Additives may adsorb to or be depleted from the surfactant monolayer and thereby affect the phase behaviour. For example, the depletion of NaCl can be compared with the depletion of a water soluble oligomer. The former is depleted for electrostatic reasons, while the oligomer is depleted by steric interactions with the ethylene oxide 'brush' with similar effects on phase behaviour. Similarly, a fundamentally opposite behaviour is also observed when adding adsorbing or non-adsorbing polymer chains, respectively, to a microemulsion system. Ionic surfactants with multivalent counterions.
Electrostatic effects dominate the phase equilibria of ionic surfactant systems.
Presently, we are investigating specific ion effects on the phase behaviour and microstructure in two and three component systems with surfactants of the didodecyldimethylammonium type. The general properties of these systems are found to be drastically dependent on the type of counterion used. Divalent surfactants are important from a technical point of view. They are, for example, used as stabilisers for asphalt emulsions and for disinfection. They are also interesting from a more fundamental point of view since a full understanding of the factors that govern self-association and other properties of charged surfactants in water and other polar solvents require a deep insight into the electrostatic interactions.