Effects of Surfactant Purity and Concentration, of Surfactant Counterion, and of Different Ions on the Water Uptake of Dioctyldimethyl Ammonium Salt-Decanol-Isooctane Reverse Micellar Systems

Abstract

The effects of surfactant purity and concentration, of surfactant counterion, and of different ions on the water uptake of a dioctyldimethyl ammonium salt-decanol-isooctane reverse micellar phase in equilibrium with an aqueous phase were investigated. The initial organic phase containing the surfactant and decanol, as cosurfactant, in isooctane was contacted with an aqueous phase containing electrolytes. The water uptake at equilibrium was measured at several surfactant concentrations for different electrolytes with pure dioctyldimethyl ammonium chloride (DODMAC), dioctyldimethyl ammonium bromide (DODMAB), and their commercial forms. The purity of the surfactant had no effect on the water uptake per mole of pure surfactant. Changing the cation of a salt did not have any effect on water uptake while changing the anion altered the water uptake considerably. Systems containing mixed anions can show a maximum in the water uptake. The water uptake increased when either the charge number of the anion of the salt increased or the ionic radius of the anion decreased. The water uptake was a linear function of surfactant concentration at each salt concentration. A mathematical model was developed which accounts for the exchange of the surfactant counterion with the anions in the aqueous solution. The model accurately represented the water content in the organic phase both for single and multiple salt systems.