Molecular insight on ultra-confined ionic transport in wetting films: the key role of friction

TL;DR

The paper addresses ionic transport in ultrathin wetting films on silica, where classical PB/Stokes descriptions fail under sub-nanometer confinement. It uses molecular dynamics with TIP4P/2005 water and Madrid/IFF force fields to compute ion distributions $c_+(z)$, electro-osmotic flows $v_x(z)$, and cation mobilities $μ_+(z)$ as a function of film thickness $h$. A key finding is that interfacial ion adsorption imposes ion-specific friction, elevating the electro-osmotic viscosity to $η_ ext{EO}$ values well above the bulk and leading to reduced conductance, especially for $K^+$, while the intrinsic solvent viscosity remains near bulk value; a 1D continuum model using these MD-informed profiles reproduces the conductance $G(h)$ without additional fitting. The study demonstrates that a simple corrected continuum framework remains predictive for nanoscale transport, clarifying the role of interfacial structure and offering guidance for nanofluidic device design.

Abstract

Nanofluidic transport is ubiquitous in natural systems from extra-cellular communication in biology to geological phenomena, and promotes the emergence of new technologies such as energy harvesting and water desalination. While experimental access to ultraconfined fluids has advanced rapidly, their behavior challenges conventional theoretical descriptions based on Poisson-Boltzmann theory or the Stokes equation whose possible extension remains an open question. In this work, we use molecular dynamics simulations to investigate ionic transport within wetting films of water confined on silica surfaces down to the sub-nanometer scale. We then analyze these results using a simple one-dimensional theoretical framework. Remarkably, we show that this model remains valid even at confinement close to the molecular scale. Our results reveal that the dynamic of ion plays a key role in ionic transports, through ion adsorption at the water-silica interface. Adsorbed cations do not participate in ionic conduction, but instead generate molecular-scale roughness and transmit additional frictional forces to the substrate. This mechanism produces an apparent viscosity increase in electrostatically driven flows, reaching up to four times the bulk value in the case of potassium. Our findings highlight the critical role of interfacial ion adsorption in nanoscale hydrodynamics and provide new insights for interpreting experiments and designing nanofluidic systems.

Figures (9)

• Figure 1: Numerical system showing two water films on opposite sides of a charged SiO$_2$ substrate. Periodic boundaries are set in all three dimensions. Counter-ions are represented as green spheres, Si atoms as yellow, O atoms as red, H atoms as white, and water molecules as a blue translucent envelope. The schematic on the right illustrates the simplified one-dimensional system used in the theoretical framework.
• Figure 2: Conductance $G$ as a function of the thickness of the water film $h$ for three different cations (Na$^+$, K$^+$ and Li$^+$). Solid lines correspond to theoretical fits from a simple continuum model proposed in Ref. allemand_2023_cond.
• Figure 3: Typical cation concentration profiles as a function of the vertical position $z$ in the wetting film, for Na$^+$ (circles) and K$^+$ (triangles) cations with $h = \qty{1.73}{\nano\meter}$. a) amorphous SiO$_2$; b) crystalline SiO$_2$. $z_\mathrm{sl}$ locates the solid-liquid interface and $h$ is the wetting film thickness (see Materials and Methods). Two layers are shown of thickness $\delta$ and $\delta_\mathrm e$, which are discussed in the main text. The colored solid lines correspond to an electrostatic model further discussed in the main text.
• Figure 4: Evolution of the ion profiles parameters with the film thickness. (a) Shift between solid-liquid and surface charge planes $\delta$; (b) Image charge repulsion layer $\delta_\mathrm e$; and (c) Effective surface charge density $\Sigma_\mathrm{PB}$ normalized by the nominal surface charge $\Sigma$.
• Figure 5: (a) Typical electro-osmotic velocity profile $v_x(z)$ for Na$^+$ at $E_x = \qty{34}{\milli\volt\per\angstrom}$ on an amorphous SiO$_2$ substrate. The colored solid line corresponds to the theoretical profile. (b) Typical pressure induced velocity profile $v_x(z)$ for K$^+$ (green data) and Li$^+$ (light blue data cations) for an applied force per particle $|f| = \qty{8.0}{\calorie\per\angstrom\per\mole}$ on an amorphous SiO$_2$ substrate. The velocity profile for Na$^+$ is similar to the Li$^+$ one. These profiles are extracted from a water film of thickness $h = \qty{1.7}{\nano\meter}$. The colored solid lines correspond to hydrodynamic model curves (see main text). In both graphs, the black solid vertical lines correspond to the positions $z=z_\text{sl}$ and $z=z_\text{sl}+h$. The dotted black lines correspond to the position of the no-shear plane $z=z_\text{sl}+\delta_\text{s}^i$, with $i=\varnothing,$P (more details in the text below) and the dotted blue vertical line is the position $z=z_\text{sl}+h-\delta_\text{e}$.