Frictional Dissipation Pathways Mediated by Hydrated Alkali Metal Ions

Abstract

Frictional energy dissipation between sliding solid surfaces in aqueous media may proceed by different pathways. Using a surface force balance (SFB), we have examined systematically how such dissipation is mediated by the series of hydrated cations M(+) = Li(+), Na(+), and K(+) that are trapped between two atomically smooth, negatively charged, mica surfaces sliding across the ionic solutions over many orders of magnitude loading. By working at local contact pressures up to ca. 30 MPa (∼300 atm), up to 2 orders of magnitude higher than earlier studies, we could show that the frictional dissipation at constant sliding velocity, represented by the coefficient of sliding friction μM+, decreased as μLi+ > μNa+ ≳ μK+. This result contrasts with the expectation (in conceptual analogy with the Hofmeister series) that the lubrication would improve with the extent of ionic hydration, since that would have led to the opposite μM+ sequence. It suggests, rather, that frictional forces, even in such simple systems, can be dominated by rate-activated pathways where the size of the hydration shell becomes a dissipative liability, rather than by the hydration-shell dissipation expected via the hydration lubrication mechanism.

Lubricating properties of single metal ions at interfaces

“…The ionic sequence found here for the effective friction coefficients contrasts with SFB findings. 31 This discrepancy could suggest that the two techniques probe different physical phenomena, due to the different sizes in the nano-confined areas they explore. The mesoscale order visible for K + ions in Fig.…”

“…37 Adding cations helps break the global symmetry of the confined system, leading to a faster relaxation. 31 We note that τ must be understood as an order of magnitude since it was not possible to explore different oscillation frequencies, ω.…”

“…Numerous studies 2,6,7,[12][13][14][15]17,[20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] have examined the behaviour of nano-confined aqueous solutions in various systems and under different circumstances. Experimentally, the two main approaches are based on the surface force apparatus (SFA) and atomic force microscopy (AFM) with each family of methods offering different modes of operation.…”

“…The SFA-related methods rely on a well-defined confinement geometry between atomically flat surfaces over a large area (typically several μm 2 ). 22,28,31,33,[38][39][40][41] The distance between the confining surfaces can be measured in an absolute manner using interferometry, and dynamical measurements of the viscoelastic properties of confined aqueous solutions are possible over a range of frequencies (typically 1-3 Hz) using surface force balances (SFB), 42 or SFA with resonance detection. 33,38,40,41 In contrast, AFM-based methods probe a small number of liquid molecules, typically located between a nanometre sharp tip and an atomically surface.…”

“…Recently, both techniques have been used to investigate the effects of dissolved metal ions on the behaviour of nano-confined water. 31,32,39,42,[48][49][50] SFB studies indicate that ultrapure water remains mostly fluid even when confined to gaps <3.5 nm, resulting in a 3-5 fold increase in viscosity. 22,28,42,49 This result is supported by some theoretical studies, 51,52 and explained by a fast rotational and translational dynamics of water molecules under extreme confinement.…”