The Gaines–Thomas activity model for ion exchange compositions.

An instance of this class can be used to control how activities of the ion exchange species are calculated using the Gaines–Thomas activity model.

The activity of ion exchange species are calculated using the either equivalent fractions

\[\beta_{i}=-\dfrac{x_{i}z_{\mathrm{e},i}}{\sum_{j}x_{j}z_{\mathrm{e},j}},\]

where \(x_{i}\) and \(x_{j}\) are species mole fractions and \(z_{\mathrm{e},i}\)} and \(z_{\mathrm{e},j}\)} are exchanger equivalents (or cation charges) in ion exchange species, or the molar fractions

\[\beta^M_{i}=-\dfrac{x_{i}}{\sum_{j}x_{j}}.\]