A research team at the University of Leeds has quantified, at atomistic resolution, why potassium-based amino acid salts absorb CO2 faster than their sodium equivalents. The neutron diffraction study examined aqueous K and Na glycinate at 2.17 mol/kg H2O before and after CO2 loading, published 12 March 2026 in Nature Communications.

What did the team measure?

Researchers chose amino acid salts as model solvents for their lower environmental health risks and reduced regeneration energy relative to monoethanolamine, the industry standard. The group, led by Lorna Dougan with EPSRC funding (EP/P02288X/1) and a UKRI Frontier Research Fellowship (EP/X023524/1), prepared seven hydrogen/deuterium isotope variants of each salt.

The team loaded samples on the NIMROD and SANDALS diffractometers at the ISIS Neutron and Muon Source, then capped CO2 loading at 0.35 mol equivalents relative to glycinate (0.76 mol CO2/kg H2O). That ceiling kept bicarbonate formation out of the dataset. Refinement R factors reached 0.00026 and 0.00020 for unloaded K and Na glycinate, dropping to 0.00012 and 0.00011 for the CO2-loaded samples.

Why does K outperform Na?

Smaller ionic radius gives Na+ a higher charge density. Na+ therefore approaches the glycinate amine nitrogen more closely than K+. The measured pair-interaction energy runs -367 ± 1 kJ/mol for N–Na+ versus -318 ± 1 kJ/mol for N–K+.

That tighter grip creates a larger barrier for approaching CO2 molecules during the two-step capture mechanism. Coordination numbers around the amine group are equal at 0.14, so the kinetic penalty sits in the energy gap, not the count.

Amine-water interactions remain the dominant contributor by weighted energy. The amine-water pair interaction measures -14.05 ± 0.07 kJ/mol for K glycinate and -13.2 ± 0.2 kJ/mol for Na glycinate. Both fall short of the pure-water benchmark of -17.71 ± 0.08 kJ/mol, signalling disrupted hydrogen bonding near the amine.

What changes when CO2 loads?

CO2 loading shifts the dominant picture. Carbamate-water pair interactions run -29.4 ± 0.2 kJ/mol (K) and -28.3 ± 0.3 kJ/mol (Na), with coordination numbers of 10.12 and 10.42 respectively. Carbamate-cation interactions measure -420 ± 2 kJ/mol (K) and -487 ± 1 kJ/mol (Na).

Bulk water structure collapses after loading. Water-water hydrogen-bond energy drops to -16.04 ± 0.09 kJ/mol (K) and -16.17 ± 0.08 kJ/mol (Na). About 26.8% of water molecules fall in the "bulk" category for both salts, up from 24.2% (K) and 25.0% (Na) before loading. NMR confirms slower water diffusivity in Na glycinate.

Glycine carbamate-zwitterion pairing reverses the cation trend. Coordination numbers run 0.07 (K) and 0.14 (Na), but the K interaction energy reaches -164.9 ± 0.9 kJ/mol versus -106 ± 2 kJ/mol for Na.

What decides what happens next?

The authors deposited all raw diffraction data, refined structures, and analysis scripts in a public repository at the University of Leeds archive (10.5518/1691) and the ISIS facility (RB2410275, RB2220355).

C-Capture Ltd, the Harrogate-based carbon capture company that co-lists two of the authors, hosted vapour-liquid equilibrium measurements that validated the kinetic conclusions. The Leeds team flags blends, non-amine solvents, and higher-concentration parameter space as the next research targets.

The milestone is reproducibility. External groups can now re-run the EPSR refinement against the deposited isotope variants before committing capital to pilot-plant trials of amino-acid-based post-combustion capture.