To understand the distinction between the effects of zwitterionic. anionic. and cationic l-leucine upon adsorption and lateral interactions at air/water surface. the surface tensions of aqueous solutions of L-leucine-L-deucinehydrochloride and L-leucine-sodium L-leucinate mixtures were measured as a function of concentration and composition at 25 degrees C. The Surface activity decreases in the order L-leucine > L-leucine hydrochloride > sodium L-leucinate. Both L-leucine hydrochloride and sodium L-leucinate form gaseous adsorbed films through the experimentally accessible concentration range, while the adsorbed film of zwitterionic 1,L-leucine shows a transition between gaseous and expanded film. (c) 2005 Elsevier Inc. All rights reserved.