MOBILITY OF SURFACTANTS IN AND BETWEEN ADSORBED MONOLAYERS

Using X-ray photoelectron spectroscopy (XPS), the lateral interdiffusi on of ionic surfactants in adsorbed monolayers and their exchange betw een two contacting monolayers were measured under varying conditions o f temperature and humidity. Measured interdiffusion coefficients at 25 degrees C were of order (1-10) x 10(-9) cm(2)/s, and exchange half-ti mes were of order 1 h (even longer for totally dry conditions). These low diffusion and exchange rates result from the complex rate-limiting interdependence of different migrating surfactant molecules and their counterions. The rate-limiting factors to both diffusion and exchange were found to be (1) steric constraints, i.e., the need to conserve t he packing density or average molecular area when molecules move or ex change positions, (2) electroneutrality constraints, i.e., the need to maintain local electroneutrality at all places at all times, (3) conf inement constraints in trapped monolayers, i.e., mid (low pressure) co mpressive confinement seems to have only a small effect on diffusion r ates, and (4) temperature and humidity, i.e., higher temperatures and humidity generally increase both the diffusion and exchange rates.