The Solution Chemistry of Mixing States Probed via Fluctuations: a Direct Description of Inhomogeneity in Mixing

Abstract

Abstract Radial distribution functions are commonly used to represent the structures of solutions, which represent the probability of finding another atom in the shell at a distance r from the atom of interest. This method has been used to study the structures of many non-crystalline materials. However, the information gained in this method is microscopic, and is limited to the first or second nearest neighbors from the featured atom. The present author proposed a completely different method to describe the solution structure by expressing the inhomogeneity in distribution of atoms and molecules and in concentration as “density fluctuation” and “concentration fluctuation”, respectively; namely the structure of a solution is described in terms of the “mixing state” or “mixing scheme.” This paper introduces density fluctuation and concentration fluctuation, as well as Kirkwood-Buff Integrals. Fluctuations of solutions become more pronounced in the mesoscale region. The relationship with solution thermodynamics, which represents the macroscopic limit, is also discussed. The features and cautions of experiments to measure the fluctuations are described. Finally, as analytical examples, temperature and concentration dependences of mixing schemes for two solution systems with upper critical and lower critical solution temperatures are presented.