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 m ica surface during spreading, compression, and subsequent Langmuir-Blo dgett deposition of an insoluble monolayer of DSPE (distearoylphosphat idylethanolamine) spread at the water-air interface, and on reimmersio n of the surface in pure water. While such systems are essentially non equilibrium and irreversible in various aspects of their behavior, we show that fundamental principles behind Langmuir-Blodgett phenomena ca n be understood by consideration of the thermodynamic equilibrium at t he three-phase line, At large areas per molecule, long before the mono layer at the liquid-vapor interface is condensed, it undergoes spontan eous condensation and compression at the three-phase line. By this the rmodynamically driven mechanism the contact angle increases and the la yer is transferred onto the surface of the solid. This is a general ef fect which occurs also for soluble surfactants. Free energies of trans fer of a lipid molecule between the interfaces that coexist at the thr ee-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 interacti ons in such systems. The assumption that monolayers of insoluble surfa ctants are stable in aqueous environments so long as the contact angle is large is shown to be erroneous. A consequence is that long range a ttractions seen between such surfaces in water are related to capillar y condensation.