Reorganization and desorption of catanionic monolayers.: Kinetics of π-t and A-t relaxation

Equimolar catanionic monolayers were formed at the air-water interface by mixing double- or single-tailed cationic amphiphiles (with chain lengths varying from C-12 to C-18) with single-tailed anionic amphiphiles (from C-12 to C-16). The mixed systems investigated show strong synergistic behavior between their components: the major part form solid or liquid condensed monolayers, with low isothermal compressibilities and high collapse surface pressures. The kinetics of monolayer relaxation, after stopping its compression, was studied for the five most uncompressible systems using two methodologies: (i) surface pressure relaxation, pi -t, at constant area A and (ii) area relaxation, A-t, at constant pi. The relaxation data could be fitted to a sum of three exponentials. The two shorter lifetimes (of few minutes and tens of minutes) were assigned, respectively, to short- and long-range monolayer reorganization processes at the interface. The third, much longer lifetime (of hundreds to thousands of minutes) was assigned to a monolayer desorption into the subphase. The two relaxation methods (at constant A or pi) gave equivalent reorganization lifetimes, within the experimental error. However, different desorption lifetimes were obtained by one or the other procedure. The experimental pi -A, pi -t, and A-t isotherm data were interconnected by Euler's chain relation at a common point of the three isotherms where desorption can be ignored, {pi (0), A(0), t(0)}.