Underscreening in ionic liquids: a first principles analysis

Rotenberg, Benjamin; Bernard, Olivier; Hansen, Jean‐Pierre

doi:10.1088/1361-648x/aaa3ac

Cited by 72 publications

(105 citation statements)

References 29 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…It must be stressed, however, that the Casimir force reported here is not directly related to the "underscreening" phenomenon discovered recently in experiments [13][14][15][16][17]. Note that the "first principles" theory of this phenomenon [18] is based on the same microscopic model and on the same theoretical tools employed in this paper. The present calculations of the Casimir force can be readily extended to asymmetric electrolytes (ions of different valences and diameters), as well as to models of ionic solutions with explicit solvent [18], within the same theoretical framework presented in Sections II and III.…”

Section: Discussionmentioning

confidence: 65%

Casimir force in dense confined electrolytes

et al. 2018

Self Cite

View full text Add to dashboard Cite

Understanding the force between charged surfaces immersed in an electrolyte solution is a classic problem in soft matter and liquid-state theory. Recent experiments showed that the force decays exponentially but the characteristic decay length in a concentrated electrolyte is significantly larger than what liquid-state theories predict based on analysing correlation functions in the bulk electrolyte. Inspired by the classical Casimir effect, we consider an alternative mechanism for force generation, namely the confinement of density fluctuations in the electrolyte by the walls.We show analytically within the random phase approximation, which assumes the ions to be point charges, that this fluctuation-induced force is attractive and also decays exponentially, albeit with a decay length that is half of the bulk correlation length. These predictions change dramatically when excluded volume effects are accounted for within the mean spherical approximation. At high ion concentrations the Casimir force is found to be exponentially damped oscillatory as a function of the distance between the confining surfaces. Our analysis does not resolve the riddle of the anomalously long screening length observed in experiments, but suggests that the Casimir force due to mode restriction in density fluctuations could be an hitherto under-appreciated source of surface-surface interaction.

show abstract

Section: Discussionmentioning

confidence: 65%

Casimir force in dense confined electrolytes

et al. 2018

Self Cite

View full text Add to dashboard Cite

show abstract

“…Recently, Gavish et al [42] and Rotenberg et al [43] suggested alternative theoretical explanations of the underscreening effect. Their arguments were based, respectively, on the phenomenological density-functional approach and the mean-spherical approximation.…”

Section: Nature Of Compacity and Overscreening Vs Underscreeningmentioning

confidence: 99%

Free and Bound States of Ions in Ionic Liquids, Conductivity, and Underscreening Paradox

et al. 2019

View full text Add to dashboard Cite

Using molecular dynamics simulations and theoretical analysis of velocity-autocorrelation functions, we study ion transport mechanisms in typical room-temperature ionic liquids. We show that ions may reside in two states: free and bound with an interstate exchange. We investigate quantitatively the exchange process and reveal new qualitative features of this process. To this end, we propose a dynamic criterion for free and bound ions based on the ion trajectory density and demonstrate that this criterion is consistent with a static one based on interionic distances. Analyzing the trajectories of individual cations and anions, we estimate the time that ions spend in bound "clustered states" and when they move quasifreely. Using this method, we evaluate the average portion of "free" ions as approximately 15%-25%, increasing with temperature in the range of 300-600 K. The ion diffusion coefficients and conductivities as a function of the temperature calculated from the velocity and electrical-current autocorrelation functions reproduce the reported experimental data very well. The experimental data for the direct-current conductivity (constant ionic current) is in good agreement with theoretical predictions of the Nernst-Einstein equation based on the concentrations and diffusion coefficients of free ions obtained in our simulations. In analogy with electronic semiconductors, we scrutinize an "ionic semiconductor" model for ionic liquids, with valence and conduction "bands" for ions separated by an energy gap. The obtained band gap for the ionic liquid is small, around 26 meV, allowing for easy interchange between the two dynamic states. Moreover, we discuss the underscreening paradox in the context of the amount of free charge carriers, showing that the obtained results do not yet approve its simplistic resolution.

show abstract

“…However, since this elegant picture is based on defects in the crystalline state, which occurs for aqueous NaCl solutions above 6 M [38], while the experiments show an increase in the screening length even around 1 M, the understanding of the fluid state is still incomplete. The scaling law has motivated several other recent theoretical works [39][40][41], but has not yet been fully understood. While these works rely on different assumptions and yield slightly different results, all of them introduce in a similar way some non-Coulomb short-range interactions between the ions (and possibly, the solvent).…”

Section: Introductionmentioning

confidence: 99%

Screening length for finite-size ions in concentrated electrolytes

et al. 2019

View full text Add to dashboard Cite

The classical Debye-Hückel (DH) theory clearly accounts for the origin of screening in electrolyte solutions and works rather well for dilute electrolyte solutions. While the Debye screening length decreases with the ion concentration and is independent of ion size, recent surface-force measurements imply that for concentrated solutions, the screening length exhibits an opposite trend; it increases with ion concentration and depends on the ionic size. The screening length is usually defined by the response of the electrolyte solution to a test charge, but can equivalently be derived from the charge-charge correlation function. By going beyond DH theory, we predict the effects of ion size on the charge-charge correlation function. A simple modification of the Coulomb interaction kernel to account for the excluded volume of neighboring ions yields a non-monotonic dependence of the screening length (correlation length) on the ionic concentration, as well as damped charge oscillations for high concentrations.

show abstract

Underscreening in ionic liquids: a first principles analysis

Cited by 72 publications

References 29 publications

Casimir force in dense confined electrolytes

Casimir force in dense confined electrolytes

Free and Bound States of Ions in Ionic Liquids, Conductivity, and Underscreening Paradox

Screening length for finite-size ions in concentrated electrolytes

Contact Info

Product

Resources

About