PHASE-DIAGRAM OF THE CESIUM PENTADECAFLUOROOCTANOATE (CSPFO)/H2O SYSTEM AS DETERMINED BY CS-133 NMR - COMPARISON WITH THE CSPFO/D2O SYSTEM

The high-resolution phase diagram of the cesium pentadecafluorooctanoate (CsPFO)/H2O system has been mapped out using mainly Cs-133+ NMR spectroscopy. It is this system, rather than the CsPFO/D2O one whose phase diagram has earlier been established, which is often used in experimental studies of phase transitions. While the two diagrams are qualitatively similar, there are quantitative differences. Substituting H2O for D2O lowers both T(NI) and T(NL) at corresponding concentrations: at phi = 0.15, T(NI)(D2O) - T(NI)(H2O) is 3.9 K and T(NL)(D2O) - T(NL)(H2O) is 4.3 K; these differences decrease-with-increasing phi (temperature) and eventually vanish at phi almost-equal-to 0.41 (80-degrees-C). This behavior arises from differences in micelle size: the micelles are a little larger in D2O than in H2O at corresponding concentrations and temperatures below 80-degrees-C. This ''isotope effect'' on micelle size is attributed to tighter binding of Cs+ to surface carboxylate groups via bridging water molecules in the case of D2O. The cmc's occur at higher concentrations in D2O, in contrast to the behavior of the sodium alkyl sulfates and alkyltrimethylammonium bromides which have lower cmc's in D2O, It is argued that this is because the PFO- monomer has a higher free energy in H2O than in D2O, while for hydrocarbon surfactants the opposite applies. This contrasting behavior reveals fundamental differences in the interactions of hydrocarbon and perfluorocarbon chains with water. The free energy associated with creating a ''cavity'' in the solvent appears to be dominant in the case of perfluorocarbon chains, while for hydrocarbon chains it is the free energy associated with the chain-water intermolecular forces which dominates.