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Solvent-induced ion clusters generate long-ranged double-layer forces at high ionic strengths

Ribar, David LU orcid ; Woodward, Clifford E. and Forsman, Jan LU (2025) In Soft Matter 21(27). p.5562-5572
Abstract

Recent experimental results by the surface force apparatus (SFA) have identified a dramatic deviation from previously established theories of simple electrolytes. This deviation, referred to as anomalous underscreening, suggests that the range of electrostatic interactions increase upon a further addition of salt, beyond some threshold concentration (usually about 1 M). In this theoretical work, we explore an extension of the restricted primitive model (RPM) wherein a short-ranged pair potential of mean force (sPMF) is added to the usual Coulombic interactions so as to mimic changes of the hydration as two ions approach one another. The strength of this potential is adjusted so that the modified RPM saturates at a realistic... (More)

Recent experimental results by the surface force apparatus (SFA) have identified a dramatic deviation from previously established theories of simple electrolytes. This deviation, referred to as anomalous underscreening, suggests that the range of electrostatic interactions increase upon a further addition of salt, beyond some threshold concentration (usually about 1 M). In this theoretical work, we explore an extension of the restricted primitive model (RPM) wherein a short-ranged pair potential of mean force (sPMF) is added to the usual Coulombic interactions so as to mimic changes of the hydration as two ions approach one another. The strength of this potential is adjusted so that the modified RPM saturates at a realistic concentration level (within a range 4-7 M, typical to aqueous 1 : 1 salts). We utilise grand canonical simulations to establish surface forces predicted by the model and compare them directly with SFA data. We explore different sPMF models, which in all cases display significant clustering at concentrations above about 1 M. In these models, we find significant double-layer repulsion at separations that significantly exceed those expected from standard RPM predictions. We do not, however, observe an increase of the screening length with salt concentration, but rather that this screening length seemingly saturates at a (rather high) value. The simulated long-ranged interactions are shown to correlate with ion cluster formation, implicating the important role of accompanying cluster-cluster interactions. In particular, steric interactions between clusters (manifested in density-density correlations) are quite relevant in these systems.

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author
; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Soft Matter
volume
21
issue
27
pages
11 pages
publisher
Royal Society of Chemistry
external identifiers
  • pmid:40548438
  • scopus:105009057388
ISSN
1744-683X
DOI
10.1039/d5sm00027k
language
English
LU publication?
yes
additional info
Publisher Copyright: © 2025 The Royal Society of Chemistry.
id
0cbca55d-f197-480d-9fdd-9cf2cc4b2aa1
date added to LUP
2025-07-28 12:46:50
date last changed
2025-08-13 03:36:36
@article{0cbca55d-f197-480d-9fdd-9cf2cc4b2aa1,
  abstract     = {{<p>Recent experimental results by the surface force apparatus (SFA) have identified a dramatic deviation from previously established theories of simple electrolytes. This deviation, referred to as anomalous underscreening, suggests that the range of electrostatic interactions increase upon a further addition of salt, beyond some threshold concentration (usually about 1 M). In this theoretical work, we explore an extension of the restricted primitive model (RPM) wherein a short-ranged pair potential of mean force (sPMF) is added to the usual Coulombic interactions so as to mimic changes of the hydration as two ions approach one another. The strength of this potential is adjusted so that the modified RPM saturates at a realistic concentration level (within a range 4-7 M, typical to aqueous 1 : 1 salts). We utilise grand canonical simulations to establish surface forces predicted by the model and compare them directly with SFA data. We explore different sPMF models, which in all cases display significant clustering at concentrations above about 1 M. In these models, we find significant double-layer repulsion at separations that significantly exceed those expected from standard RPM predictions. We do not, however, observe an increase of the screening length with salt concentration, but rather that this screening length seemingly saturates at a (rather high) value. The simulated long-ranged interactions are shown to correlate with ion cluster formation, implicating the important role of accompanying cluster-cluster interactions. In particular, steric interactions between clusters (manifested in density-density correlations) are quite relevant in these systems.</p>}},
  author       = {{Ribar, David and Woodward, Clifford E. and Forsman, Jan}},
  issn         = {{1744-683X}},
  language     = {{eng}},
  month        = {{06}},
  number       = {{27}},
  pages        = {{5562--5572}},
  publisher    = {{Royal Society of Chemistry}},
  series       = {{Soft Matter}},
  title        = {{Solvent-induced ion clusters generate long-ranged double-layer forces at high ionic strengths}},
  url          = {{http://dx.doi.org/10.1039/d5sm00027k}},
  doi          = {{10.1039/d5sm00027k}},
  volume       = {{21}},
  year         = {{2025}},
}