see, e.g., Refs. 1-3 and references therein). These references deal with systems consisting of significant fractions of oil and water and focus on the different phases and intricate microstructures that develop in such systems as the surfactant and salt concentrations are varied. In contrast, the present chapter focuses exclusively on simulations, particularly on a first level introduction to the use of lattice Monte Carlo methods for modeling self-association and phase equilibria in surfactant solutions with and without an oil phase. Although results on phase equilibria are presented, we spend a substantial portion of the review on micellization in surfactant-water mixtures, as this forms the necessary first step in the eventual identification of the most essential parameters needed in computer models of surfactant-water-oil systems. 

Note added in proof, recently, two review papers covering intermediate phases of surfactant-water mixture [406] and fluctuating Euler characteristics in lamellar and microemulsion phases [407] were published. 

The synthesis of M41S-type materials can be carried out under hydrothermal conditions using alkyltrimethylammonium cations C TMA" as structure-directing agents and tetraethoxysilane (TEOS) as silica source. Similarly to the lyotropic phases formed in surfactant-water mixtures, the surfactants form micellar structures. Their self-assembly is strongly influenced by the presence of silicate anions which form by the hydrolysis of TEOS. After the synthesis, the space in between the surfactant micelles is filled by silica which develops by polycondensation of the silicate anions and which forms walls of a thickness of ca. 8 A [3,4]. 

H02 Holyst, R., Staniszewski, K., Patkowski, A., and Gapinski, J., Hidden minima of the Gibbs free energy revealed in a phase separation in polymer/surfactant/water mixture, J. Phys. Chem. 5,109, 8533, 2005. 

BE1 Bergfeldt, K. and Piculell, L., Phase behavior of weakly charged polymer/surfactant/ water mixtures, J. Phys. Chem., 100, 5935, 1996. 

FIGURE 3.6 In a lyotropic system such as surfactant (white) and water (purple), different phases become stable at different volume fractions. We can relate this idea to a variety of systems, such as surfactant-water mixtures or block copolymers (the polymer equivalent of a surfactant). 

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