Henry's Constant Analysis for Water and Nonpolar Solvents from Experimental Data, Macroscopic Models, and Molecular Simulation

Abstract

Experimental data, equations of state (EoS), and Monte Carlo simulations are used to analyze the Henry's law constant of solutes in water and in organic solvents at different temperatures. EoS are incapable of correlating the experimental data for light hydrocarbons dissolved in water. Novel simulation methodologies are used for methane in water and in ethane. Results are analyzed with respect to the free energy of cavity formation for hosting the solute molecule in the solvent and the free energy of interactions between the solute molecule and the solvent. It is shown that the hydrophobic phenomenon is driven, to a large extent, by the weak intermolecular interactions between water molecules and nonpolar solute molecules. creases with temperature for relatively low-temperature values, goes through a maximum, and then decreases for higher temperatures. A relatively high Henry's constant value corre- sponds to low solubility and vice versa. Traditionally, the low solubility of a solute in a solvent has been associated with differences in energetic interactions between solvent molecules and solute molecules or differences in molecular size or both. A typical example of the first case is the dissolution of n-alkanes in water. N-alkane molecules interact through weak London forces, whereas water molecules exhibit strong hydrogen- bonding interactions resulting in three-dimensional structures. A representative example for the second case is that of light gases (hydrogen, nitrogen, etc.) dissolved in a heavy n-alkane. In this work, a generalized thermodynamic framework is presented for the analysis of the Henry's constant and its variation with temperature. Henry's constant is expressed as a product of two terms: The first term involves pure solvent thermodynamic properties only. The second term is a function of the excess chemical potential of the solute at the state conditions examined. The latter is subdivided into a term associated with the formation of a cavity in the solvent to host the solute molecule and a term that accounts for the free energy of inserting the solute into the cavity by turning on the energetic interactions between solute and solvent molecules. In this way, detailed analysis of the various factors affecting Henry's constant is performed. Experimental data for two representative systems, methane in water 3 and methane in n-hexadecane, 3 are analyzed