The phase behavior and microstructure of silicone oil microemulsions

The phase behavior and microstructure of silicone oil microemulsions is measured and placed within the paradigm of the general patterns of phase behavior for nonionic surfactant solutions. The binary phase behavior of silicone oils with the nonionic ethoxylated alcohol surfactants (C_iE _j's) indicates an UCST is present at temperatures above what is measured with alkane oils. The corresponding ternary mixtures also indicate that higher temperatures are needed for the surfactant to partition into the silicone oil phase. Small angle neutron scattering (SANS) indicates that silicone oil penetrates the surfactant film less than alkane oils, consistent with more highly curved structures and a higher phase inversion temperature.; The addition of a cationic cosurfactant increases the efficiency of nonionic surfactants by increasing the stability of the single phase microemulsion with respect to an aqueous excess phase. The destabilization of the excess phase allows single phase microemulsions to be formed with one half to one quarter the surfactant concentration of using nonionic surfactant alone.; The microstructure of a cationic - nonionic surfactant mixture is measured in micelles, droplet, and bicontinuous microemulsions. The size and shape of mixed micelles and microemulsion droplets is measured by SANS and analyzed using both a model-based approach and the generalized inverse Fourier transform method (GIFT). Both techniques indicate that the ionic surfactant co-assembles with the nonionic surfactant and only changes the interparticle potential. The microstructure of cationic - nonionic bicontinuous microemulsions is measured by SANS and indicates that the addition of charge increases the correlation between surfactant layers while the distance between successive surfactant layers decreases. The results are in agreement with theoretical calculations for the microstructure of charged surfactant monolayers.; SANS from efficient microemulsions have a component of multiple scattering. The amount of multiple scattering is controlled by the pathlength and contrast of the sample and may be accounted for either experimentally or numerically.