Environmentally ubiquitous, nanopores range from pores in subsurface rocks to biological channels in transmembrane proteins. Engineered nanopores are used in catalysts, separations, sensors, and fuel cells, taking advantage of their size exclusion and ion selectivity. To predict and manage such subsurface processes as geologic CO<sub>2</sub> sequestration, oil and gas recovery, geothermal operations, and radionuclide management, we need a thorough understanding of ion behaviors, their transport from bulk solutions into nanopores, and their interactions with the local surface functional groups in nanopores. Previous studies have reported that physical and chemical properties in nanopores (e.g., the melting point of chemicals and water, ion coordination chemistry, and ion affinity) differ from those in the bulk. However, our understanding of the most fundamental water chemistry parameter, pH, in nanopores is still in its infancy.  In this talk, using surface-enhanced Raman scattering spectroscopy and a Poisson–Boltzmann numerical model, I will show how pH in nanopores is different from that in the bulk. To control the bulk pH, we utilized multiple buffers (phosphate, borate, Tris, and 2-amino-2-methyl-1,3-propanediol). In nanopores, we found that anion concentrations are higher than cation concentrations, attracting more protons. This effect is intensified at higher salinity, where it can surpass buffer capacities. This finding will refine our understanding of the aqueous chemistry in nanopores in complex geochemical environments. Knowing the exact pH can help us to predict the exact speciation of chemicals in nanopores in sediments, membranes, catalysts, and energy storage materials.

How does pH in nanopores differ from the bulk pH in high salinity water?

ACS Fall 2021

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