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

Abstract

The dependence of the surface pressure vs area (π−A) isotherms of fatty primary amine monolayers spread on a subphase of an aqueous solution of various Hn[MO4]n- acids (M = S6+, P5+, Cl7+) under ionic conditions of low pH on several temporal parametersincluding compression/expansion rateshave been studied as a function of the chain length of the fatty amines. A parameter, RT/C, has been used as a qualitative measure of the relative strengths of the tail−tail interaction as compared to the subphase−head-group (acid−base) complex-forming interaction. The changes in the π−A isotherm due to waiting for different periods of time after spreading as well as the changes in the surface pressure, π, with time at various barrier positions have been studied. The results show an increase in the lift-off area, ALO, with time that is not due to an external contaminant. The compression/expansion curves always show hysteresis, the extent of which may decrease (for larger RT/C) or show a counterintuitive increase (for smaller RT/C) with decreasing compression/expansion rates. The results are analyzed in terms of a new two-state model involving clusters and individual complexed amine molecules.