Fluctuations in dilute binary supercritical mixtures

The density fluctuation and the fluctuations in the number of particles of a binary mixture are expressed in terms of measurable thermodynamic quantities and the obtained equations are used in the dilute range to compare the behaviors of the CO2 / naphthalene and CO2 / ethane mixtures. When a small amount of naphthalene is added to the supercritical CO2, the density fluctuation and those in the number of particles exhibit sharp increases, with increasing naphthalene concentration, compared to the corresponding ideal mixture. In contrast, the fluctuations in the mixture CO2 / ethane almost coincide to those of the corresponding ideal mixture. The opposite behaviors regarding the partial molar volume in dilute solutions of ethane in CO2 and CO2 in ethane, which have positive partial molar volumes for the solute, and naphthalene in CO2, which has a negative partial molar volume for the solute, are explained on the basis of general statistical thermodynamic equations. The degrees of nonideality are responsible for the different behaviors of the above mixtures. (C) 2002 Elsevier Science B.V. All rights reserved.