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 methylene blue (MB) molecule is a prerequisite for a precise determination of the specific surface area. There is, however, some disagreement about the correct value due to the unknown orientation of the adsorbed MB molecules. In the present study the electronic structure and orientation of MB on muscovite mica is investigated with x-ray photoelectron spectroscopy and the synchrotron-based technique of near edge x-ray absorption fine structure spectroscopy. It was found that the MB molecules are tilted, with their largest face inclined at 65-70° with respect to the surface. This value disagrees with common a priori assumptions that MB lies flat on the surface, i.e., the specific surface area of mica is overestimated in many cases by a significant factor. The results, however, agree with a recent study proposing an apparent surface area of 66 Å2 for MB molecules on mica.