Linearly polarized IR-spectroscopy of partially oriented solids as a colloid suspension in nematic host: a tool for spectroscopic and structural elucidation of the embedded chemicals

Abstract

Colloid suspensions in nematic liquid crystal were employed for the first time in 2004 as a tool for the partial orientation of solids, to be examined by linearly polarized IR-(IR-LD) spectroscopy. It has been found is found that a partial orientation (15–20%) of suspended particles, is adequate for the recording of reasonable linearly polarized IR-spectra is achieved when: 5 ± 1% by weight of the given solid compound (organic, inorganic, transition metal complex or glass) with particle size within the limits 0.3–0.9 μm is mixed with a nematic liquid crystal substance (ZLI 1695, ZLI 1538 or MLC 6815) suitable for IR spectroscopy and the slightly viscous suspension obtained is phase pressed between two KBr-plates. These latter are roughened in one direction prior to use with fine sandpaper (C800) (size 5 μm). Then, the KBr-plattes and pressed suspension are moved repeatedly with 3 μm/s for 100 times. The optimal cell thickness is 100 μm. If mathematical procedures are used for polarized IR-spectra interpretation, then it is possible to perform structural elucidation of the embedded compounds, independently of their melting point, crystalline or amorphous state, and the quality of the monocrystals or polycrystalline of the e sample. The method permits the study of organic and inorganic compounds, transition metal complexes and glasses. Here we will discuss the fundamental questions concerning the above state such as the morphology of the suspended particles, the particle size, the influence of the physical chemistry properties of liquid crystal medium on the degree of orientation of suspended particles; the velocity of the shearing of the suspension, the degree of the roughening of the KBr-plates and their effects on the degree of orientation, the influence of the space group on the orientation parameter, the nature and balance of the forces acting on the suspended particles; their degree of orientation, the mathematical model used. Conventional and linearly polarized IR-spectroscopy and electron microscopy are used for elucidation of these points. Statistical approaches have also been applied in order to estimate the impact of the experimental parameters (size, velocity, thickness) on the IR-signal for each of the 13 systems studied. An experimental design of the type involving full factorial design on two levels of variation of the input factors is presented.