Neutron Scattering Study of Pressure Effect on C₁₂E₅/d-octane/D₂O Microemulsion Structure

Dobrin P. Bossev,¹ Markus Lesemann,² Simone Ferdinand,³ ÊNicholas Rosov,¹ and Michael E. Paulaitis⁴

¹NCNR, NIST, Gaithersburg, MD 20877, ²W. R. Grace & Co., Columbia, MD 21044,

³Merck & Co., Rahway, NJ 07065, ⁴Johns Hopkins University, Baltimore, MD 21218

C₁₂E₅/octane/D₂Omicroemulsion (3.7 wt % C₁₂E₅, 4.3 wt % octane, and 92.0 wt % water) in the L₁ phase region is investigated by small angle neutron scattering (SANS) and neutron spin echo (NSE) techniques at a temperature of 26.2 *C and different pressuresto monitor the changes of the microemulsion structure in the vicinity of the L₁/L_a phase boundary. The SANS scattering intensities are analyzed as a product of a form factor and a structure factor. The form factor indicates that the size and polydispersity of the microemulsion droplets do not change significantly as a function of pressure. At high pressures the structure factor of the microemulsion droplets resembles that of hard-spheres. When the pressure is decreased this microemulsion approaches the L₁/L_a phase boundary and the structure factor exhibits significant pressure dependence. The pair distribution function indicates interpenetration of the microemulsion droplets near the L₁/L_a phase boundary, that is assumed to be pressureöinduced through reducing the interfacial tension and/or changing the spontaneous curvature of the surfactant film. NSE data helps to analyze the surfactant film rigidity as a function of pressure.