Phase Behaviour of Ph-Dependent Microemulsions At High Temperature and Salinities

Abstract

This paper describes the formulation principal for a model microemulsion system which exhibits pH dependent phase behaviour. The system investigated consists of octane, brine, alkyl ether carboxylic acid surfactants and short chain alcohols. The CMCs of these surfactants were lower in acid form of the surfactant (COOH) than for the salt form (COO-), also the micelles formed in acid solutions were smaller than for the salt. Furthermore the surface and interfacial tensions were found to increase with increasing pH. Increasing pH ionises the carboxylic acid head group thereby making the surfactant more hydrophilic. The effect of an increase in pH can be counterbalanced by increasing the electrolyte concentration. Measurements of ultralow interfacial tensions at different salinities and temperatures have been made using a spinning drop apparatus. Three phase microemulsion systems were studied as a function of temperature and pH. It was observed that the presence of ethylene oxide (EO) moiety in the surfactant molecule made the surfactants less sensitive to salinity than anionic surfactants. In addition, the carboxylic ionic head group made the surfactant more stable to temperature than simple EO nonionic surfactants. Thus these surfactants are more robust than either simple anionic or nonionic surfactants and thus these materials have potential in the field of surfactant flooding for tertiary oil recovery.