THE WETTING OF MONOLAYER FILMS EXPOSING IONIZABLE ACIDS AND BASES

This paper describes the wetting by water of monolayer films formed by the spontaneous assembly of long-chain alkanethiols, HS(CH2)10X, onto gold surfaces, where X = carboxylate, phosphonate, 5-tetrazolo, 1-imidazolo, or 2-imidazolo. Mono layers were also generated from mixtures of HS(CH2)10X and HS(CH2)10-CH3. The wetting by water of the single-component and the mixed surfaces was examined as a function of pH. The surfaces uniformly became more wettable at values of pH at which the interfacial groups were charged. Surfaces rich in carboxylic acid groups exhibited an unexpected behavior: the surfaces were less wettable at values of pH between 3 and 7 than at lower or higher values of pH. We attribute this unexpected behavior to conformational changes of the terminal parts of the chain including the carboxylate groups upon partial ionization.