ROLE OF TAIL-TAIL INTERACTIONS VERSUS HEAD-GROUP-SUBPHASE INTERACTIONS IN PRESSURE-AREA ISOTHERMS OF FATTY AMINES AT THE AIR-WATER-INTERFACE .2. TIME-DEPENDENCE

The dependence of the surface pressure vs area (pi-A) isotherms of fat ty primary amine monolayers spread on a subphase of an aqueous solutio n of various H-n[MO4](n-) acids (M = S6+, P5+, Cl7+) under ionic condi tions of low pH on several temporal parameters-including compression/e xpansion rates-have been studied as a function of the chain length of the fatty amines. A parameter, R-T/C, has been used as a qualitative m easure of the relative strengths of the tail-tail interaction as compa red to the subphase-head-group (acid-base) complex-forming interaction . The changes in the pi-A isotherm due to waiting for different period s of time after spreading as well as the changes in the surface pressu re, pi, with time at various barrier positions have been studied. The results show an increase in the Lift-off area, A(LO), with time that i s not due to an external contaminant. The compression/expansion curves always show hysteresis, the extent of which may decrease (for larger R-T/C) or show a counterintuitive increase (for smaller R-T/C) with de creasing compression/expansion rates. The results are analyzed in term s of a new two-state model involving clusters and individual complexed amine molecules.