Ion mobility measurements in dielectric liquids

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Abstract
Ion mobility measurements were made on n-hexane and a 0.22M solution of biphenyl in n-hexane using a time-of-flight technique. A collimated beam of x rays from a flash x-ray generator was used to produce a layer of ions near one electrode of a parallel-plate conductivity cell. Part of the charge layer was swept under the influence of an electric field to the opposite electrode, and the motion of the charge was observed by means of a current-versus-time measurement. Mobilities have been determined by fitting theoretical current–time curves to experimental ones. The theoretical current–time curves were obtained by solving numerically the differential equations governing carrier motion during a mobility measurement. Diffusion, bimolecular recombination, motion under an applied electric field, and induced bulk liquid motion were included in the theoretical model. The measured spatial dose distribution, radiation yield data, applied voltage, and electrode separation were input constants to the theoretical calculation, while values for the anion mobility, cation mobility, and induced liquid acceleration were variable parameters. The theoretical model yields the spatial distribution of charge carriers and its time dependence during the charge transport. It also describes the electric field between the electrodes as a function of space and time and demonstrates the effects of space charge. The current–time curves generated from the model are in good agreement with those obtained experimentally. The theoretical model that includes the effects of induced bulk liquid motion yields values of mobility that are significantly lower than those determined experimentally by use of inflection or ’’half-steady-state’’ points. Measurements made on purified n-hexane showed that a radiation pulse produces more than one slow carrier of each sign. When biphenyl was added to the sample, one negative and two positive species were observed. Our theoretical fits to experimental curves obtained from the irradiation of a 0.22M solution of biphenyl in n-hexane yields a mobility value for the negative species of 3.6×10−4 cm2 V−1⋅sec−1. This negative species is believed to be the biphenylide anion.