Kinetics and thermodynamics of surfactant adsorption at model interfaces: Evidence of structural transitions in the adsorbed films

An in situ method, based on two liquid phase contact angle measurements, is developed to investigate the mechanisms of simultaneous adsorption of surfactant at solid-liquid and liquid-liquid interfaces. Well-characterized model systems (methyl-terminated monolayer-hydrocarbon drop-aqueous surfactant solution) and (methyl-terminated monolayer-air bubble-aqueous surfactant solution) have been used to allow reliable connection between surface characteristics and observed phenomena. The adsorption kinetics at such complex interfaces was analyzed through the balance of interfacial forces. For both nonionic surfactants (C(8)E(10) and C(12)E(7)) and air bubble used in these studies,,we have found that the adsorption kinetics exhibit intermediate transition plateaus at some critical bulk concentrations (around the cmc). These transitions were related to structural rearrangements in the adsorbed surface phase.