Correlated volume fluctuations in binary liquid mixtures from isothermal compressions at 298.15 K

New fluctuation-related properties are introduced based on the link between the mean-squared fluctuation of the volume and the isothermal compressibility of binary liquid mixtures by applying the methods of chemical thermodynamics. Some weaknesses have been pointed out in previous similar attempts by other authors. A different result is obtained when Pfeiffer and Heremans' model for correlated volume fluctuations is worked out in terms of the PTN ensemble (i.e. at constant pressure and temperature). An expression is derived for estimating adjusted correlation coefficients in pairwise volume fluctuations from the knowledge of excess molar isothermal compressions, and it is shown that this expression predicts bounding values for the latter property in real mixtures. Ultrasound speeds in 31 aqueous mixtures of 2-(ethylamino)ethanol are reported at 298.15 K. The adjusted correlation coefficients for volume fluctuations are graphically depicted at several compositions of two nonaqueous mixtures and eleven aqueous amphiphile mixtures at 298.15 K. This procedure is shown to be an effective means of monitoring interactions between unlike molecules in binary liquid mixtures. A surprisingly high limiting adjusted correlation coefficient is found for water in neat amino alcohols and alcohols possessing a tertiary functional group.