1. Laboratory for Chemical Surface Science, Department of Chemistry, Physical Chemistry, Royal Institute of Technology S-100 44 Stockholm, Sweden, Institute for Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden, Faculty of Chemistry, Department of Physical Chemistry, Blvd. J Bourchier 1, 1126 Sofia, Bulgaria, Akzo Nobel Surface Chemistry AB, S-444 85 Stenungsund, Sweden, and Groupe Colloids, Service de Chimie Moleculaire, Centre d'Etudes de Saclay, 91191 Gif sur Yvette, France