E. Ruckenstein et Bq. Li, PHASE-TRANSITION FROM A LIQUID EXPANDED TO A LIQUID CONDENSED SURFACTANT MONOLAYER, Journal of physical chemistry, 100(8), 1996, pp. 3108-3114
Liquid monolayers of insoluble surfactants at the air/water interface
are treated as a two-dimensional solution made up of molecules with di
sordered chains and clusters of molecules with ordered chains. The Flo
ry-Huggins equation is used to calculate the activities of the species
. On this basis, the phase transition from the Liquid expanded to the
liquid condensed state is predicted. Depending on the value of the int
eraction parameter between disordered and ordered molecules and the ex
tent of clustering, the theoretical pi-A isotherms exhibit either a fi
rst-order phase transition or a nonhorizontal, second-order, phase tra
nsition. For the tetradecanoic, pentadecanoic, hexadecanoic, and hepta
decanoic fatty acids, the theoretical pi-A isotherms are in agreement
with experimental data. It should be emphasized that the theory is pre
dictive since all the parameters are calculated on the basis of the mo
lecular structure.