Surface Forces in Bicontinuous Microemulsions: Water Capillary Condensation and Lamellae Formation

Abstract

We present results from measurements of forces between macroscopic mica surfaces immersed in a bicontinuous microemulsion (AOT/n-decane/brine). At separations of 40−60 nm a force barrier appears, followed by two or three more. These indicate the presence of lamellar-like structures. The relative position of the sample within the phase diagram (at constant surfactant volume fraction) has profound impact on the transition region. Moving closer to the excess water phase boundary, we observe, apart from the layered structure, water droplets condensing out from the bulk phase, leading to an attractive background in the force profile due to the capillary forces. This attraction becomes more and more dominant over the repulsive barriers closer to the microemulsion/water phase boundary. Finally, on the border, only a long-range (∼120 nm) attraction remains. In contrast to other examples of capillary condensation, when it leads to an immediate jump of the two surfaces into contact, we could, because of the low mic...