An Electrolyte Equation of State Based on openCOSMO-RS: From Aqueous Solutions to Non-Aqueous Systems
Abstract
Predictive COSMO-RS-based electrolyte models such as COSMO-RS-ES describe electrolyte solutions from quantum-chemical information, but treat the liquid as incompressible and give no access to densities or pressure effects. This work presents openCOSMO-RS-ES-Phi, to our knowledge the first open-source implementation of a predictive electrolyte equation of state based on openCOSMO-RS. It combines the COSMO-RS-based equation of state openCOSMO-RS-Phi with the ion interaction energies and the long-range term of COSMO-RS-ES, which is first refitted on the same simplified short range. With this refit, a dielectric decrement improves the mean ionic activity coefficients (MIAC), whereas a Born term counts the solvation of the ions a second time and is therefore left out. The equation of state then adds only volume, dispersion and temperature parameters of the ions, fitted to aqueous MIAC and densities. openCOSMO-RS-ES-Phi reproduces COSMO-RS-ES in dilute aqueous solutions, improves on it in concentrated ones and also yields solution densities. Extrapolating from aqueous systems, it predicts the MIAC in water + cosolvent mixtures as accurately as the fitted COSMO-RS-ES, as well as the MIAC and bubble points of a benchmark database of water-alcohol-salt systems. Salting-out of organic components is not described as accurately, and salts come out too soluble in organic solvents. Overall, the interaction model developed for COSMO-RS-ES carries over to a large extent to an equation of state, which makes openCOSMO-RS-ES-Phi a sound base model for further development, with ion transfer between solvents as a clear target for fitting to non-aqueous data.
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BibTeX
@misc{rausdzus2026electrolyte,
title = {{An Electrolyte Equation of State Based on openCOSMO-RS: From Aqueous Solutions to Non-Aqueous Systems}},
author = {Leander Rausdzus and Simon Müller},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.01686},
eprint = {2611.01686},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.01686},
note = {CC BY-NC-ND 4.0},
}
RIS
TY - UNPB
TI - An Electrolyte Equation of State Based on openCOSMO-RS: From Aqueous Solutions to Non-Aqueous Systems
AU - Leander Rausdzus
AU - Simon Müller
PY - 2026
DA - 2026/11/01
PB - arXiv
DO - 10.48550/arxiv.2611.01686
UR - https://arxiv.org/abs/2611.01686
AB - Predictive COSMO-RS-based electrolyte models such as COSMO-RS-ES describe electrolyte solutions from quantum-chemical information, but treat the liquid as incompressible and give no access to densities or pressure effects. This work presents openCOSMO-RS-ES-Phi, to our knowledge the first open-source implementation of a predictive electrolyte equation of state based on openCOSMO-RS. It combines the COSMO-RS-based equation of state openCOSMO-RS-Phi with the ion interaction energies and the long-range term of COSMO-RS-ES, which is first refitted on the same simplified short range. With this refit, a dielectric decrement improves the mean ionic activity coefficients (MIAC), whereas a Born term counts the solvation of the ions a second time and is therefore left out. The equation of state then adds only volume, dispersion and temperature parameters of the ions, fitted to aqueous MIAC and densities. openCOSMO-RS-ES-Phi reproduces COSMO-RS-ES in dilute aqueous solutions, improves on it in concentrated ones and also yields solution densities. Extrapolating from aqueous systems, it predicts the MIAC in water + cosolvent mixtures as accurately as the fitted COSMO-RS-ES, as well as the MIAC and bubble points of a benchmark database of water-alcohol-salt systems. Salting-out of organic components is not described as accurately, and salts come out too soluble in organic solvents. Overall, the interaction model developed for COSMO-RS-ES carries over to a large extent to an equation of state, which makes openCOSMO-RS-ES-Phi a sound base model for further development, with ion transfer between solvents as a clear target for fitting to non-aqueous data.
ER -