Gas diffusion and solubility measurements in liquid-filled porous media



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Diffusivities and solubilities of ethane in eight solvents and solvent-saturated Vycor and silica-alumina were determined by measuring gravimetrically the transient response of the samples to a step change in the surrounding gas pressure. The solvents included saturated hydrocarbons, an alcohol, a halogenated compound, a triester, an oxy compound, and a tertiary amine, all with molecular weights between 100 and 400. Diffusivities were obtained by a nonlinear least-squares fit of the rate of approach to equilibrium data to a Fickian diffusion model. Solubilities were obtained by a material balance. Bulk diffusivities and solubilities of ethane in the various solvents were determined by an identical experiment in which the solvents were immobilized in a fritted glass disc of known tortuosity. Characteristics of diffusion of ethane in the solvent-saturated porous media were expressed as apparent tortuosities, defined as the ratio of the bulk diffusivity to the effective diffusivity. Ethane solubilities in the liquid-filled pores were expressed as solute distribution coefficients, ratios of the ethane solubility in the pores to ethane solubility in the bulk liquid. Solvent molecular sizes ranged from 10 to 22 percent of the pore diameter of Vycor; yet theoretical predictions of steric effects on diffusion and solubility did not agree with the qualitative trends of the data for the various solvent-solid systems. Interactions with the solid surface, both chemical and physical, seemed to dominate all other effects. The ratio of ethane concentration inside pores to the ethane concentration in the bulk at equilibrium was found to be approximately unity for saturated hydrocarbon solvents, and significantly less than unity for the solvents expected to selectively adsorb on silica surfaces. The apparent tortuosities were proportional to the solute distribution coefficients as predicted by a simple diffusion-adsorption model, suggesting that solute adsorption had occurred. The effect was observed even when no enhancement of the gas solubility occurred, an observation usually indicative of solute adsorption. To account for this apparent adsorption, a model was developed to predict adsorption factors from experimental solute distribution coefficients and system properties. A qualitative model was also presented to illustrate the alteration of pore structure by strongly adsorbing species. Mechanisms by which the liquid molecular structure might be affected by the presence of an extensive solid surface were proposed to explain anomalies in the concentration dependence of the apparent tortuosity of Vycor. Structural effects were also proposed to explain the unusually low tortuosity values observed for Vycor saturated with primary decanol.