Concentration fluctuations in fluid mixtures

The B matrix of the Kirkwood-Buff theory of solutions describes number fluctuations of the component species. We show how, when the eigenvalues of the B matrix are widely different in magnitude, the description of the fluctuations can be simplified by restriction to a lower dimensional subspace of composition space. This simplification entails a loss of accuracy, which is argued to be often acceptable. The development is illustrated by application to water-urea and water -2,2,2 trifluoroethanol solutions. It is concluded that in these cases, the fluctuations of the components are coupled primarily by keeping the amount of void space in the solution almost constant.