Electrostatic surface potentials of cationic and anionic oil-in-water microemulsion droplets free from added electrolyte

Abstract

The electrostatic surface potentials, ψ 0, of oil-in-water droplets in the microemulsion system sodium dodecyl sulphate (SDS)/pentan-1-ol/n-dodecane/water and also in the system hexade-cyltrimethylammonium bromide (CTAB)/butan-1-ol/n-hexadecane/water were measured by using a solvatochromic acid-base indicator technique. The conductivity of the CTAB system was also measured. It was found that |ψ 0| first decreased with decreasing volume fraction of water and then reached a plateau (for both systems), whilst the conductivity at first increased and then decreased over the same volume fraction range (for the CTAB system). The observed behaviour has been successfully interpreted by using a cell model: ψ 0 falls due to increased electrostatic screening caused by increasing concentrations of dissociated counter-ions, whilst at lower volume fractions of water a slight increase in the size of the droplets maintains ψ 0 sufficiently high for the system to remain stable. Using simple additional assumptions the model also predicts the qualitative changes in conductivity. Lack of quantitative agreement will persist until a more complete theory for the conductivity of concentrated microemulsion systems is developed.