- Ruhr-Universität Bochum

Properties of liquids at interfaces and in confinement are different from those in bulk and are relevant to a series of phenomena of natural and technological interest. Water interfaces are key to atmospheric and geochemical processes, while confined electrolyte solutions and confined ionic liquids (IL) play a crucial role in energy devices. A microscopic, atomistic understanding of solid/liquid interfaces is therefore crucial to advance both our fundamental understating of natural phenomena, as well as to improve technological devices. Molecular dynamics simulations, at atomistic resolution and at different level of theory, can provide a dynamical picture of interfaces “in operando” and help the microscopic interpretation of experiments.

In this short contribution I would like to highlight some recent work from our group on atomistic simulation of metal/liquid interfaces at the nanoscale. Peculiar to these systems is the role of electronic polarization which needs to be accurately included in the simulations. We have recently developed a simple method to introduce metal polarization in atomistic simulations, where harmonically coupled core-shell charge pairs are combined to Lennard Jones potentials on each metal atom. Such approach proved accurate in reproducing image charge potentials, compatible with atomistic simulations packages and inexpensive, so it can be used on large scale simulation containing hundred thousands atoms [1]. We have used this approach to investigate IL confined between gold electrode and observed quite some interesting phenomena. For example, we have shown that although structural modifications affect the interfaces only for a couple of nanometers, dynamical properties are instead modified on much longer scale, and e.g. bulk diffusion is only recovered 10 nm away from the gold surface [2]. Simulations also permitted to compute interfacial free energy and to evaluate the impact of the metallic polarization on the shift of the freezing temperature in confinement. Extending the simulation to the non-equilibrium realm, we have also investigated the impact of shear flow on the same type of systems, finding that the portion of fluid closer to the metal surface behaves as a glassy solid, which extends for a few nanometres. An analysis of friction showed that, thanks to the strong interaction with the surface, the confined IL resists the “squeeze out” and remains in place at higher pressures [3]. We have also shown, for different electrolyte solutions, that the metal polarisation enhances the interfacial capacitance with differences between the cathode and anode, depending on the ion’s size and solvation shell structures [4].

To explicitly consider reactivity, a description which also includes the electronic structure is required, as in ab initio molecular dynamics simulations. Electronic structure-based simulations permit to describe chemisorption at interfaces, as well charge transfer, also as function of a variable interfacial potential. In our recent work [5] we used ab initio molecular dynamics simulations in combination with the charge unbalance method to obtain the atomistic structure of the Pt-water double layer in response to an applied potential, in realistic solution conditions. The simulations permitted to evaluate the interface capacitance and the absolute electrode potential for different values of the charge on the electrodes. We showed that the metal polarisation is responsible for interfacial charge transfer and oscillations, which modulate the water coverage at the surface and in turn the interface dipole. Our results pave the way to the development of realistic models for catalytic processes at the Pt-water interface..

References:
1 Geada I. L., et al., Nature Communications (2018), 9, 716.
2 Ntim S. and Sulpizi M., Physical Chemistry Chemical Physics (2020), 22 (19), 10786-10791.
3 Ntim S. and Sulpizi M., Physical Chemistry Chemical Physics (2021), 23, 24357-24364.
4 Ntim S. and Sulpizi M., Physical Chemistry Chemical Physics (2023), 25, 22619- 22625.
5 Khatib R., et al., Electrochimica Acta (2022), 391, 138875.

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