SURFACE AND INTERFACIAL BEHAVIOR OF NICOTINIC-ACID, NICOTINAMIDE AND NIPECOTAMIDE

The results of an adsorption study of Nc, NcA and NpA at the free surf ace of water are compared with those previously obtained at the unchar ged Hg/solution interface. The saturation surface concentrations of ad sorbates differ at the two interfaces. For Nc and NcA at the air/solut ion interface the surface concentration is smaller than that of NpA (i .e. NpA achieves full saturation coverage, but Nc and NcA do not), whe reas the opposite effect is observed at the uncharged Hg surface. The lateral interaction has been found to be essentially attractive at the free surface, whereas such an interaction is repulsive at the Hg surf ace; this is interpreted in terms of hydrogen bonding of the polar gro up to neighbouring water molecules in the adsorption layer. The former probably results from the easier replacement of water molecules from the surface layer. This effect however, decreases in the cases of Nc a nd NcA (particularly in the presence of 0.1 hi KF) as referred to NpA molecules.