Dilute Vapor Absorption: A New Accurate Technique for Measurement of the Limiting Activity Coefficient of Water in Hydrophobic Solvents of Lower Volatility

Abstract

An accurate experimental method for the determination of the limiting activity coefficient of water in hydrophobic solvents was described theoretically; the pertinent apparatus was built and its performance was duly verified. The method is based on continuously saturating the solvent by a gas stream of a constant and known humidity up to the attainment of the steady state and subsequent determination of the water content in the solvent by Karl Fischer titration. Using this new method, accurate values of the limiting activity coefficient of water were determined in eight selected solvents. The measurements were carried out at several temperatures and from these data the infinite dilution dissolution enthalpy and entropy of water in the solvents were evaluated. They both exhibit highly positive values reflecting the breakage of the hydrogen bonding network of water upon its dissolution in the solvents. Prediction of the water limiting activity coefficients was further attempted using the Modified UNIFAC method and found to fail dramatically, the calculated values being lower than the experimental ones by a factor ranging from 2 to 10. General lack of reliable limiting water activity coefficient data and their unreliable prediction indicate the significance of both the new method and the acquired data.

Activity Coefficients of Water at Infinite Dilution in Common Oxygenated Solvents

“…As a result, the behavior of water as a solute in organic solvents is insufficiently understood and generally difficult to predict. To contribute to an improvement, we have recently developed a new experimental technique (Dilute Vapor Absorption, DVA) 1 well applicable for measurement of the limiting activity coefficient of water in rather hydrophobic solvents of lower volatility and reported the data for a representative set of solvents of this type.…”