Characterization of crystallisation processes with electrical impedance spectroscopy

摘要

The surfaces of crystalline particles are often charged and therefore crystals have an electrical double layer associated with their surfaces when, for example, suspended in an aqueous medium. Under the influence of an external electric field, an electrical dipole moment can be induced due to polarization of the electrical double layer. The complex impedance, conductivity, relaxation time and phase shift caused by the polarization are all related to particle characteristics, e.g. the size, polymorph and concentration. The method of electrical impedance spectroscopy (EIS) provides, potentially, a capability for characterizing the particles in colloidal suspensions. Here some preliminary observations of physical-chemistry fundamentals relating to the use of electrical impedance spectroscopy, potentially, for particle sizing in crystallisation processes are reported. Firstly, the effects of L-glutamic acid (LGA) solids concentration, solute concentration and temperature on the measured EIS spectra were studied. The results show that the effects of solid LGA concentration and solute concentration on the relaxation frequency and phase angle are very small. Secondly, the temperature effect was analysed. By deducting the contribution due to the changes in concentration and temperature from the total change in relaxation frequency, the experimental observations, although preliminary in nature, suggest that the crystal size can be related directly to relaxation frequency (in this study, the shape of LGA crystals is assumed to be spherical so the crystal size means the diameter of the sphere). The experimental observations can be explained with either static polarization models or particle vibration models. On-line measurement of electrical impedance spectra associated with LGA nucleation-growth processes are presented. The information content of the complex electrical impedance, phase angle shift and relaxation times of the crystal suspensions during a crystallisation process is assessed. The results show that during nucleation the imaginary part of the electrical-impedance response changes significantly and the relaxation frequency decreases during the growth of crystals. The longer-term objective is to develop a new spectroscopic measurement method and further, a spectroscopic tomography technique for on-line measurement of the spatial distribution of particle size in particle suspensions in liquid media.