Polar head group interactions in mixed Langmuir monolayers

We have investigated the miscibility in the mixed monolayers of cholesterol (Ch)-octyl cyano biphenyl (8CB) and cholesteryl acetate (ChA)-8CB using surface manometry and epifluorescence microscopic techniques. The main skeleton is the same both in Ch and ChA, whereas the polar head group is alcohol in Ch and ester in ChA. The 8CB molecule has a polar cyano as a terminal group and we probe its interaction with the polar group of Ch or ChA molecules in the mixed monolayers. Both Ch-8CB and ChA-8CB mixed monolayers exhibit two collapse pressures. In the case of the Ch-8CB mixed monolayer, the lower collapse pressure varies after 0.6 mole fraction (MF) of Ch in 8CB and the higher collapse pressure is nearly independent of composition. In ChA-8CB mixed monolayer, the lower collapse pressure varies continuously with the composition of ChA while the higher collapse pressure is nearly independent of the composition of ChA. In both these mixed monolayers, above the lower collapse pressure, 8CB gets squeezed out of the monolayer and forms multilayers. We find that in the case of Ch-8CB there is a phase separation in the monolayer occurring in the range of 0.15-0.9 MF of Ch. However, in ChA-8CB, the monolayer phase is miscible in all the proportions (except at very high concentration of ChA) below the lower collapse pressure. We attribute this better miscibility in the ChA-8CB compared to the Ch-8CB to the role played by the ester and cyano polar head group interactions.