1. Laboratory of Thermodynamics and Physicochemical Hydrodynamics, Faculty of Chemistry, University of Sofia, 1126 Sofia, Bulgaria, Colgate-Palmolive Company, Technology Center, 909 River Road, Piscataway, New Jersey 08854-5596, and Colgate-Palmolive R&D, Inc., Avenue Du Parc Industriel, B-4041 Milmort, Belgium