  Recently an improved long-range model for electrolyte solutions was developed
that is applicable from infinite dilution to pure salt. This paper tests this
claim for the first time applying it to the calculation of liquid-liquid
equilibria for mixtures of different ionic liquids (ILs) and water. The
conventional Pitzer-Debye-H\"uckel (PDH) equation is compared to two of its
new, thermodynamically consistent extensions. Both development stages, the
extended PDH term and the modified-extended PDH, account for concentration
dependent mixture properties instead of using solvent properties. The latter
one additionally introduces a modified parameter of closest approach which
improves the overall performance of the model for high electrolyte
concentrations in systems with variable or low permittivities. To account for
the short-range interactions, these long-range models are coupled with the
UNIversal QUAsi-Chemical (UNIQUAC) model. Three modeling strategies were tested
for the short-range contribution. First, the UNIQUAC parameters were adjusted
to each system individually, then the binary interaction parameters were the
same for each binary interaction type for all the systems and lastly a linear
function of the carbon number was used where possible. For all systems and all
modeling strategies tested, the predictive performance increased from PDH to
E-PDH and then to ME-PDH. Overall, an introduction of concentration dependent
properties and the modification added to ME-PDH enhanced modeling performance
when describing these systems, showing the general applicability of this novel
long-range term.
