1. Max-Planck-Institut für Kolloid- und Grenzflächenforschung, Rudower Chaussee 5, 12 489 Berlin, Germany, Institute of Biophysics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 21, 1113 Sofia, Bulgaria, and Institute of Applied Physics, Technical University, 1156 Sofia, Bulgaria