Salt effects on adsorbed nonelectrolytes

Abstract

An approach to the study of the influence of electrolytes on adsorbed nonelectrolytes at liquid surfaces is described. The adsorption of tetrabutylammonium bromide (Bu4NBr) from aqueous solution to the interfaces with octane, decanol, and air has been determined. Results are presented for the effects of some inorganic salts (NaCl, NH4Br, and Na2CO3), and of Bu4NBr on monolayers of butanol at the air – aqueous solution interface, and of Bu4NBr on dodecanol adsorbed at the octane – aqueous solution interface. The interfacial salt effects differ from the bulk effects in the cases studied. The inorganic salts, which salt-out butanol (and alkanols generally) in aqueous solution, have little or no effect on adsorbed butanol. On the other hand, Bu4NBr which salts-in alkanols in bulk aqueous solution has a strong salting-out effect on dodecanol at the liquid–liquid interface; a similar but less marked effect is observed for butanol at the liquid–vapour surface. Salting-in of alkanols by Bu4NBr in bulk has previously been ascribed to hydrophobic interactions between cations and alkyl groups of the alkanol, whereas the surface effect is assumed to result from interactions between alcoholic OH groups and cations.