arXiv subjects

Pietro Ballone

Publications and source records attributed to Pietro Ballone.

4 recordsLinked to original sources

arXiv2025

Hidden long-range correlations in the ion distribution at the graphite / [bmim][NTf$_2$] electrified interface

A capacitor consisting of the [bmim][NTf$_2$] ionic liquid (IL) confined in between planar graphite electrodes has been investigated by molecular dynamics based on an all-atom, unpolarizable force field. Despite a few peculiarities due to the size and complexity of the ions, properties such as the density of ions throughout the capacitor, the screening of the surface charge on the electrodes by the IL and exact sum rules for the radial distribution functions of cations and anions generally comply with the results of time honored theories of the electrostatic double layer. This soothing regularity may conceal hidden correlations still compatible with the static screening rules, propagating far inside the IL the information on the state of charge of the capacitor. Evidence in this respect might have been detected by vibrational spectroscopy (see, for instance, Langmuir 2021, vol. 37, 5193-5201) showing changes in optical properties of the IL far from the charged electrodes. We show that grouping the [bmim]$^+$ and [NTf$_2$]$^-$ ions into instantaneous neutral pairs reveals an intriguing long range ordering of ions normal to the interface, driven by the capacitor state of charge. These correlations manifest themselves through the parallel orientation of the dipole moments of the neutral ion pairs. We speculate that this effect changes the average intensity of fluctuating electric fields deep in the IL, while average, static fields vanish in agreement with well established screening laws. This effect, which could change the spectroscopic properties of the IL, is present in the simulated [bmim][NTf$_2$] / graphite capacitor, but too small to be unambiguously confirmed by the present simulations with a safe margin over the error bar. The conceptual interest in these effects, however, will motivate further studies of the same or similar electrode / ionic liquid interfaces.

physics.chem-ph↗

arXiv2025

A kinetic model to simulate charge flow through an electrochemical half cell

A kinetic model of the electron transfer at the electrode / electrolyte solution interface is developed, implemented in a Monte Carlo framework, and applied to simulate this process in idealised systems consisting of the primitive model of electrolyte solutions limited by an impenetrable conducting surface. In the present implementation, a charged, spherical interface surrounding an equally spherical sample of electrolyte solution is introduced to model a single-electrode system, providing the computational analog to the conceptual half-cell picture that is widely used in electrochemistry. The electron transfer itself is described as a simple surface hopping process underlying a first order reaction corresponding to one of the coupled M/M$^+$ and X$^-$/X half reactions. Then, the electron transfer at the interface is combined with the self-diffusion of ions in the electrolyte solutions whose role is to supply reagents and disperse products, allowing the system to settle in a stationary non-equilibrium state. Simulations for the primitive model of electrolyte in contact with a charged impenetrable surface show that, after a brief transient, the samples sustain a steady current through the electrolyte solution. The results quantify the dependence of the current on: the overall charge of the electrode, the electrolyte concentration, the solvent viscosity and the kinetic parameter $k_e$ that represents the rate of the electron transfer for each ion in contact with the electrode. Since the simulated interface is very idealised, strategies to overcome the limitations of the present model are outlined and briefly discussed.

physics.chem-ph↗

arXiv2023

Random lasing in a solution of reflective colloidal particles: the effect of interfaces and inter-particle correlations

The propagation of light across 2D and 3D slabs of reflective colloidal particles in a fluid-like state has been investigated by simulation. The colloids are represented as hard spheres with and without an attractive square-well tail. Representative configurations of particles have been generated by Monte Carlo. The path of rays entering the slab normal to its planar surface has been determined by exact geometric scattering conditions, assuming that particles are macroscopic spheres fully reflective at the surface of their hard-core potential. The analysis of light paths provides the transmission and reflection coefficients, the mean-free path, the average length of transmitted and reflected paths, the distribution of scattering events across the slab, and the angular spread of the outcoming rays as a function of dimensionality and thermodynamic state. The results highlight the presence of a sizeable population of very long paths, which play an important role in random lasing from solutions of metal particles in an optically active fluid. The output power spectrum resulting from the stimulated emission amplification decays asymptotically as an inverse power law. The present study goes beyond the standard approach based on a random walk confined between two planar interfaces and parametrised in terms of the mean-free path and scattering matrix. Here, instead, the mean free path, the correlation among scattering events, and memory effects are not assumed a priori but emerge from the underlying statistical mechanics model of interacting particles. Moreover, the approach joins smoothly the ballistic regime of light propagation at low density with the diffusive regime at high density of scattering centres. These properties are exploited to investigate the effect of weak polydispersivity and of large density fluctuations at the critical point of the model with the attractive potential tail.

cond-mat.soft↗

arXiv2021

First-principles wavevector- and frequency-dependent exchange-correlation kernel for jellium at all densities

We propose a spatially and temporally nonlocal exchange-correlation (xc) kernel for the spin-unpolarized fluid phase of ground-state jellium, for use in time-dependent density functional and linear response calculations. The kernel is constructed to satisfy known properties of the exact xc kernel, to accurately describe the correlation energies of bulk jellium, and to satisfy frequency-moment sum rules at a wide range of bulk jellium densities, including those low densities that display strong correlation and symmetry breaking. These effects are easier to understand in the simple jellium model than in real systems. All exact constraints satisfied by the recent MCP07 kernel [A. Ruzsinszky, et al., Phys. Rev. B 101, 245135 (2020)] are maintained in the new revised MCP07 (rMCP07) kernel, while others are added. The revision $f_\mathrm{xc}^\mathrm{rMCP07}(q,ω)$ differs from MCP07 only for non-zero frequencies $ω$. Only at densities much lower than those of real bulk metals is the frequency dependence of the kernel important for the correlation energy of jellium. As the wavevector $q$ tends to zero, the kernel has a $-4πα(ω)/q^2$ divergence whose frequency-dependent ultranonlocality coefficient $α(ω)$ vanishes in jellium, and is predicted by rMCP07 to be extremely small for the real metals Al and Na.}

cond-mat.mtrl-sci↗