The critical micelle concentration (cmc) of the nonionic surfactant C8E5 in D2O was measured at 30 °C and pressures up to 350 MPa using 1H-NMR chemical shifts. The cmc was found to increase with pressure up to approximately 150 MPa and then decrease at higher pressures. This characteristic pressure dependence is similar to that reported for several ionic surfactant solutions and is in quantitative agreement with the effect of pressure on the transfer of hydrocarbons from water into nonpolar solvents. We conclude, therefore, that the observed behavior is independent of the hydrophilic headgroup of the surfactant and that it reflects a pressure-induced destabilization of the hydrophobic micelle interior at low pressures and enhanced stabilization at high pressures.

In the "large aggregates" region of the phase diagram of the system C 12 E 5 /D 2 O the mutual diffusion coefficient of mixtures with critical and noncritical composition is determined by dynamic light scattering (compositions: 0.23 × 10 -2 e y e 10.01 × 10 -2 ; y c ) 1.08 × 10 -2 ; y, mass fraction of C 12 E 5 ). In all experiments the diffusion coefficient decreases with increasing temperatures, approaching the liquid/liquid coexistence curve at fixed values of y. This is a system independent property observed in "simple" as well as in "complex" binary mixtures with a miscibility gap. The change of size and shape of C 12 E 5 micelles with composition and temperature in aqueous solutions reported for the "large aggregates" region appears to be the result of concentration fluctuations existing in a wide temperature and composition region below the binodal curve. Approaching T p (T c ) the diffusive property of the mixtures is dominated by the dynamics of concentration fluctuations.

We present the results of a high-pressure small-angle neutron scattering study of the effect of pressure on surfactant microstructure. The study was carried out on a solution of 1 wt % C 8 E 5 in D 2 O at 29.4 °C and pressures up to 310 MPa. The C 8 E 5 micelles that form under these conditions are noninteracting. We find that applying pressure leads to a pronounced decrease in the micelle radius of gyration and the forward scattering intensity over the pressure range from ambient to 150 MPa. The partial molecular volume of the surfactant and the extent of hydration of the surfactant head groups in the micelle were also determined using the method of solvent contrast variation. Both quantities decrease with the application of pressure up to 150 MPa. Core-shell model fits to the scattering spectra over the entire q-range indicate that the shell radius decreases, while the hydrophobic core radius increases slightly with pressure. The pressure dependence of the shell radius is notably similar to that observed for the radius of gyration. Collectively, these observations lead to the conclusion that the effect of pressure on C 8 E 5 micellization is to induce the dehydration of surfactant head groups and the collapse of the hydrophilic micelle shell at pressures between ambient and 150 MPa.

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