Molecular structure-interfacial activity relationship of N-substituted amino acids. Adsorption of N-dansylglycine at the mercury-aqueous solution interface

The adsorption of N-dansylglycine (5-dimethylaminonaphthalene-1-sulphanylglycine) in monoanionic and dianionic forms has been characterized in aqueous solution at a mercury electrode. At pH 6 (only the monoamonic form present in the bulk), N-dansylglycine is arranged on the metal surface in a perpendicular manner through the naphthyl moiety, and an attractive interaction factor among the adsorbed molecules is observed. At pH 12 (dianionic form), the molecules still arranged in a perpendicular way show a greater molecular area and no interaction factor is revealed. These features, together with those regarding a series of other N-substituted glycine derivatives previously studied, are related to some calculated structural indices, and a relationship is obtained between the adsorption Gibbs energy and ?El-h (the energy difference between the lowest unoccupied molecular orbital and the highest occupied one). Moreover, some local theoretical indices (net atomic charges, partial net ? charges) are used in a tentative molecular interpretation of the disposition of the sorbate on the surface and of the significance of the Frumkin interaction factor. To offer a more general grounding to the argument, a group of naphthyl and benzoyl derivatives studied by Bockris and co-workers (J. Phys. Chem., 65 (1961) 2000) in very similar conditions is also considered.