Thermodynamic behavior and collapse kinetics of mixed DPPC/n-hexadecanol monolayers at the air/water interface

The thermodynamic behavior and collapse kinetics of mixed DPPC/n-hexadecanol monolayers at the air/water interface were studied. Surface pressure-area isotherms were obtained for mixed DPPC/n-hexadecanol monolayers at 15°C, 25°C , and 37°C by using a computer-controlled film balance. The analysis indicated that, at all surface pressures and compositions, the areas of monolayers exhibited significant negative deviations from the ideal values. Excess free energy changes of mixing had been calculated and the most stable state of the mixed monolayer with x_DPPC = 0.5 was found. The collapse kinetics of the mixed monolayers in the temperature range of 15-37°C was investigated by the relaxation measurements of the surface pressure at the collapse point as a function of time. It was shown that the collapse process which occurred in two stages can be represented by a nucleation and growth mechanism. The first stage is described by the Prout-Tompkins law and the second stage is described by a second order kinetic law.