Electrostatics in Concentrated Electrolytes
- Alpha Lee (University of Cambridge, United Kingdom)
- Benjamin Rotenberg (CNRS and Sorbonne Université, France)
- Paul A. Madden (Queen's College, Oxford, United Kingdom)
- Susan Perkin (University of Oxford, United Kingdom)
Concentrated electrolytes are ubiquitous in biology and industry – from ion channels to fuel cells and supercapacitors. Although the chemical physics of electrolyte solutions is often considered a solved problem, a series of recent experimental and computational results suggest that the conventional wisdom is fundamentally incorrect.
Recent surface force measurements have shown that the interaction between charged surfaces in a concentrated electrolyte is long-ranged  and the screening length increases with ion concentration . This is at odds with the intuition given by mean-field Poisson-Boltzmann theory and classic models of correlations in Coulomb fluids [3,4,5]; non-local  and concentration-dependent  dielectric models may shed light on this phenomenology. Moreover, the dynamics of ion transport in ionic liquids display signatures of slow, possibly glass-like, relaxation [8,9], problematizing the conventional continuum equations of ion transport (e.g. ). Both effects are significant for industrial applications because the electrolytes used in energy storage are typically concentrated. Although the experimental evidence on the structure and ion dynamics in concentrated electrolytes is relatively well-established, the microscopic picture underlying those phenomena is far from clear.
The conventional picture of ions near charged interface, the electrical double layer, has also been challenged by recent experiments and simulations. Molecular dynamics simulations show that the ionic liquid/metal interface displays a voltage-induced phase transition , which is corroborated by experimental studies (for a review, see ). This phase transition could cause hysteresis and energy loss in electrical double layer supercapacitors. In addition, the differential capacitance at the point of zero charge is experimentally shown to be a non-monotonic function of ion concentration for ionic liquid-solvent mixtures . Both phenomena cannot be explained by Poisson-Boltzmann or related theories. In fact, even classical density functional theories cannot adequately capture ion correlations parallel to the electrode , yet much simpler theories could predict the capacitance reasonably well . What is crucially missing from simulations and experimental studies is an assessment of whether the non-trivial interfacial behavior is generic for concentrated electrolytes, or specific to ionic liquids.
In fact, although many quantum ab initio studies have been done to probe the structure of ionic liquids (e.g. [16,17]), there are significant challenges in bridging the lengthscales - from isolated ion pairs to all atom force fields, coarse-grained force fields and effective continuum theories. Microscopic quantum calculations of ion pairs are known to correlate with macroscopic properties such as viscosity  and solubility of salts . In addition, all atom MD simulations revealed the existence of microphase-separated regions corresponding to packing of the alkyl chains on the cations [20-23]. However, those atomistic insights are seldom carried over to theory development. Moreover, a more general conceptual question arises as to whether ionic liquids are merely very concentrated electrolytes, or are the intricacies of the non-electrostatic interactions between ionic liquid ions the most dominant physics.
Our workshop will bring computational scientists and experimentalists together to consolidate known results, identify gaps in understanding, and formulate an action plan for the community.
Concentrated electrolytes are often viewed as “perturbations” to dilute electrolytes; in fact, many theories are constructed with this in mind. Recent experiments and simulations suggest that distinct phenomenologies are present in highly concentrated electrolytes, thus the conventional wisdom needs to be revisited.
Although ionic liquids have received considerable attention in recent years, our discussion will not focus on ionic liquids per se. Rather, we will take a broader view and discuss the extent to which the physics of ionic liquids, concentrated electrolyte solutions, molten salts etc. are similar, and how can we understand their differences.
Specifically, we have identified the following key topics to be addressed during the workshop:
- Nanostructure and microstructure in bulk electrolytes: to what extent do the non-Coulombic interionic interactions affect the macroscopic properties of electrolytes?
- Effect of specific ion-solvent interactions on electrolyte structure: what is the role of the solvent in determining the structure of bulk electrolytes?
- Properties of bulk electrolytes as solvents: how can we design electrolytes and ionic liquids to dissolve particular substrates?
- The electrostatic screening length in bulk electrolytes: what is the microscopic reason behind the anomalously large screening length measured experimentally?
- Dynamics of ion transport in concentrated electrolytes: what are the timescales and length-scales of ion transport in concentrated electrolytes?
- Concentrated electrolytes far-from-equilibrium: are there instabilities or coherent structures that arise when concentrated electrolytes are driven away from equilibrium via, for example, AC fields or electrochemical reactions?
- Structure of electrolytes near charged and polarised surfaces: how does ions arrange itself near electrified surfaces? To what extent do Coulomb correlations parallel to the surface modify the structure of the electrical double layer?
We do not propose to structure our workshop around specific applications of ionic materials (e.g. supercapacitors, ionic lubricants, polyelectrolyte actuators etc.) We believe complicated intricacies necessarily arise in each specific application, which obscure the more fundamental physics of concentrated electrolytes.
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