Electric linear dichroism. A powerful method for the ionic chromophore–colloid system as exemplified by dye and montmorillonite suspensions

The electric linear dichroism (ELD) measures the anisotropic absorption of a chromophore in solution under externally applied electric pulse field, which orients the solute molecule (particle) rotationally to the field direction. The ELD method is used to study at 20°C the binding mode and electrooptic properties of a nonlinear optical molecule (DAES), 4-[2-[4-(dimethylamino)phenyl]ethenyl]-1-ethylpyridinium, in the presence of montmorillonite K10 (MK-10) in the absorption region at mixing ratios (D/S) between 0.1 and 0.97, where D is the concentration of DAES and S is the cation exchange capacity of MK-10. Descriptions are given on a new home-built ELD apparatus. The measured quantities are the wavelength (600–300 nm) and field-strength (0–3 kV cm−1) dependence of the reduced dichroism, ΔA/A. The optical transition moment directions of bound DAES is determined from the observed ELD spectra relative to the symmetry axis of the field-oriented disklike MK-10 particle. Discussion is given both on the field-strength dependence of ΔA/A and on the ELD spectral analysis in terms of the tilt, roll, and inclination angles of the molecular plane of DAES bound to the surface and/or interlayer of MK-10 particle. The DAES molecule is not bound flatly on the surface or between the layers, but inclined at 35–39°, tilted by ±21°, and rolled by ±(27–33)° relative to the clay plane. The DAES–MK-10 system is no more associative than the clay itself on the basis of the electric-dichroism-average relaxation time, evaluated from the field-off decay signal, of DAES bound to MK-10 particle in solution.