Solute-Solute Interactions in Aqueous Solutions

Solute‐solute interactions in aqueous solutions of nonelectrolytes are interpreted using both lattice and distribution function theories of the dissolved state. Experimental activity data of high precision can be obtained from the literature for aqueous solutions of many nonelectrolytes. If the logarithm of the solvent activity coefficient ${\mathrm{(\gamma }}_1)$ is expressed as a power series in the mole fraction of the solute ${\mathrm{(x}}_2\mathrm{),}\ln {\gamma }_1{\mathrm{\hspace{0.17em}=\hspace{0.17em}Bx}}_2^2{\mathrm{\hspace{0.17em}+\hspace{0.17em}Cx}}_2^3\mathrm{\hspace{0.17em}+\hspace{0.17em}···}$ , then the coefficients $B$ and $C$ can be determined analytically from the experimental measurements. Values of $B$ were obtained for 52 aqueous mixtures; values of $C$ were obtained for 39 of these mixtures. The solutes considered include aliphatic alcohols, amines, amides, ketones, fatty acids, amino acids, and sugars. In some cases, experimental data were available from which the temperature dependence of the quantities $B$ and $C$ could also be determined. The effect of solute size on the coefficients $B$ and $C$ was investigated using the lattice theories of Flory, Huggins, and Guggenheim and McGlashan. More detailed conclusions concerning solute‐solute interactions can be drawn by using experimental activity data in conjunction with the McMillan‐Mayer theory of solutions. The minimum attractive contributions to the second virial coefficient of the osmotic pressure were estimated for most of the systems studied in this paper. Pairwise attractions seem to increase with (1) increasing aliphatic chain length (from the temperature dependence of the coefficient $B$ it is also found that pairwise association due to aliphatic chains increases with increasing temperature); (2) an increase in the number of solute functional groups capable of participating in hydrogen‐bond formation; (3) increasing dipole moment of the solute molecule for a series of amino acids and peptides. The minimum attractive contributions to the third virial coefficient were determined and interpreted for 12 of the systems. Finally, the relative importance of pairwise versus triplet interactions in aqueous solution was investigated.