ORIENTATION AND ELECTRONIC-STRUCTURE OF ION-EXCHANGED DYE MOLECULES ON MICA - AN X-RAY-ABSORPTION STUDY

Dye molecules are frequently used to determine the specific surface ar ea and the ion exchange capacity of high-surface-area materials such a s mica. The organic molecules are often considered to be planar and to adsorb in a flat orientation. In the present study we have investigat ed the orientation and electronic structure of crystal violet (CV) and malachite green (MG) on muscovite mica, prepared by immersing the sub strates for extended periods into aqueous solutions of the dyes of var ious concentrations, The K+ ions of the mica surface are replaced by t he organic cations via ion exchange. X-ray photoelectron spectroscopy reveals that only one amino group is involved in the interaction of CV and MG with the muscovite surface, i.e., certain resonance structures are abolished upon adsorption. With near edge X-ray absorption fine s tructure spectroscopy a significant tilt angle with respect to the sur face was found for all investigated species. A flat orientation, as ha s often been proposed before, can effectively be ruled out. Hence, our results are in marked contrast to the often quoted orientation and su ggest that the specific surface areas determined with dyes may, in gen eral, be overestimated. (C) 1998 Academic Press.