1. Laboratory for Physical Chemistry and Colloidal Science, DreijenPlein 6, 6700 EK Wageningen, The Netherlands, Rhodia, Centre de Recherches 52, Rue de la Haie Coq, 93308 Aubervilliers, France, and Rhodia, CRL 85, Avenue des Frères Perret, 69191 Saint Fons Cedex, France