Can we describe charged nanoparticles with electrolyte theories? Insight from mesoscopic simulation techniques

Abstract

Electrolyte theories enable to describe the structural and dynamical properties of simple electrolytes in solution, such as sodium chloride in water. Using these theories for aqueous solutions of charged nanoparticles is a straightforward route to extract their charge and size from experimental data. Nevertheless, for such strongly asymmetric electrolytes, the validity of the underlying approximations have never been properly challenged with exact simulation results. In the present work, well established mesoscopic numerical simulations are used to challenge the ability of advanced electrolyte theories to predict the electrical conductivity of suspensions of charged nanoparticles, in the salt-free case. The theories under investigation are based on the Debye-Fuoss-Onsager treatment of electrolyte transport. When the nanoparticles are small enough (about one nanometer large), the theoretical results agree remarkably well with the simulation ones, even in the high concentration regime (packing fraction of in nanoparticles larger than 3% ). Strikingly, for highly charged nanoparticles, the theory is able to capture the non-monotonic variation of the ratio of the electrical conductivity to its value at infinite dilution (ideal value) as a function of the concentration. However, the tested theories fail to describe the conductivity of suspensions containing larger nanoparticles (e.g. of diameter 4 nm). Finally, only small charged nanoparticles can be considered as ions, as far as electrolyte theories are concerned. arXiv:2001.11722v1 [cond-mat.soft] 31 Jan 2020

Confined Electrolytes Show Bulk Dynamics Modulated by Hydrodynamic Couplings with the Walls

“…In a previous article, we have shown that analytical theories of ionic transport can lead to predictions of bulk electrical conductivity in very good agreement with values computed by MPCD simulations, for simple 1−1 electrolytes up to several moles per liter. 28 The present article offers new perspectives in the use of such theories. Our results suggest that the nonideal ionic velocities in a charged slit pore under an electroosmotic flow may be recovered by the superposition of two independent effects: (1) the electrostatic and hydrodynamic corrections computed for bulk electrolytes, which only depend on the ionic concentration, and (2) the hydrodynamic influence of the walls, which only depends on the geometry of the pore.…”

“…More precisely, it is slightly lower, as the values in bulk are close to 0.91D°for bulk ions in a MPCD solvent. 28 For ions in the surface layers, the diffusion coefficients remain in a narrow range, between 0.58 and 0.68D°for all the surface charges and ionic concentrations. They are slightly decreased by an increase of the surface charge of the walls but almost unaffected by the added salt concentration.…”

“…In the low concentrations regime, the corrections of conductivity from the ideal behavior scale as the square root of concentration. This has been first derived from a theoretical point of view by Debye, Fuoss, Onsager, and Falkenhagen. − In a recent work, we used mesoscopic simulations to prove the relevance of modern versions of Fuoss–Onsager treatments ,,, for 1–1 bulk electrolytes (NaCl, KBr, KCl) up to 2 mol/L . These descriptions of electrolytes rely on the primitive model of electrolytes that assumes that the solvent is a continuous medium characterized by its dielectric constant and its viscocity, whereas ions are charged spheres.…”

“…19−21 In a recent work, we used mesoscopic simulations to prove the relevance of modern versions of Fuoss−Onsager treatments 8,23,26,27 for 1−1 bulk electrolytes (NaCl, KBr, KCl) up to 2 mol/L. 28 These descriptions of electrolytes rely on the primitive model of electrolytes that assumes that the solvent is a continuous medium characterized by its dielectric constant and its viscocity, whereas ions are charged spheres. Also, two main contributions are assumed to influence the dynamics of ions: electrostatic interactions (usually referred to as electrostatic relaxation forces) and hydrodynamic couplings.…”

“…In the present paper, we propose to quantify the effect of confinement of electrolytes on the electrical conductivity through the use of an efficient mesoscale simulation method, namely, multiparticle collision dynamics (MPCD). , MPCD allows one to reproduce hydrodynamic interactions in systems in which thermal fluctuations play a role. In our group, we have worked on the adaptation of MPCD to electrolyte solutions. ,− Another mesoscale method suited to the simulation of the dynamics of solutions of charged species is Brownian dynamics with hydrodynamic interactions (BD). − BD has some limitations that are overcome by MPCD. MPCD is moreover also well suited to the simulation of confined electrolytes.…”