Orientation of Adsorbed Dyes in the Interlayer Space of Clays. 1. Anisotropy of Rhodamine 6G in Laponite Films by Vis-Absorption with Polarized Light

X-ray diffraction and Vis-absorption spectroscopy with linearly polarized light are used to determine the orientation of rhodamine 6G (R6G) dye in the interlayer space of supported films of Laponite B (Lap) clay with different dye loadings. XRD profiles suggest that the d 001 interlayer space increases with the relative amount of dye in the clay surface, probably due to a more perpendicular disposition of the adsorbed molecules with respect to the clay layers in high dye content films. This orientation reduces the covering area per adsorbed molecule, providing a more compact and packed arrangement of the dye molecules. This is confirmed by linearly polarized absorption spectra, from which the orientation angle of the adsorbed molecules with respect to the clay layer can be evaluated. Indeed, R6G/Lap supported films present an anisotropy effect and, depending on the dye loadings, different evolutions of the dichroic ratio with the twisted δ angle between the normal to the film and the incident beam are observed. For very diluted R6G/Lap films, the R6G monomers adopt an inclined disposition of around 62° with respect to the film normal. Increasing the loadings, dimer, and higher-order aggregates of R6G can be disposed in two preferential orientations: an inclined angle of around 60°, common in all loading samples, favors long-displaced coplanar aggregates, characterized by an absorption J-band, and a second more perpendicular disposition of the monomer units as the dye content is increased leading to short-displaced coplanar aggregates, identified by an absorption H-band.