Pressure effects on the nanostructure of bicontinuous propane microemulsions with extended surfactants: a SANS study

In our recent work, we investigated the influence of pressure on the temperature-dependent phase behavior of symmetric, application-relevant microemulsions containing propane, stabilized by an extended surfactant mixture. By means of high-pressure small-angle neutron scattering, the present study provides further insights by unraveling the impact of pressure and propane on the nanostructure of these microemulsions near their optimum point. Despite the obvious presence of multiple scattering, all recorded scattering curves show the typical characteristics of symmetric bicontinuous microemulsions. Analysis of the scattering data using the Teubner-Strey model and Porod's law for diffuse interfaces provided the periodicity d_TS, the correlation length ξ_TS, and the specific interface S/V, as well as the amphiphilicity factor f_a and the effective bending rigidity κ_eff of the amphiphilic film. The overall structural order of pure propane microemulsions was found to be markedly lower compared to the n-decane microemulsions. While the structure of n-decane-rich microemulsions only shows a weak pressure dependence, propane-rich formulations exhibit a significant increase of ξ_TS with pressure due to an increasing surfactant monolayer rigidity, caused by enhanced interactions of the compressible propane with the surfactant tails. Microemulsions containing mixtures of the two hydrocarbons behave accordingly, demonstrating that the presence of the short-chain alkane gradually amplifies the sensitivity of the amphiphilic film to pressure changes. Interestingly, the geometric prefactor a of bicontinuous structure models increases from slightly above 7 for n-decane microemulsions to a > 8 for propane formulations, owing to the increasing disorder.