A simple method for determining brine-gas interfacial tensions
Résumé
It is well known that most inorganic electrolytes dissolved in water have the effect to raise the interfacial tension (IFT), whereas all compressed gases but helium have the opposite effect, which corresponds respectively to negative adsorption (depletion) of the salt and to gas adsorption on the two opposite sides of the interface. By using Gibbs' adsorption equation and reasonable assumptions, we show that those two effects are independent: the presence of inorganic electrolytes in the aqueous phase has a negligible impact on the gas-related IFT decrement, and the compressed gas does not in turn alter the IFT increment due to the salt dissolved in the aqueous phase. As a consequence the IFT at a given pressure and temperature can be approximated by the brine surface tension at the same temperature, minus the gas-related IFT decrement of pure water surface tension at those pressure and temperature. The two latter quantities are easier to determine experimentally, and have been the subject of numerous experimental and theoretical investigations. The proposed approximation is consistent with the available experimental data, including when the 'gas' is a supercritical fluid or a compressible condensate (liquid) made up of compounds sparingly soluble in water, such as CO 2 at respectively supercritical or subcritical temperatures.