Preferential interactions in aqueous solutions of urea and KCl Original Research Article

A quantitative characterization of the thermodynamic effects due to interactions of salt ions and urea in aqueous solution is needed for rigorous analyses of the effects of changing urea concentration on biopolymer processes in solutions that also contain salt. Therefore, we investigate preferential interactions in aqueous solutions containing KCl and urea by using vapor pressure osmometry (VPO) to measure osmolality as a function of the molality of urea (component 3) over the range 0.09⩽m3⩽1.65 m at two fixed molalities of KCl (component 2) (m2=0.212 and 0.427 m). With this experimental input and corresponding VPO measurements on solutions that contain only urea or KCl, we evaluate approximately the chemical potential derivative μ23=(∂μKCl/∂murea)T,P,mKCl=(∂μurea/∂mKCl)T,P,murea=μ32 and hence the preferential interaction coefficients Γμ3 and Γμ1,μ3. These results show that for water–KCl–urea solutions neither of these coefficients is determined primarily by contributions from thermodynamic nonideality to μ23. In aqueous solutions containing a biopolymer and a small solute, the contribution of ideal mixing entropy to μ23 is negligible in comparison with the experimental uncertainty, whereas in KCl–urea solutions the contribution due to ideal mixing entropy accounts for at least half of the magnitude of μ23. For comparison, we analyze literature data for NaCl–urea interactions and find again that nonideality makes a smaller contribution to μ23 than does ideal mixing entropy. In contrast, for aqueous solutions of urea and the protein bovine serum albumin, the experimentally determined contribution of nonideality to μ23 exceeds the contribution of ideal mixing by a factor of ∼2×102.