Time Dependent Anchoring of Adsorbed Cationic Surfactant Molecules at Mica/Solution Interface

Abstract

The nature of adsorbed cationic amphiphiles at the mica/solution interface was studied by XPS and contact angle measurements. The elemental analyses of freshly cleaved mica surfaces by XPS showed that the potassium atoms on the surface lattice of mica are not necessarily distributed equally to each surface on cleavage. The adsorbed cationic amphiphile molecules remaining on mica surfaces after rinsing with distilled water were found to be anchored to the surface by ion-exchange, replacing surface potassium and/or other cations. The ratio of adsorbed cationic amphiphile molecules with single alkyl chains to the maximum potassium ions on mica surface was estimated to be twice as large as that of amphiphiles having two alkyl chains. The contact angle of water drops placed on the adsorbed surface showed a gradual decrease with the elapse of time due to the dissolution of adsorbed surfactant into the water drop; however, the decrease was not observed for those mica surfaces when aged for more than 3 days in the adsorption bath. The anchoring of adsorbed molecules by ion-exchange was found to occur extremely slowly, however; the anchored molecules may not easily be desorbed when rinsed with deionized water. The time dependent anchoring of adsorbed molecules was studied in terms of adsorption time, alkyl chain length, and concentration of cationic surfactant.