Effect of pressure on microstructure of C12E5/n-octane-in-D2O microemulsions

Forced Rayleigh scattering was used to measure the self-diffusion coefficie nts of oil droplets dispersed in the water-continuous L-1 phase of the C12E 5/n-octane/D2O microemulsion. A single microemulsion composition of 3.7 wt % surfactant, 4.3 wt % alkane, and 92.0 wt % water was studied at atmospher ic pressure as a function of temperature from 17.3 to 24.5 degreesC and at 26.2 degreesC as a function of pressure from 100 to 534 bar. Droplet self-d iffusion coefficients were found to decrease by a factor of similar to2 wit h increasing temperature from the emulsification failure boundary to the ph ase boundary for this L-1 phase and the lamellar phase. This decrease is at tributed to a transition from spherical to larger nonspherical oil droplets in water, i.e., a decrease in the spontaneous curvature of the oil/water i nterface with increasing temperature. The effect of increasing pressure, li ke decreasing temperature, in this region of the phase diagram is to increa se the oil droplet self-diffusion coefficients by a factor of similar to2-3 between 220 and 540 bar. This increase is likewise attributed to a transit ion from nonspherical to smaller spherical oil droplets in water or an incr ease in the spontaneous curvature of the oil/water interface with increasin g pressure. We conclude that the spontaneous curvature of the oil/water int erface is sensitive to pressure, with increasing curvature corresponding to increasing pressure. This conclusion is consistent with the pressure-induc ed (2) over bar --> 3 --> 2 sequence of phase transitions observed for mixt ures of CiEj surfactants, liquid alkanes, and water. Our results demonstrat e the utility of forced Rayleigh scattering as a complementary experimental technique to small:angle neutron, X-ray, and light scattering in experimen tal studies of microemulsion microstructures at high pressures.