Surfactant binding, saturation

In a study of interaction between a small molecule and a macromolecule, equilibrium dialysis is a fundamentally important method and it has been applied to many studies. But in the case of surfactant ligands, it has a drawback as much as a week may be required to attain an equilibrium.5>33 Gel chromatography provides another technique for obtaining binding isotherm, but has been used mostly to determine an amount of binding saturation. It is supposed that absorption of surfactants into the gel phase may obstruct reaching equilibrium at lower surfactant concentrations. 

Results illustrating the use of a decyl sulfate electrode to study interaction with PVP are given in Figure 4 (20). Note that the surfactant by itself (lowest plot) leads to an emf/log concentration plot, up to the c.m.c., with the required Nernstian slope. Before the breakpoints (TO that occur in the presence of the polymer one can see that the slope is virtually unaffected by polymer beyond Tu i.e., in the binding zone, the slope is lower, implying binding of surfactant. After saturation, i.e., at concentrations above T2... 

It seems likely that the cationic CPC micelles, which have a large positive charge at or near the micellar surface, interact attractively with the n-molecular orbital system of benzene, and that this interaction contributes to the fact that the solubilization constant for benzene in CPC is approximately twice as large as that in SDS micelles. A preferential interaction between cationic surfactants and aromatic solutes has been reported by several groups of investigators (25-27), and recent work in our laboratory shows that 1-hexadecyltrimethylammonium bromide micelles also solubilize benzene more effectively than do the anionic alkylsulfate surfactant micelles (28). Thus, the tendency of benzene molecules to solubilize near the surface of the cationic micelles, at low XB values, may lead to a partial saturation of surface "sites" by benzene, diminishing the ability of additional benzene molecules to bind near the surface. Such an effect could be responsible for the initial increase in activity coefficient that occurs, particularly in the CPC solutions, as Xg increases. 

It is important to realize, however, that the determination of the substrate-micelle binding constant from solubility data relies entirely on data for saturated solutions and that, in the case of ionic surfactants, differences in the counterion interactions with the micelle and the micelle-substrate complex and activity coefficient effects may seriously complicate the results. In these respects, distribution studies with varying substrate and surfactant concentrations are certainly preferable. In view of the assumptions involved in the derivation and application of equations (10) and (11), the agreement between the K values obtained from kinetic data (equation 10) and those obtained from solubility measurements (equation 11) for several substrate-micelle interactions is certainly both remarkable and significant. 

Gels that are usually employed in studies of surfactant uptake are previously saturated with the same solvent as that of surfactant solutions. When such gels are immersed in a surfactant solution, the binding should take place through the gel surface. The size and shape of the gel are then regarded as the primary factor directly affecting the uptake amount because it depends on the available surface area of the gel. 

Table III gives typical results from an adsorption run. In all cases the heat effects were small, which suggests that only certain sites bind the surfactant and not the entire surface. In addition, there was no evidence that the sites on the surface were saturated, even when a 2% surfactant solution was used.

Figure 4 Schematic surface tension vs. surfactant concentration plots in the presence (- - - ) and absence (—) of a polymer. T1 indicates the critical concentration of binding. T2 the saturation concentration, and T2 the formation of surfactant micelles. (Reproduced from Rades et al. [53].)...

The previous section discussed reactions in which a reagent, e.g., a nucleophilic anion, is added to a cationic micelle of a surfactant whose counteranion is chemically inert, so that the two anions compete for the micelle. However one has a conceptually simpler situation when the only micellar counterion is also the reactant. There is then no interionic competition, and if the micelle is saturated with reactive counterions, i.e., if /8 is constant, the observed first order rate constants should increase as the substrate binds to the micelle, and will be constant once the substrate is fully micellar bound. 

Water molecules can be bound to both ethylene oxide (EO) groups of Brij 97 and to the hydroxyl (OH) groups of butanol. Unfortunately, it is not known for this system how many moleeules are bound to eiflier alcohol or surfactant. Therefore, two curves are presented in Fig. 35. The curves demonstrate the number of bound water molecules calculated per EO group and per EO -I- OH group versus the total water content, respectively. As the water content increases, the number of water molecules bound to EO or EO + OH groups increases and reaaches a maximum at 2.5 molecules for one curve and 1.5 for the other. This increase means that the water-binding centers do not become saturated until 0.6 of the total water volume fraction is ob-... 

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