1. Laboratory of Thermodynamics and Physico-chemical Hydrodynamics, Faculty of Chemistry, University of Sofia, 1126 Sofia, Bulgaria, Colgate-Palmolive Research and Development, Inc., Avenue Du Parc Industriel, B-4041 Milmort (Herstal), Belgium, and Colgate-Palmolive Company, Technology Center, 909 River Road, Piscataway, New Jersey 08854-5596