Chapter 3 Immiscible Langmuir monolayers: Interactions between the polar head groups of cholesterol and octylcyanobiphenyl

Mixed Langmuir monolayers are ideal systems to mimic biological membranes [1, 2]. Studies on mixed monolayers provide information about the interactions involved, miscibility and stability of the monolayer. It is known that cholesterol(Ch) is found in bilayer membranes and lipoproteins. Ch regulates the transport and barrier properties of the membranes [3]. Lung surfactant in the human body is composed of different kinds of lipids and proteins. This forms Langmuir monolayer at the alveolar air-water interface [4, 5, 6]. These mixed surfactants tend to reduce the surface tension to nearly zero. The mixed monolayer is required to be stable without going to the collapsed state during expiration. Lack or deficiency of these surfactants lead to respiratory distress syndrome(RDS) in the human body. Recent studies on mixtures of saturated lipid, unsaturated lipid with different concentration of cholesterol to mimic raft formation in membranes bring in some correlation between monolayer and unilamellar vesicles [7]. There are many reports on the mixed monolayer of cholesterol with biologically active lipids [8]. The mixed monolayer of cholesterol and oleic acid shows good miscibility [9]. This is attributed to Ch filling the voids in the bent shape molecule, oleic acid. With increase