1. Department of Chemistry, Dainton Building University of Sheffield Brook Hill South Yorkshire S3 7HF UK
2. Present address: Unilever Research & Development Colworth Laboratory, Colworth House Sharnbrook Bedford MK44 1LQ UK
3. Laboratory of Physical Chemistry Department of Chemical Engineering & Institute for Complex Molecular Systems Eindhoven University of Technology P.O. Box 513, 5600 MB Eindhoven The Netherlands