On interpretation of quasielastic neutron scattering experiments from droplet microemulsions

Abstract

Quasielastic neutron scattering from droplet microemulsions is considered. Independent spherical fluid droplets are immersed in another fluid, the bulk fluids being separated by a surface layer fluctuating in shape. As distinct from the previous theories, the intermediate scattering function is calculated assuming small but nonzero thickness of the interface. The three components of the system are characterized by different scattering length densities. Consequently, the theory is applicable also for the scattering on emulsions containing large droplets with negligible thickness of the layer. With the aim to determine the Helfrich's elasticity characteristics of the surface layer, we apply the obtained formulas to describe the neutron spin-echo scattering experiments on microemulsions in which the droplet shape fluctuations have been detected. A quantitative agreement with the experiments can be reached only if the values of bending elasticity coefficient κ are smaller (in some cases significantly) than those determined in previous works. We conclude that the coefficients κ extracted so far from the neutron scattering experiments are not reliable and the theoretical description of these experiments should be essentially improved.