Ionic features in free liquid films of non-ionics

Abstract

The thickness of thin liquid films stabilized by heptaethylene glycol mono n-dodecyl ether (C 12E 7) is measured as a function of NaCl concentration. This thickness passes through a maximum at around 1 M, similar to what has been found with other non-ionic surfactants. In films stabilized by a mixture of C 12E 7 and sodium dodecyl sulfate (SDS) or cetyl trimethyl ammonium chloride (CTAC), the maximum is much less pronounced, and there appears an enhanced thickness at lower NaCl concentration. We have extended the self-consistent field lattice theory of Böhmer, Evers and Scheutjens for the adsorption of polyelectrolytes to liquid films stabilized by a mixture of non-ionic and ionic surfactants. This theory accounts for the presence of electric fields and for the excluded volumes of surfactant segments and ions. In this way, the thickness of the double layer is increased and, therefore, the enhanced thickness at low salt concentration can be explained by electrostatic repulsion. When the thickness of the mixed film is corrected for this repulsion, the result of pure C 12E 7 films is retrieved, re-establishing the effect of electrolytes on non-ionic surfactants. The maximum at high salt concentration for non-ionic films in the absence of ionic additives cannot be explained in this way. We suggest an explanation for this maximum in terms of contributions of monolayer fluctuations leading to an additional repulsion that is dependent on the salt concentration.