Kirkwood–Buff’s Theory: Changes of the Solutes Chemical Potentials with the Concentrations at Constant Pressure and Temperature

Abstract

Kirkwood–Buff’s theory (1951) provides new relations between some thermodynamic quantities such as the chemical potential and the spatial pair correlation functions G _αβ also called Kirkwood–Buff integrals. It, especially, offers expressions linking the chemical potential changes of solution components and their concentrations, at constant pressure and temperature. These expressions are all the more interesting as this kind of solutions is the most often encountered.

Expressions stemming from this theory probably set up the best means to begin to grasp the physical significance of activities and of their coefficients. This point is tackled in the next three following chapters.

The chapter is devoted to the sole study, according to this theory, of binary, homogeneous and isotropic mixtures of nonelectrolytes. It is set up within the grand ensemble framework. The starting point of the reasoning leading to the theory is the setting up of two mathematical relations expressing the concentration fluctuations. One of these relations links the fluctuations to the Kirkwood–Buff integrals G _αβ , the other links the same fluctuations to the partial derivatives of the mean number of particles with respect to the different chemical potentials.