NOVEL PHASE-DIAGRAM OF A MIXED LANGMUIR MONOLAYER OF OCTYLCYANOBIPHENYL AND STEARIC-ACID

We have studied the phase diagram of a mixed monolayer system of 4'-n- octyl-4-cyanobiphenyl (8CB) and stearic acid (SA) at the air-water int erface by surface manometry and optical microscopic techniques. We fou nd that in the mixture, a liquid condensed phase (also known as L-2) w as induced which does not occur in the monolayers of the individual co mponents. The induction occurs over a large range of molar concentrati ons viz. 3% to 95% of SA in 8CB. More interestingly, we observed a pha se separation of the liquid expanded (LE) phase (also known as L-1) in to an 8CB-rich LE phase and an SA-rich LE phase for molar concentratio ns in the range 55% to 95% of SA in 8CB. We also observed a co-existen ce of three stable phases.