Liquid expanded-liquid condensed phase transition in Langmuir films of discotic molecules

We have measured the surface pressure isotherms of Langmuir monolayers of benzene-hexa-n-alkanoates (BH-n) spread at an air-water interface. These disc-like molecules are composed of six aliphatic chains attached to the central benzene nucleus by six ester bonds. The hydrophilic ester groups act as pinning points and hold the benzene nucleus flat on the water surface. For the lower homologs BH-5 to BH-7, we observe that the so-called liquid expanded phase obtained at high areas per molecule transforms into the liquid condensed phase upon lateral compression. The transition is approximately first-order, with a plateau region of finite slope and a single discontinuity in the monolayer compressibility. The associated heat of transition, as calculated from the Clapeyron equation, decreases linearly with temperature and extrapolates to zero at the critical temperature Tcrit. Above Tcrit, the transition disappears and the liquid expanded phase is observed at all areas per molecule. Contrary to the phospholipid and single chain fatty acid cases, T crit is found to decrease when the chain length increases. We interpret the phase change as an order-disorder transition of the aliphatic chains from a gauche to an all-trans conformation. Based on the areas per molecule occupied in each monolayer state, we can also infer that the chains get lifted up from an orientation parallel to an orientation vertical, relative to the interface.