Influence of secondary and tertiary amines on the stability of stearic acid monolayers

Acid-base interactions and the influence of multivalent cations on the stability of stearic acid monolayers were studied by means of the Langmuir technique. The surface activity of secondary and tertiary amines increases with increasing size of the hydrocarbon moiety. The Langmuir study revealed that the dibutylamine-stearic acid complex is water-soluble, whereas the complexes formed when stearic acid interacts with tributylamine, dihexylamine, trihexylamine or trioctylamine are not sufficiently polar to be water-soluble. The presence of Ca(2+)ions in the subphase results in a reduced penetration of the monolayer. Relaxation measurements show that salt in the bulk solution stabilizes the monolayer. (C) 1997 Elsevier Science B.V.