Ionic surfactants with divalent counterions

Abstract

The properties of surfactant systems with divalent and monovalent counterions are compared with a special emphasis on electrostatic effects. It is argued that the high Krafft temperatures with divalent ions are due to strong interactions in the crystals. The lower critical micelle concentration with divalent counterions is due to a smaller positive electrostatic free energy in forming an electrical double layer. The effect is satisfactorily described by the Poisson-Boltzmann (PB) equation. Micelle growth is sometimes induced by divalent ions. For this effect the PB equation does not work satisfactorily. Phase equilibria are rather similar with divalent and monovalent ions, but with divalent counterions lamellar phases do not swell in water. This is due to an attractive correlation component to the force. The PB equation fails completely in predicting the force between two charged surfaces under the conditions found in these lamellar phases. In systems with a mixture of monovalent and divalent counterions the divalent ions tend to expel the monovalent ones. However, at short range an image charge effect reduces the preference for divalent ions.