Speciation and hydration forces in sodium carbonate/bicarbonate aqueous solutions nanoconfined between mica sheets

TL;DR

This study investigates how carbonate and bicarbonate ions influence electrostatic and hydration forces in nanoconfined aqueous solutions between mica sheets, revealing altered surface charge, screening length, and hydration features due to confinement effects.

Contribution

It provides experimental force measurements and a modified Poisson-Boltzmann model to understand carbonate chemistry and hydration forces in nanoscale confinement, an area less explored compared to bulk solutions.

Findings

Bicarbonate increases surface negative charge and repulsive pressure.

Screening length is shorter for Na2CO3 than NaHCO3 at same concentration.

Hydration forces with characteristic sizes are observed below 2 nm.

Abstract

The equilibrium between hydrated and hydrolysed forms of CO2 in water is central to a multitude of processes in geology, oceanography and biology. Chemistry of the carbonate system is well understood in bulk solution, however processes such as mineral weathering and biomineralisation frequently occur in nano-confined spaces where carbonate chemistry is less explored. For confined systems, the speciation equilibria are expected to tilt due to surface reactivity, electric fields and reduced configurational entropy. In this discussion paper we provide measurements of interaction force between negatively charged aluminosilicate (mica) sheets across aqueous carbonate/bicarbonate solutions confined to nanoscale films in equilibrium with a reservoir of the solution. By fitting the measurements to a Poisson-Boltzmann equation modified to account for charge regulation at the bounding walls, we discuss features of the bicarbonate speciation in confinement. We find that (i) the presence of bicarbonate in the bulk reservoir causes a repulsive excess pressure in the slit compared to pH-neutral salt solutions at the same concentration, arising from a higher (negative) effective charge on the mica surfaces; (ii) the electrostatic screening length is lower for solutions of Na2CO3 compared to NaHCO3 at the same bulk concentration, due to a shift in the speciation equilibria with pH and in accordance with Debye-H\"uckel theory; (iii) hydration forces are observed at distances below 2 nm with features of size 0.1 nm and 0.3 nm; this was reproducible across the various bicarbonate electrolytes studied, and contrasts with hydration forces of uniform step size measured in pH-neutral electrolytes.