Size, surfactant aggregate, related

It is noted that the investigation of a mixed adsorption layer of CioEs and TPeAB (tetrapentyl ammoniiun bromide) [35] shows evidence for attractive forces / > 0), which suggests that the presence of the ionic surfactant can prevent aggregation in the extended S-L adsorption layer. Therefore, the main question of interest concerns how the Frumkin model and the aggregation model are related. One can find from Eq. 29 that the size of the elementary adsorption cell increases with the aggregation munber resulting in a reduction in the munber of cells. Negative has the same effect of de-... 

To make the significance of the NMR technique as an experimental tool in surfactant science more apparent, it is important to compare the strengths and the weaknesses of the NMR relaxation technique in relation to other experimental techniques. In comparison with other experimental techniques to study, for example, microemulsion droplet size, the NMR relaxation technique has two major advantages, both of which are associated with the fact that it is reorientational motions that are measured. One is that the relaxation rate, i.e., R2, is sensitive to small variations in micellar size. For example, in the case of a sphere, the rotational correlation time is proportional to the cube of the radius. This can be compared with the translational self-diffusion coefficient, which varies linearly with the radius. The second, and perhaps the most important, advantage is the fact that the rotational diffusion of particles in solution is essentially independent of interparticle interactions (electrostatic and hydrodynamic). This is in contrast to most other techniques available to study surfactant systems or colloidal systems in general, such as viscosity, collective and self-diffusion, and scattered light intensity. A weakness of the NMR relaxation approach to aggregate size determinations, compared with form factor determinations, would be the difficulties in absolute calibration, since the transformation from information on dynamics to information on structure must be performed by means of a motional model. 

In the following years some more studies appeared in the literature concerning trypsin activity in reverse micelles in relation to various characteristics of the reaction medium. Fadnavis et al. studied the pH dependence of hydrolytic activity of trypsin in CTAB reverse micelles toward a positively charged model ester substrate [74]. It was found that enzyme activity variations as a function of w are pH dependent. In 2005, Dasgupta and coworkers related the catalytic activity of trypsin in reverse micelles formulated with cationic surfactants with the concentration of the water-pool components and the aggregate size to delineate the independent role of both parameters [75]. Finally, in 2006, the influence of ethylene glycol on the thermostability of trypsin in AOT reverse micelles was examined and was found to exhibit a positive effect [76]. 

When both oil (o) and water (w) are present, o/w microemulsions will be formed when v/a0lc < 1 w/o microemulsions when v/ac c>U and lamellar phases when v/a0lc 1 (Israelachvili etal., 1980 Mitchell and Ninham, 1981). The v/a ratio depends on the surfactant chemical structure (lc and v) and on surface repulsions between headgroups (aD), (Mitchell and Ninham, 1981). When repulsions increase, the surfactant parameter (v/aGlc) increases and micelles get smaller. As a consequence, size and CMC are related surfactants with low CMC aggregate into large molecules, while the higher the CMC, the smaller the micelles. 

Important parameters that control the size of micelles are the degree of polymerization of the polymer blocks, NA and NB, and the Flory-Huggins interaction parameter %. The micellar structure is characterized by the core radius Rc, the overall radius Rm, and the distance b between adjacent blocks at the core/shell-interface as shown in Fig. 1. b is often called grafting distance for comparisons to polymer brush models, b2 is the area per chain which compares to the area per head group in case of surfactant micelles. In the case of spherical micelles, the core radius Rc and the area per chain b2 are directly related to the number of polymers per micelles, i.e., the aggregation number Z=4nR2clb2. 

In all the measurements carried out in this study, for different ratios of NaDDS NaDOC, the dilution curves (Figure 1) of surfactant solution exhibited a clear break that corresponded with the c.m.c. as determined by other methods. This observation agrees with literature reports (1-11,18). The present data, however, show for the first time that mixed micelle systems also behave the same way as pure micellar systems, as measured by calorimetry. Further, because the aggregation number, N, of NaDDS Is much larger than that of NaDOC (16), Table I, the variation of enthalpy around the c.m.c. region is not related to the size of micelles. 

---