Wertheim association theory for ion pairing in electrolytes: effect of neutral clusters
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| Format: | Preprint |
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2025
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| _version_ | 1866912671830376448 |
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| author | Warren, Patrick B. Masters, Andrew J. |
| author_facet | Warren, Patrick B. Masters, Andrew J. |
| contents | We address the problem of the vapor-liquid phase transition in the restricted primitive model (RPM) using Wertheim's statistical associating fluid theory to capture the effects of ion pairing which dominate the low-temperature vapor phase. For this we employ a reference system in which ion-pairing is suppressed by a judicious modification of the interaction between unlike charges from 1/r to erf(kappa r)/r, where kappa is a state-dependent parameter chosen so that the Helmholtz free energy A is at a null point (dA/d(kappa) = 0). Unlike the original RPM, this reference fluid admits real solutions to the hypernetted-chain (HNC) closure of the Ornstein-Zernike equations over a wide range of densities and temperatures. In the present study, we go beyond previous work [M. Li, Ph.D. thesis, University of Manchester (2011)] to allow for isodesmic assembly of ion pairs into neutral clusters. We find this has the potential to improve significantly the agreement with the Monte-Carlo results for the RPM vapor phase boundary. We can also match recent results on anomalous underscreening in the RPM [Härtel et al., Phys. Rev. Lett. 130, 108202 (2023)] assuming that only the free ions contribute to the screening length. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2510_22856 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Wertheim association theory for ion pairing in electrolytes: effect of neutral clusters Warren, Patrick B. Masters, Andrew J. Soft Condensed Matter Statistical Mechanics We address the problem of the vapor-liquid phase transition in the restricted primitive model (RPM) using Wertheim's statistical associating fluid theory to capture the effects of ion pairing which dominate the low-temperature vapor phase. For this we employ a reference system in which ion-pairing is suppressed by a judicious modification of the interaction between unlike charges from 1/r to erf(kappa r)/r, where kappa is a state-dependent parameter chosen so that the Helmholtz free energy A is at a null point (dA/d(kappa) = 0). Unlike the original RPM, this reference fluid admits real solutions to the hypernetted-chain (HNC) closure of the Ornstein-Zernike equations over a wide range of densities and temperatures. In the present study, we go beyond previous work [M. Li, Ph.D. thesis, University of Manchester (2011)] to allow for isodesmic assembly of ion pairs into neutral clusters. We find this has the potential to improve significantly the agreement with the Monte-Carlo results for the RPM vapor phase boundary. We can also match recent results on anomalous underscreening in the RPM [Härtel et al., Phys. Rev. Lett. 130, 108202 (2023)] assuming that only the free ions contribute to the screening length. |
| title | Wertheim association theory for ion pairing in electrolytes: effect of neutral clusters |
| topic | Soft Condensed Matter Statistical Mechanics |
| url | https://arxiv.org/abs/2510.22856 |