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

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-Hückel 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.

Figures (4)

• Figure 1: (a) Fractional concentrations of the three dissolved inorganic carbon (DIC) species present in bicarbonate-containing solutions as a function of electrolyte pH. Calculated using the equilibrium constants as in equation \ref{['eqn:bicarbonate']} as shown in the SI. (b) Schematic diagram of the SFB used to measure interactions between mica sheets across electrolyte solution. The mica-coated lens setup is immersed in bulk solution, so that confined system is in equilibrium with a reservoir.
• Figure 2: Interaction free energy per unit area, $W^{||} = F/2\pi R$, between mica sheets as a function of their separation distance (relative to closest separation), $D_0$, measured across 10 mM Na2CO3 aqueous solution at 294 K. Interactions measured on approach (blue curve) and retraction (red curve) are shown. The force profiles are reversible in the range down to 1 nm, then small steps and a hysteresis are seen (enlarged in the inset). At the smallest separations the force is again reversible, showing absence of adhesion.
• Figure 3: Interaction energy per unit area measured between mica sheets as a function of separation distance. (a) 1 mM NaHCO3 compared to 1 mM KClAgg2025. (b) 1 mM NaHCO3 compared to 10 mM NaHCO3 and 10 mM Na2CO3. In all cases the dashed lines are fits to equation \ref{['eqn:DLVO']}, with fitting parameters in Table \ref{['tab:1']}.
• Figure 4: High resolution measurement of the interaction as a function of separation below $\approx2$ nm, in (a)-(c) for each of the solutions discussed above, showing discontinuities attributed to structural changes in the film. In each case a step of 0.1 nm precedes a step of 0.3 nm.