Primitive model electrolytes in the near and far field: Decay lengths from DFT and simulations

P. Cats*, R. Evans, A. Härtel, R. Van Roij

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

45 Citations (Scopus)


Inspired by recent experimental observations of anomalously large decay lengths in concentrated electrolytes, we revisit the Restricted Primitive Model (RPM) for an aqueous electrolyte. We investigate the asymptotic decay lengths of the one-body ionic density profiles for the RPM in contact with a planar electrode using classical Density Functional Theory (DFT) and compare these with the decay lengths of the corresponding two-body correlation functions in bulk systems, obtained in previous Integral Equation Theory (IET) studies. Extensive Molecular Dynamics (MD) simulations are employed to complement the DFT and IET predictions. Our DFT calculations incorporate electrostatic interactions between the ions using three different (existing) approaches: one is based on the simplest mean-field treatment of Coulomb interactions (MFC), while the other two employ the Mean Spherical Approximation (MSA). The MSAc invokes only the MSA bulk direct correlation function, whereas the MSAu also incorporates the MSA bulk internal energy. Although MSAu yields profiles that are in excellent agreement with MD simulations in the near field, in the far field, we observe that the decay lengths are consistent between IET, MSAc, and MD simulations, whereas those from MFC and MSAu deviate significantly. Using DFT, we calculated the solvation force, which relates directly to surface force experiments. We find that its decay length is neither qualitatively nor quantitatively close to the large decay lengths measured in experiments and conclude that the latter cannot be accounted for by the primitive model. The anomalously large decay lengths found in surface force measurements require an explanation that lies beyond primitive models.

Original languageEnglish
Article number124504
JournalJournal of Chemical Physics
Issue number12
Publication statusPublished - 28 Mar 2021

Bibliographical note

Funding Information:
We thank C. Holm for sharing a preprint of Ref. 68 with us and P. S. Salmon for sending Ref. 65. We are grateful to R. Roth for insightful comments and a referee who pointed us to the pertinent literature, which improved our analysis and discussion. This work is part of the D-ITP consortium, a program of the Netherlands Organisation for Scientific Research (NWO) that is funded by the Dutch Ministry of Education, Culture and Science (OCW). It forms part of the NWO programme “Data-driven science for smart and sustainable energy research” with Project No. 16DDS014. A.H. acknowledges support by the state of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through Grant No. INST 39/963-1 FUGG (bwForCluster NEMO) and through Project No. 406121234. R.E. was supported by the Leverhulme Trust through Grant No. EM 2020-029/4.

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© 2021 Author(s).


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