Gaseous diffusion in liquids

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Virginia Polytechnic Institute and State University

Diffusivity of nonreactive gases in liquids provides a means of interpreting structure in the liquid state. Structural models of the liquid state include Hildebrand's condensed gas model and Eyring's pseudo-lattice model. The former model predicts a linear dependence of diffusivity with temperature while the latter model predicts linear dependence of log(D) versus 1/T. The limited temperature dependent diffusivity data to date with a typical precision of ± 5% do not permit distinguishing which temperature dependence is more linear. However, the present investigation shows that diffusivities of one gas solute in two nonpolar liquids indirectly supports a linear diffusivity temperature dependence by a Graham's law like relation. At a fixed temperature this relation equates relative diffusivities to the square root of the inverse molecular weights of the respective liquids.

Diffusion of gases into nonpolar liquids have previously been measured by two techniques: (1) a pseudo-steady state technique developed by Hildebrand with diffusion through multiple capillaries and (2) a method by Walkley with diffusion through an open tube. Each of these methods requires prior knowledge of solubility of the gas in the liquid. An apparatus is constructed which combines these methods into a single experiment. Simultaneous solution of the two equations which describe the combined experiment yields both the solubility and diffusion coefficient. Diffusivities and solubilities of nitrogen, argon and oxygen into liquids of carbon tetrachloride and benzene as well as oxygen into water have been studied. The results compare favorably with the Literature.

The diffusion cell for this technique consists of a capillary disk, which is flooded with liquid. Gas is admitted into the space over the open solvent. With temperature and pressure constant, volume uptake of the gas in the solvent is monitored. Time-volume uptake data is evaluated by the two diffusion equations. Although the experiment is conceptually easy, a small gas volume change over a prolonged period of time poses problems in data collection and experiment control. The data collection and control is simplified by dedicating a Microcomputer Interfaced Data Acquisition System (MIDAS) to the experiment.