Periodic Reporting for period 2 - ELECTROLYTE (The Electrolytic Revolution: Harnessing Coulomb Physics and Soft Matter Chemistry to Design Electrolyte Materials)
Reporting period: 2023-04-01 to 2024-09-30
Electrolytes are salts which flow and so allow the conduction of charge. The simplest household electrolyte is table salt dissolved into water, as is so often used in the kitchen. There are many other commonplace electrolytes such as the oceans and the fluid inside our cells or in our blood. The common feature of all these well-known examples is that they include water as the solvent; the salt (e.g. sodium chloride, NaCl) is dissolved in the water in the sense that the individual ions break apart into positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). Each of the ions in these examples is surrounded by water molecules. However, there are many types of electrolyte where there is no water solvent: some salts are fluid without any solvent at all (these are called ionic liquids), some salts can be dissolved in other salts, or in other molecules such as polymers. These kind of electrolytes don’t usually occur naturally (although there are examples, ranging from exoplanets to volcanic magma) but they are important for technological applications such as battery electrolytes.
Owing to the charges on the ions, for example positive on the sodium and negative on the chloride in NaCl, interactions between the ions (repulsions and attractions) are quite strong and so are ‘felt’ by the other ions in the electrolyte over long distances. If the electrolyte is ‘dilute’, meaning that there is only a small amount of salt in a large quantity of water, the effect of these interactions reasonably well understood using mathematical theories. But most of the important salts, like those used in batteries or even the ones in the sea or in our bodies, are too concentrated for the known theories to apply well. In this project we are working towards a better understanding of many different sorts of electrolytes from a fundamental perspective – that is to say, we are looking for general principles, that can be explained using physics and maths, that describe the properties of the electrolytes. The results are of use to scientists working in a wide range of other disciplines, ranging from marine chemistry to battery electrolyte development.
Owing to the charges on the ions, for example positive on the sodium and negative on the chloride in NaCl, interactions between the ions (repulsions and attractions) are quite strong and so are ‘felt’ by the other ions in the electrolyte over long distances. If the electrolyte is ‘dilute’, meaning that there is only a small amount of salt in a large quantity of water, the effect of these interactions reasonably well understood using mathematical theories. But most of the important salts, like those used in batteries or even the ones in the sea or in our bodies, are too concentrated for the known theories to apply well. In this project we are working towards a better understanding of many different sorts of electrolytes from a fundamental perspective – that is to say, we are looking for general principles, that can be explained using physics and maths, that describe the properties of the electrolytes. The results are of use to scientists working in a wide range of other disciplines, ranging from marine chemistry to battery electrolyte development.
The project is divided into three Work Packages: (1) Fundamental studies on structure and interactions in electrolytes, (2) nanostructure, phase behaviour / phase separation in electrolyte mixtures, and (3) biological aspects including osmotic effects and bio-inspired materials. So far, we have made some progress on all three work packages. Regarding WP1, we have been studying the decay of electrostatic correlations in electrolytes, in particular electrolytes at high concentration. Experimental measurements of interaction potential of mean force across electrolytes reveals interactions over longer range than previous theoretical work anticipated. We are making careful studies of various electrolyte systems and aligning our theoretical interpretation to recent work by R. Kjellander (Gothenburg) which helps interpret the multiple decay modes observed. An example of the work is published in Faraday Discussions 2023 (first author C. Fung). Regarding WP2, we worked with an interesting class of concentrated electrolyte known as ‘deep eutectic solvents’, initially to map out phase behaviour and subsequently to make measurements of the dynamic properties of thin films of these electrolytes. We worked out how the hydrogen bonding in these mixtures determines the friction properties within and across the liquid. (see publications in Langmuir and Chemical Communications, first author Hayler.) Regarding WP3 we have investigated the effect of zwitterionic solutes on the interaction energy between particles in electrolyte solutions. Zwitterions are common throughout biological electrolytes, but they have not been taken into account in theoretical descriptions until now. We found several interesting features, such as enhancement of surface charge and modification of the dielectric properties, compared to electrolytes in water alone. Results of this work are published in Proc. Nat. Acad. Sci. 2023 (first author Hallett). Spanning all work packages, we have also written a comprehensive review article describing the “surface force balance” (SFB) , the apparatus we work with and develop for these measurements. This article is published in the Reports on Progress in Physics 2024.
I will discuss some progress beyond the pre-existing ways of thinking that we have developed during this project in two areas. (i) Until now, it was generally accepted that the decay of electrostatic interactions in electrolytes was determined by a single parameter, the “screening length”. Classical theories predict the screening length in dilute electrolytes, while more modern approaches from liquid state theory predicted how the screening length would deviate for more concentrated electrolytes. However, in our measurements we found that there is no such thing as a singe screening lenth in electroltes; instead, interactions are characterised (parameterised) by more than one decay length. Our experiments are contributing towards the new picture, revealing how these more subtle features can be understood. (ii) The second area of progress in thinking comes from our work with zwitterions in electrolyte solutions. It is long known that zwitterions contribute towards balancing the osmotic pressure inside cells, but their role in mediating interactions was not much considered. Instead, the classical ideas from colloid science which are typically used to explain interactions only consider the salt concentration and permittivity of the environment. Now, we are showing how the nature of small (overall uncharged) molecules can have a substantial impact on the electrostatic interactions: for example by adjusting the effective surface charge of a particle or surface.