SURFACE FORCES IN BICONTINUOUS MICROEMULSIONS - WATER CAPILLARY CONDENSATION AND LAMELLAE FORMATION

We present results from measurements of forces between macroscopic mic a surfaces immersed in a bicontinuous microemulsion (AOT/n-decane/brin e). At separations of 40-60 nm a force barrier appears, followed by tw o or three more. These indicate the presence of lamellar-like structur es. The relative position of the sample within the phase diagram (at c onstant surfactant volume fraction) has profound impact on the transit ion region. Moving closer to the excess water phase boundary, we obser ve, apart from the layered structure, water droplets condensing out fr om the bulk phase, leading to an attractive background in the force pr ofile due to the capillary forces. This attraction becomes more and mo re dominant over the repulsive barriers closer to the microemulsion/wa ter phase boundary. Finally, on the border, only a long-range (similar to 120 nm) attraction remains. In contrast to other examples of capil lary condensation, when it leads to an immediate jump of the two surfa ces into contact, we could, because of the low microemulsion surface t ension, measure the whole force profile, on both approach and separati on. In a simple thermodynamic model we explain the observed phenomena.