The bending elasticity of surfactant monolayers and bilayers and its effect on the swelling of free liquid films

The present study was carried out to obtain more insight into the unusual swelling behaviour of surfactant layers. Here, swelling means that the thickness of the water layer between two surfactant layers increases. It was shown recently that high salt concentrations can bring about swelling in aqueous multilayer vesicles and free liquid films of nonionic surfactants (polyoxyethylenated n-dodecyl alcohols). By this so-called salt-induced swelling the water layer becomes thicker up to relatively high salt concentrations (1-2 kmole/m <sup>3</SUP>), reaches a maximum and, beyond these concentrations, shrinks again. In multilayer vesicles, the maximum swelling can be substantial (extreme swelling): the water layer thickness can easily be 10 times the surfactant layer thickness.Until now, an explanation for salt-induced swelling in terms of interaction forces between the surfactant layers was not available. The Van der Waals force is relatively unaffected by the ionic strength of the water layer. On the basis of steric repulsion, shrinking is expected due to salting-out of the ethylene oxide head groups of the nonionic surfactant. This could explain the decreasing part of the swelling curve. A problem with this explanation is that the bilayers in the vesicles are too far apart for the head groups to interact. When pushed to extremes, electrostatic repulsion could lead to some swelling if salt ions adsorb specifically onto the surfactants. In that case, as a function of salt concentration two regimes can be distinguished: in low concentrations the electrostatic double layer is built up by an increasing surface charge, leading to swelling, whereas in high salt concentrations the double layer is compressed, leading to shrinking. In chapter 3 this mechanism is shown to be quantitatively insufficient to account for the thickness variation of free liquid films, let alone for the extreme swelling in multilayer vesicles.Helfrich has suggested that thermal undulations of nearby surfactant layers provide an additional steric repulsion between the layers. Since this is a long range force, it could be responsible for the extreme swelling in multilayer vesicles. Quantitative analyses are required to test this suggestion. A parameter that dominates the magnitude of the Helfrich force is the mean bending elasticity modulus of the surfactant layer involved: a small modulus implies strong undulations and hence, large repulsions. The presence of a surface tension restricts the undulations. The surface tension of vesicles is negligible, but that of a film is significant. However, monolayers are thinner than bilayers and hence may be less rigid so that undulations can still be large enough to affect the film thickness.A major part of the present study is the modelling of surfactant monolayers and bilayers, with the aim to calculate bending moduli from the solution properties of the surfactants (see chapters four and five). Based on these calculations, it was predicted that the thickness of free liquid films containing n-alcohol instead of salt would show qualitatively the same trends if Helfrich repulsion is responsible for the salt-induced swelling. The experimental verification of this prediction is described in chapter six. The results agree very well with the proposed influence of undulations.