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