1. Max-Planck-Institut für Kolloid- und Grenzflächenforschung, D-14424 Potsdam/Golm, Germany, and Research-Group for Colloids and Interfaces Supported by the Max-Planck-Society at the Institute for Ceramic Engineering, Freiberg University of Mining and Technology, Chemnitzer Strasse 40, D-09599 Freiberg, Germany