1. Department of Physics, University of Bío-Bío, Av. Collao 1202, P.O. Box 5-C, Concepción, Chile
2. Departamento de Química Física, Universidad Complutense de Madrid, Madrid 28040, Spain
3. Chemical Engineering Department and Surface Analysis Laboratory (ASIF), University of Concepción, P.O. Box 160-C, Correo 3, Concepción, Chile
4. Institut für Theoretische Physik II: Weiche Materie, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-40225 Düsseldorf, Germany