How Electrostatics Influences Hydrodynamic Boundary Conditions: Poiseuille and Electro-osmostic Flows in Clay Nanopores.

Abstract

We report on a molecular simulation study of the origin of non-slip or slip hydrodynamic boundary conditions in clay nanopores, focussing on the role of electrostatics. We simulate hydrodynamic and electro-osmotic flows and consider both charged (montmorillonite) and uncharged (pyrophyllite) clays. We further use two commonly used force fields to analyze the effect of local interactions, in particular the effect of the polarity of the surface, in addition to the mere effect of the presence or absence of a net charge and counter-ions. For the 6 nm pore investigated here, the molecular velocity profile can be well described by continuum hydrodynamics only if (a) proper boundary conditions, with a slip or stagnation length determined from molecular simulation, are taken into account and (b) the ionic density profiles from MD simulations are used in the case of electro-osmotic flow, since the Poisson-Boltzmann equation fails to reproduce the ionic profiles, hence the force acting on the fluid. Among the considered force fields only CLAYFF predicts a hydrophobic pyrophyllite and hydrophilic montmorillonite, as expected from experimental behaviour. The non-slip or slip boundary conditions at clay surfaces strongly depend on electrostatic interactions of water molecules with the surface. The presence of a net charge results in an average electric field experienced by surface water molecules between the charged surface and the condensed layer counter-ions, which influences their orientation. The charge distribution inside the clay layer determines the polarity of the surface and hence the strength of hydrogen bonds donated by water molecules to surface oxygen atoms.

Stochastic rotation dynamics simulation of electro-osmosis

“…This suggests that Equation (1) may apply even in some cases where in-plane correlations are present, as was already observed e.g. in MD simulations [43,44]. In addition, this indicates that SRD predicts the same behaviour as the Stokes equation (note that the transient regime, not analysed here, may differ).…”

Pierre Turq, an inspirational scientist in charge and at interfaces

“…The same behaviour is observed in the presence of CO 2 , studied for applications to the sequestration of this greenhouse gas in saline aquifers [67]. The wettability of clay minerals depends on delicate balance of interactions with the surface [68] and has an important influence on the hydrodynamic behaviour (stick or slip) of the surface [69,70], as also observed, e.g.Êin silica nanopores [71]. Such a molecular scale understanding of the mineral/water interface also provided the basis of improved mesoscopic models with an implicit solvent [70,72,73], which can be used for the simulation on larger scales, as illustrated in Figure 4.…”

“…The wettability of clay minerals depends on delicate balance of interactions with the surface [68] and has an important influence on the hydrodynamic behaviour (stick or slip) of the surface [69,70], as also observed, e.g.Êin silica nanopores [71]. Such a molecular scale understanding of the mineral/water interface also provided the basis of improved mesoscopic models with an implicit solvent [70,72,73], which can be used for the simulation on larger scales, as illustrated in Figure 4. Such simulations will greatly benefit from the recent developments of imaging techniques, which should provide realistic descriptions of these complex materials, and of the modelling tools described in Section 3.…”

Molecular Dynamics Simulations of Carbon Dioxide, Methane, and Their Mixture in Montmorillonite Clay Hydrates

“…[2][3][4]34,[37][38][39][40][41][42][43][44][45][46] The swelling process typically exhibits two regimes: crystalline swelling at basal d-spacings < ∼ 19Å and osmotic swelling corresponding to basal d-spacings > ∼ 30Å. 47,48 The stable basal dspacing is about 10Å for dry clays (no water molecules in the interlayer) and increases upon contact with water to the range 11.5-12.5Å yielding a fully saturated monolayer (1W) water arrangement.…”