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

Dobrin P. Bossev,1 Markus Lesemann,2 Simone Ferdinand,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 % waterin 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/La 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/La phase boundary and the structure factor exhibits significant pressure dependence. The pair distribution function indicates interpenetration of the microemulsion droplets near the L1/La 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.