Time dependent anchoring of adsorbed cationic surfactant molecules at mice/solution interface

The nature of adsorbed cationic amphiphiles at the mica/solution interface was studied by XPS and contact angle measurements. The elemental analyses o f freshly cleaved mica surfaces by XPS showed that the potassium atoms on t he surface lattice of mica are not necessarily distributed equally to each surface on cleavage. The adsorbed cationic amphiphile molecules remaining o n mica surfaces after rinsing with distilled water were found to be anchore d to the surface by ion-exchange, replacing surface potassium and/or other cations, The ratio of adsorbed cationic amphiphile molecules with single al kyl chains to the maximum potassium ions on mica surface was estimated to b e twice as large as that of amphiphiles having two alkyl chains. The contac t angle of water drops placed on the adsorbed surface showed a gradual decr ease 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 anchori ng of adsorbed molecules by ion-exchange was found to occur extremely slowl y, however; the anchored molecules may not easily be desorbed when rinsed w ith deionized water. The time dependent anchoring of adsorbed molecules was studied in terms of adsorption time, alkyl chain length, and concentration of cationic surfactant, (C) 1999 Academic Press.