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

Equimolar catanionic monolayers were formed at the air-water interface by m ixing double- or single-tailed cationic amphiphiles (with chain lengths var ying from C-12 to C-18) with single-tailed anionic amphiphiles (from C-12 t o C-16). The mixed systems investigated show strong synergistic behavior be tween their components: the major part form solid or liquid condensed monol ayers, with low isothermal compressibilities and high collapse surface pres sures. The kinetics of monolayer relaxation, after stopping its compression , was studied for the five most uncompressible systems using two methodolog ies: (i) surface pressure relaxation, pi -t, at constant area A and (ii) ar ea 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 mono layer reorganization processes at the interface. The third, much longer lif etime (of hundreds to thousands of minutes) was assigned to a monolayer des orption into the subphase. The two relaxation methods (at constant A or pi) gave equivalent reorganization lifetimes, within the experimental error. H owever, different desorption lifetimes were obtained by one or the other pr ocedure. The experimental pi -A, pi -t, and A-t isotherm data were intercon nected by Euler's chain relation at a common point of the three isotherms w here desorption can be ignored, {pi (0), A(0), t(0)}.