SOLVENT ISOTOPE EFFECT ON THE SELF-ASSEMBLY AND LIQUID-CRYSTALLINE PHASE-BEHAVIOR IN AQUEOUS-SOLUTIONS OF AMMONIUM PENTADECAFLUOROOCTANOATE

The partial high-resolution phase diagram of the ammonium pentadecaflu orooctanoate (APFO)/H2O system (weight fraction of APFO from 0.350 to 0.630) has been established using N-14 NMR to determine the liquid cry stalline phase transition temperatures. Comparison between the APFO/H2 O phase diagram and the previously published APFO/D2O one reveals that at comparable volume fractions the phase transition temperatures are lower in the APFO/H2O system, but the difference between these decreas es with increasing volume fraction of surfactant (increasing temperatu re). This isotope effect on the phase behavior can be understood in te rms of phase transitions which are driven by hard particle interaction s together with changes in the micellar self-assembly attributed to a tighter binding of ammonium ions to surface carboxylate groups in the case of D2O, leading to larger micelles in D2O than in H2O at correspo nding volume fractions and temperatures. The magnitude of the solvent isotope effect is similar to that previously determined in the closely related cesium pentadecafluorooctanoate (CsPFO)/water systems, an obs ervation which is consistent with the presence of a counterion-surface interaction via bridging water molecules. Polynomials are presented, which give the liquid crystalline phase transition temperatures (to an accuracy of +/-0.3 K) as a function of weight fraction of surfactant, for both the APFO/H2O and APFO/D2O systems.