SMALL-ANGLE NEUTRON-SCATTERING EXPERIMENTS ON MICROEMULSION DROPLETS - RELATION TO THE BENDING ELASTICITY OF THE AMPHIPHILIC FILM

Small-angle neutron scattering (SANS) experiments and measurements of interfacial tension have been carried out on droplet type microemulsio ns in the system nonionic surfactant (C(i)E(j))/hydrocarbon/water. The SANS spectra in the shell contrast can be very well interpreted by a model of polydisperse spherical shell structured aggregates. In additi on it has been shown that a more precise description is obtained if on e takes into account the diffuseness of the amphiphilic him. However, the outcome for the structural parameters does not depend on the choic e of the particular model employed. For microemulsion droplets structu ral quantities such as radius, polydispersity index, and macroscopic i nterfacial tension can be related directly to the bending constants (b ending modulus re and Gaussian modulus <(kappa)over bar>) of the syste m. The sum 2 kappa + <(kappa)over bar> can be deduced independently fr om interfacial tension and polydispersity index (from SANS data in the shell contrast) and both are in good agreement. Variation of the chai n length of the surfactant shows that this sum increases with increasi ng thickness of the amphiphilic film. In contrast, variation of the ch ain length of the hydrocarbon does not influence the elastic propertie s of the surfactant him. A dilution series also allows for a determina tion of 2 kappa + <(kappa)over bar> from the concentration dependence of the deduced radii and polydispersity indices. Again good agreement is observed with the values obtained before and the most appropriate e ntropy of mixing term to describe such a microemulsion appears to be a random mixing approximation. Experiments both in oil-in-water (O/W) a nd water-in-oil (W/O) droplets indicate that 2 kappa + <(kappa)over ba r> decreases with increasing temperature (W/O at higher temperature). In general one can state that the properties of these microemulsions a re well described by the elastic theory for the amphiphilic film. Stru ctural parameters such as radius and polydispersity are related to the interfacial tension and if two of these quantities are known the thir d can be predicted reliably.