Thermodynamics of transfer of amphiphiles between the liquid-air interface and a solid surface-wetting tension study of Langmuir-Blodgett films

Wetting tension was determined by measuring the meniscus height on a mica surface during spreading, compression, and subsequent Langmuir-Blodgett deposition of an insoluble monolayer of DSPE (distearoylphosphatidylethanolamine) spread at the water-air interface, and on reimmersion of the surface in pure water. While such systems are essentially nonequilibrium and irreversible in various aspects of their behavior, we show that fundamental principles behind Langmuir-Blodgett phenomena can be understood by consideration of the thermodynamic equilibrium at the three-phase line, At large areas per molecule, long before the monolayer at the liquid-vapor interface is condensed, it undergoes spontaneous condensation and compression at the three-phase line. By this thermodynamically driven mechanism the contact angle increases and the layer is transferred onto the surface of the solid. This is a general effect which occurs also for soluble surfactants. Free energies of transfer of a lipid molecule between the interfaces that coexist at the three-phase line are the basic parameters. This is revealed by a unified thermodynamic and kinetic analysis which explains mechanisms involved in deposition, and the nature of instability of deposited monolayers. The results explain contact angle hysteresis and hydrophobic interactions in such systems. The assumption that monolayers of insoluble surfactants are stable in aqueous environments so long as the contact angle is large is shown to be erroneous. A consequence is that long range attractions seen between such surfaces in water are related to capillary condensation.