A CONFINED COMPLEX LIQUID - OSCILLATORY FORCES AND LAMELLAE FORMATION FROM AN L(3) PHASE

In this paper, we present the results from measurements of forces between macroscopic mica surfaces immersed in an L-3 (sponge) phase. Two different surfactant systems are investigated. As previously reported (Petrov, P.; et al. Langmuir 1994, 10, 988) for the AOT/brine/water L-3 phase, two distinct regions in the force profile are observed: At large separations (40-120 nm), the force is oscillatory due to the structure in the bulk solution. In a confined space, at separations smaller than similar to 30 nm, there is a topological transition to a layered, lamellar-like structure with a force having huge repulsive barriers with a period of 3-3.5 nm, weakly dependent on concentration. The same characteristic forces are observed for an L-3 phase made of C12E5/hexanol/water. The period of long-range oscillations, scaled by the thickness of the bilayer, is inversely proportional to the surfactant volume fraction and shows a uniform functional dependence for both L-3-phase systems, in accordance with the general scaling law. Model calculations of the force profile within the flexible surface model reproduce and rationalize all qualitative observations and give a reasonable quantitative estimate of the force. We derive generally applicable conditions for the appearance of a stable lens of lamellar phase between curved surfaces in a bulk L-3 solution and calculate the force explicitly, without invoking the Derjaguin approximation.