Interface surfactant

Eqnation 4 shows that, at constant , a change of the external parameter/ affects not only the radins but also the concentration of water-containing reversed micelles. It is also of interest that, by increasing R, the fraction of bulklike water molecules located in the core (or the time fraction spent by each water molecule in the core) of spherical reversed micelles increases progressively, whereas the opposite occurs for perturbed water molecules located at the water-surfactant interface, as a consequence of the parallel decrease of the micellar surface-to-volume ratio. 

The water structure at the water/surfactant interface depends on the nature of the surfactant head group, whereas the hydrophobic interface plays only a secondary role [91-93],... 

Indeed, the degree of binding of the counterions to the micellar surface, even in the largest aqueous core, is found to be 12% [2,94]. This means that virtually all counterions are confined in a thin shell near the surface (about 4 A), the concentration of ions in this domain is very high, and a nearly ordered bidimensional spherical lattice of charges is formed at the water/surfactant interface of ionic surfactants. 

Differential scanning calorimetry measurements have shown a marked cooling/heat-ing cycle hysteresis and that water entrapped in AOT-reversed micelles is only partially freezable. Moreover, the freezable fraction displays strong supercooling behavior as an effect of the very small size of the aqueous micellar core. The nonfreezable water fraction has been recognized as the water located at the water/surfactant interface engaged in solvation of the surfactant head groups [97,98]. 

Electrolytes are obviously solubilized only in the aqueous micellar core. Adding electrolytes in water-containing AOT-reversed micelles has an effect that is opposite to that observed for direct micelles, i.e., a decrease in the micellar radius and in the intermicellar attractive interactions is observed. This has been attributed to the stabilization of AOT ions at the water/surfactant interface [128]. 

The different location of polar and amphiphilic molecules within water-containing reversed micelles is depicted in Figure 6. Polar solutes, by increasing the micellar core matter of spherical micelles, induce an increase in the micellar radius, while amphiphilic molecules, being preferentially solubihzed in the water/surfactant interface and consequently increasing the interfacial surface, lead to a decrease in the miceUar radius [49,136,137], These effects can easily be embodied in Eqs. (3) and (4), aUowing a quantitative evaluation of the mean micellar radius and number density of reversed miceUes in the presence of polar and amphiphilic solubilizates. Moreover it must be pointed out that, as a function of the specific distribution law of the solubihzate molecules and on a time scale shorter than that of the material exchange process, the system appears polydisperse and composed of empty and differently occupied reversed miceUes [136],... 

Another example of chemical-potential-driven percolation is in the recent report on the use of simple poly(oxyethylene)alkyl ethers, C, ), as cosurfactants in reverse water, alkane, and AOT microemulsions [27]. While studying temperature-driven percolation, Nazario et al. also examined the effects of added C, ) as cosurfactants, and found that these cosurfactants decreased the temperature threshold for percolation. Based on these collective observations one can conclude that linear alcohols as cosurfactants tend to stiffen the surfactant interface, and that amides and poly(oxyethylene) alkyl ethers as cosurfactants tend to make this interface more flexible and enhance clustering, leading to more facile percolation. 

Concerning the structure of dispersed CLAs, the model originally proposed by Sebba [57] of a spherical oil-core droplet surrounded by a thin aqueous film stabilized by the presence of three surfactant layers is, in our opinion, essentially correct. However, there is still little direct evidence for the microstructure of the surfactant interfaces. From an engineering point of view, however, there is now quantitative data on the stability of CLAs which, together with solute mass transfer kinetics, should enable the successful design and operation of a CLA extraction process. 

Experiments on the stability of water/surfactant films at various pressures were performed by Exerowa et al.2,3 For a dilute aqueous solution of a nonionic surfactant,3 tetraoxyethylene decyl ether (D(EO>4,5 x 10-4 mol/dm3) or eicosaoxyethylene nonylphenol ether (NP(EO)2o, 1 x 10-5 mol/dm3), and electrolyte (KC1), thick films (with thicknesses of the order of 100 A) were observed at low electrolyte concentrations. With an increase of the electrolyte concentration, the film thickness first decreased, which suggests that the repulsion was caused by the double layer. This repulsive force was generated because of the different adsorptions of the two species of ions on the water/ surfactant interface. At a critical electrolyte concentration, a black film was formed, and the further addition of electrolyte did not. modify its thickness, which became almost independent of the external pressure, until a critical pressure was reached, at which it ruptured. While for NP(EO)2o only one metastable equilibrium thickness was found at each electrolyte concentration, in the case of D(EO)4 a hysteresis of the film thickness with increasing and decreasing pressure (i.e., two metastable minima) was observed in the range 5 x 10 4 to 3 x 10 mol/dm3 KC1. The maximum pressure used in these experiments was relatively low, 5 x 104 N/m2, and the Newton black films did not rupture in the range of pressures employed. 

