Mobility of Surfactants in and between Adsorbed Monolayers

Using X-ray photoelectron spectroscopy (XPS), the lateral interdiffusion of ionic surfactants in adsorbed monolayers and their exchange between two contacting monolayers were measured under varying conditions of temperature and humidity. Measured interdiffusion coefficients at 25 °C were of order (1−10) × 10-9 cm2/s, and exchange half-times 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 the packing density or average molecular area when molecules move or exchange positions, (2) electroneutrality constraints, i.e., the need to maintain local electroneutrality at all places at all times, (3) confinement constraints in trapped monolayers, i.e., mild (low pressure) compressive confinement seems to have only a small effect on diffusion rates, and (4) temperature and humidity, i.e., higher temperatures and humidity generally increase both the diffusion and exchange rates.