Appendix
mixtures.pl

Generally, concentrations of substances should not be expressed in mole fractions. Formula on excess Gibbs free energy of 1 mole of solution of solute B in solvent A, for EHL model, assuming pure substance state as standard state, is then:
GEm/(R·T) = xB·ln[QBA·xB/(xB+SBA·xA)]+RBA·xA·ln[(SBA·xA/(xB+SBA·xA)]-xB·ln(xB)-xA·ln(xA), where: R = gas constant; T = temperature (K); SBA = new parameter, expressing concentration of substances.
From foregoing formula on logarithm of solvent A activity coefficient may be obtained:
ln(gAA) = RBA·ln[(SBA·xA/(xB+SBA·xA)]+(RBA-SBA)·XB/(xB+SBA·xA)- ln(XA)
and formula on logarithm of solute B activity coefficient may be obtained:
ln(gBA) = ln(QBA)-ln(xB+SBA·xA)+(SBA-RBA)·XA/(xB+SBA·xA),
that is two-parameter equation on solvent A activity coefficient and three-parameter equation on solute B activity coefficient, leading to four parameters in the whole range of concentrations. The quantity of parameters may be reduced to two, for example advantaging of activity coefficient B in infinite dilution value.

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Lech Dabrowski 2012