Alkanes, spontaneous curvatures

In the formulation of the rule, the author did not distinguish between the molecular and micellar solubilities of the surfactant, simply becau.se micelles were not known at that time. If the surfactant concentration is above the CMC, the net solubility has micellar and molecular components. In the case when the critical micelle concentration is negligibly small, Bancroft s rule is reduced to the oriented wedge trend. Indeed, for positive spontaneous curvature, micelles are formed in water and the surfactant is water-soluble . Conversely, for negative spontaneous curvature, the surfactant forms micelles in oil and is oil-soluble . The situation is more complicated when the critical micelle concentration is comparable to the net micellar concentration. This type of solution behavior, for example, is shown by most C,Ey surfactants, which show a very high CMC value in alkanes, 1 wt%. Experiment shows that in these cases Bancroft s rule is violated and nature follows the oriented wedge trend. ... 

Figure 7.11 Macroemulsion inversion as a function of monolayer spontaneous curvature. The spontaneous curvature is varied by changing the ratio of two surfactants in mixture (a), by varying the equivalent alkane carbon number (EACN), i.e. by mixing two oils in different ratios (b). and by the addition of alcohol (c). All the system are studied over the 2—3—2 transition range see the bars at the top of the plots. The left-hand axis shows the macroemulsion lifetime, denoted as the time for the separation of two-thirds of the disperse phase by volume. The right-hand axis shows the macroemulsion conductivity. All emulsions were prepared at a 1 1 oil-to-water ratio other details of the compositions are shown on the plots. DDS and WITCO TRS10-80 are commercial surfactants (Reproduced by pemiission of Marcel Dekker Inc. from ref. 78)...

Many reports are available where the cationic surfactant CTAB has been used to prepare gold nanoparticles [127-129]. Giustini et al. [130] have characterized the quaternary w/o micro emulsion of CTAB/n-pentanol/ n-hexane/water. Some salient features of CTAB/co-surfactant/alkane/water system are (1) formation of nearly spherical droplets in the L2 region (a liquid isotropic phase formed by disconnected aqueous domains dispersed in a continuous organic bulk) stabilized by a surfactant/co-surfactant interfacial film. (2) With an increase in water content, L2 is followed up to the water solubilization failure, without any transition to bicontinuous structure, and (3) at low Wo, the droplet radius is smaller than R° (spontaneous radius of curvature of the interfacial film) but when the droplet radius tends to become larger than R° (i.e., increasing Wo), the microemulsion phase separates into a Winsor II system. 

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