Short range forces in surfactant systems. Specific ion-effects and ion competition

Electrostatic effects often dominate the behavior of charged surfactant systems. While descriptions on the level of the Poisson–Boltzmann equation are successful in describing many important features of such systems, they cannot predict those specific ion effects arising when changing counterions of the same valency and they often fail in predicting effects due to changing from a monovalent to a divalent counterion, everything else kept constant. We show in this communication two examples on how such specific ion effects are conveniently studied using equilibrium bulk surfactant systems. By systematically changing among alkali counterions in a lamellar liquid crystal while simultaneously measuring the NMR quadrupolar splitting of the ions, ion competition may be investigated. The evolution of the microstructure (as determined with NMR diffusometry) in a microemulsion when changing from Br− to SO42− as counterions to a cationic twin-tailed surfactant constitutes a convenient tool to investigate effects due to changing in the charge of the counterion. Finally, based on a suggestion originally due to Wennerström, we suggest an explanation on the effects observed when a monovalent counter ion is replaced with a divalent one in surfactant systems; an explanation which is based on the concept of image charges.