PRESSURE-DEPENDENCE OF THE CRITICAL MICELLE CONCENTRATION OF A NONIONIC SURFACTANT IN WATER STUDIED BY H-1-NMR

The critical micelle concentration (cmc) of the nonionic surfactant C8 E5 in D2O was measured at 30 degrees C and pressures up to 350 MPa usi ng H-1-NMR chemical shifts. The cmc was found to increase with pressur e up to approximately 150 MPa and then decrease at higher pressures. T his characteristic pressure dependence is similar to that reported for several ionic surfactant solutions and is in quantitative agreement w ith the effect of pressure on the transfer of hydrocarbons from water into nonpolar solvents. We conclude, therefore, that the observed beha vior is independent of the hydrophilic headgroup of the surfactant and that it reflects a pressure-induced destabilization of the hydrophobi c micelle interior at low pressures and enhanced stabilization at high pressures.