ORIENTATION AND ELECTRONIC-STRUCTURE OF METHYLENE-BLUE ON MICA - A NEAR-EDGE X-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY STUDY

Methylene blue adsorption from aqueous solution is widely used for the determination of the specific surface area of mica. In this procedure , knowledge of the space requirement for each ion-exchange-reacted met hylene blue (MB) molecule is a prerequisite for a precise determinatio n of the specific surface area. There is, however, some disagreement a bout the correct value due to the unknown orientation of the adsorbed MB molecules. In the present study the electronic structure and orient ation of MB on muscovite mica is investigated with x-ray photoelectron spectroscopy and the synchrotron-based technique of near edge x-ray a bsorption fine structure spectroscopy. It was found that the MB molecu les are tilted, with their largest face inclined at 65-70 degrees with respect to the surface. This value disagrees with common a priori ass umptions that MB lies flat on the surface, i.e., the specific surface area of mica is overestimated in many cases by a significant factor. T he results, however, agree with a recent study proposing an apparent s urface area of 66 Angstrom(2) for MB molecules on mica. (C) 1996 Ameri can Institute of Physics.