The Upper Consolute Boundary in Zwitterionic Surfactant-Water Systems

Article abstract of DOI:10.1021/j150669a061

Selected zwitterionic surfactants display a liquid-liquid miscibility gap having an upper critical temperature, which differs in overall shape from the "cloud point" miscibility gap displayed by single-bond and semipolar surfactants.The dependence of the location of this phase phenomenon on the molecular structure of the surfactant and NMR investigations of the isotropic solution structure in the vicinity of the gap are reported.Lengthening the lipophilic group enlarges the area spanned by both the cloud point miscibility gap and the zwitterionic surfactant-water miscibility gap.However, increasing the size of proximate substituents further enlarges the former but shrinks the latter.This gap is most evident in ammonio sulfates, less so in ammonio sulfonates, and does not exist in ammonio carboxylate-water systems.NMR investigations of the structure of 3-(nonyldimethylammonio)propyl sulfate-water solutions in the region of its miscibility gap have been carried out using the pulsed field gradient spin-echo method.In addition, 1H NMR spectra have been recorded.The self-diffusion data, just below the independently measured critical micelle concentration (cmc), show that the surfactant exists as a molecular solution.In contrast to ionic surfactants, the concentration of molecular surfactant increases slowly above the cmc.Above the cmc, the self-diffusion data suggest that small, spherical micelles exist over a wide range of concentrations and temperatures.The NMR line widths observed in the continuous-wave mode are consistent with this interpretation.Estimates of the hydration levels of the surfactant suggest that hydration decreases with increasing concentration and temperature but remains relatively high even at 80 deg C.These studies reveal no anomalies in solution structure or aggregation phenomena in the vicinity of the miscibility gap.