The free energy of surfactant interfaces is due to interactions between water and the surfactant head-groups, as well as interactions between the surfactant chains, both of which compete to set the curvatures of the interface. Consequently, all else being equal, homogeneous interfaces are preferred over other geometries for a monodisperse distribution of surfactant... 

The most water-dilute botmdaries of the microemulsion phase regions within the ternary phase triangle of our systems are invariably lines of constant surfactant to water fraction (see Fig. 4.19). It is easy to show that this implies the micellar radii at the upper water limit of the microemulsion region are constant, irrespective of the oil content. The implication is that the curvatures of the surfactant interface, which separates the hydrophilic from hydrophobic regions, are fixed within this region. 

Figure 4.20 Artist s impressiort of the interfadal geometry of a ternary microemulsion made up of a double-chain cationic surfactant "dissolved" in a mixture of water and short chain hydrocarbons. The cormectivity of the surfactant interface - which encloses the water network -decreases as water is added to the mixture (left average coordination number of four right average coordination munber of two).

As in binary surfactant-water systems considered previously, two constraints on the geometry of the surfactant interface are active a local constraint, which is due to the surfactant molecular architecture, and a global constraint, set by the composition. These constraints alone are sufficient to determine the microstructure of the microemulsion. They imply that the expected microstructure must vary continuously as a function of the composition of tile microemulsion. Calculations show - and small-angle X-ray and neutron scattering studies confirm - that the DDAB/water/alkane microemulsions consist of a complex network of water tubes within the hydrocarbon matrix. As water is added to the mixture, the Gaussian curvature - and topology -decreases [41]. Thus the connectivity of the water networks drops (Fig. 4.20). 

The formation of spheres exhausts the range of geometrically accessible monolayer structures, since the topology of spheres is lower than that of all other shapes. (In more familiar terms, spheres minimise the surface to volume ratio - thus soap bubbles form spheres. In other words, the volume associated with a unit surface area of a surface is maximised if that surface forms a part of a sphere. So we expect the surfactant interface to become spherical as the internal volume associated with each surfactant molecule becomes large.) This critical volume fraction is not particularly large for ty pical double-chain surfactants it lies between 20% and 50%. 

Figure 8.4 Schematic representation of the different types of silica-surfactant interface. Solvent molecules are not shown, except for the I °S° case (triangles) dashed lines correspond to H-bonding interactions. Reproduced with permission from [5 ]. Copyright (2002) American Chemical Society...

As already stated, the limiting form of the governing mass transfer problem for this limit of insoluble surfactant is (7-270). Thus, in this case, we do not need to consider either the bulk transport or surfactant adsorption-desorption processes and the problem is greatly simplified. The governing equation (7-270) requires that either us or T be zero at every point on the drop interface. To verify this fact, we may note that the surfactant interface concentration is axisymmetric so that the solution of (7-270) reduces to the form... 

The second objective Is to examine the Influence of reversed micellar solution parameters, Including the Interaction of substrates with the surfactant Interface, on observed Initial rate kinetics. This Is of Interest because a number of reports have Indicated that enzymes In reversed micellar solutions exhibit an enhanced reactivity, or "super-activity" (7-9I. As a model system, the hydrolysis reactions of synthetic substrates of a-chymotrypsln were studied In a reversed micellar solution. Nuclear magnetic resonance was used to examine the Interactions between these substrates and the micellar environment. 

Substrate Localization. To determine how the substrate molecules are associated with the surfactant interface layer, chemical shifts and spin-lattice relaxation rates for substrate molecule protons in reversed micellar and in aqueous solutions were measured. Both substrate molecules, GPANA and BTPNA, have three subunits the... 

Chapter 7 deals with the basic concepts and properties of very specific technological media, namely, foam systems. Important processes such as surfactant interface accumulation, syneresis, and foam rupture are considered. 

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