1. Center for Ultrastructure Research, Universität für Bodenkultur Wien, Gregor-Mendel-Strasse 33, A-1180 Vienna, Austria, Technical Research Centre of Finland, Chemical Technology, P.O. Box 14021, FIN-33101 Tampere, Finland, and Department of Physical Chemistry, Åbo Akademi University, Porthaninkatu 3-5, FIN-20500 Turku, Finland