Rotator phases, surfactants

A very different kind of surfactant molecule obtains if the carboxylate head group of an alkanoic acid is replaced by one that is both hydrophobic and lipophobic, like a perfluoroalkyl group. Such diblock molecules, F(CF2) (CH2) H(FmHn), have been shown to form normal and reverse micelles in perfluoroalkanes and alkanes, respectively [68,69]. On the bases of the well-known antipathy between hydrocarbons and perfluorocarbons, the disparity between cross-sectional areas and volumes of CH (18.5 and 10.22 A ) [70, 71] and CF (28.3 A and 16.04 A ) [70, 71] groups, and the tendency of long perfluoroalkyl chains to adopt helical conformations, it is expected and found that organized assemblies of FmH/i molecules exhibit some strange properties. For instance, the alkyl portions of many of these molecules melt before their perfluoroalkyl portions [72], and the allowed motions resemble closely those of -alkanes in their rotator phases [73, 74]. 

In PAH solubilization tests with soil, batch-test soil-aqueous samples with nonionic surfactant and 14C-PAH were rotated on a tube rotator periodically to maintain the soil in suspension during equilibration. The samples were centrifuged, and aliquots were expressed through preconditioned 0.22- xm Teflon filters to reduce soil-derived colloidal substances. The extent of PAH solubilization in nonionic surfactant solution without soil was assessed in batch tests as a function of surfactant dose to confirm the value of PAH aqueous solubility (S) and to determine the values of Scmc and MSR. Nonionic surfactant sorption onto soil was evaluated for sub-CMC (or sub-CAC) aqueous-phase concentrations by surface-tension measurements. Supra-CMC sorption of nonionic surfactant to determine or supra-CAC isotherm was assessed either with azo dye solubilization and spectrophotometric analysis, or by measurement of chemical oxygen demand, from which the amount of surfactant in bulk solution could be inferred (10). 

Within the gel phase, the bilayers have rigid, mostly a -trans alkyl chains, as shown by a sharp, wide-angle X-ray spacing of about 4.2 A and a large transition heat on melting, typically 25-75% of the crystalline surfactant melting transition. This indicates restricted chain motions, mostly limited to rotation about the long axis only. In contrast, the water (polar medium) is in a liquid-like state, with fast rotational and translational mobility. (Since the structure contains both crystalline and liquid domains it is a true LIQUID CRYSTAL ). 

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