Surface Forces in Complex Liquid Systems

Abstract: Surface force measurements, combined with ellipsometry, were performed on several colloidal liquid systems. We studied the forces between mica surfaces in presence of dilute aqueous solutions of ethylenediamine (divalent) and spermidine (trivalent) hydrochlorides. With increasing the diamine concentration the electrostatic repulsion is reduced and later it disappears completely. At equivalent triamine concentrations the force is first attractive, but becomes more repulsive as the concentration increases, indicating a reversal of the double-layer charge. Adsorption of cationic hydrophobically modified hydroxyethyl cellulose (Quatrisoft LM 200) was measured at hydrophilic mica and silica surfaces. Addition of surfactant effects the structure and composition of the adsorbed layer. SDS interacts most strongly with the polyelectrolyte due to changes in the electrostatic interaction with the polymer-surfactant complexes formed. C₁₂E₅ only forms large clusters on the hydrophobe polymer chains. No evidence of interaction between the polycation and TTAB was observed. Instead, TTAB adsorbs at the surface in competition with the polyelectrolyte. Forces between mica surfaces were measured in a L₃ (sponge) phase and bicontinuous microemulsion. For AOT/brine/water and C₁₂E₅/hexanol/water L₃ phases at large separations the force was oscillatory due to structure in the bulk solution with period showing an uniform functional dependence. At shorter separations there was a topological transition to a lamellar structure. In the case of bicontinuous microemulsions (AOT/n-decane/brine) we could not resolve any long range oscillatory forces, but at short separations there was a continuous transition to lamellar-like structure. Moving closer to the excess water phase boundary we observed a decrease in the number of lamellar layers for the sponge phases, while for the microemulsions condensation of a water droplet was favored, causing an attractive capillary interaction.