Phase Behaviour of Amphoteric Surfactant/Electrolyte/Cosurfactant/Alkane Systems

This paper describes the effect of pH on the phase behaviour of an amphoteric surfactant (N^α, N^α-dimethyl-N^α-lauroyl lysine, DMLL) in electrolyte (NaCl)/alkane (n-octane)/cosurfactant (n-pentanol) systems which are of possible pertinence to enhanced oil recovery. Three-phase microemulsion systems, which are composed of water, alkane, and microemulsion phases, are formed with n-octane containing/i-pentanol over the entire pH range of 1-9. The pH of the aqueous surfactant solutions is found to have a considerable effect on the cosurfactant requirement for middle phase microemulsion formation. The optimal cosurfactant requirement for middle phase microemulsion formation shows two minima at two pH values, where different types of DMLL coexist in the multiphase microemulsion system, but is almost independent of temperature in the vicinity of the isoelectric region (net charge of DMLL molecules becomes almost zero).

It is found that the change of optimal concentration of cosurfactant with changing pH is caused by the dissociation of the surfactant and the binding of ions (Na+ and Cl^-) with the hydrophilic groups of DMLL molecules.