Hydroxide diffuses slower than hydronium in water because its solvated structure inhibits correlated proton transfer

Abstract

Proton transfer via hydronium and hydroxide ions in water is ubiquitous. It underlies acid-base chemistry, certain enzyme reactions, and even infection by the flu. Despite two centuries of investigation, the mechanism underlying why hydroxide diffuses slower than hydronium in water is still not well understood. Herein, we employ state-of-the-art density-functional-theory-based molecular dynamics-with corrections for non-local van der Waals interactions, and self-interaction in the electronic ground state-to model water and hydrated water ions. At this level of theory, we show that structural diffusion of hydronium preserves the previously recognized concerted behaviour. However, by contrast, proton transfer via hydroxide is less temporally correlated, due to a stabilized hypercoordination solvation structure that discourages proton transfer. Specifically, the latter exhibits non-planar geometry, which agrees with neutron-scattering results. Asymmetry in the temporal correlation of proton transfer leads to hydroxide diffusing slower than hydronium.

Complete steric exclusion of ions and proton transport through confined monolayer water

“…It is given by d p = (kT/eFC p )  410 -6 cm 2 s -1 where C p is the proton concentration in HCl or Nafion, and F is the Faraday constant 18 . This value is more than an order of magnitude smaller than d p  910 -5 cm 2 s -1 for bulk water 9,26 which is also obvious from the difference between the red and black curves in Fig. 2B, if one recalls that the conductivity of bulk HCl is  80% dominated by protons 8 .…”

“…3A). The highlyordered hydrogen bonding in 2D water should suppress rotation of water molecules with respect to 3D water, which is an essential but slowest step for proton transport, according to the Grotthuss mechanism 25,26 . This is in contrast to 1D water that exhibits a string-like hydrogen bonding, which was argued 5,11 to enhance proton transport ( Supplementary Fig.…”

Free energy of proton transfer at the water–TiO2 interface from ab initio deep potential molecular dynamics

“…We estimated the free energy difference between molecular and dissociated water on aqueous TiO 2 from enhanced sampling techniques. Due to the complex nature of proton transfer reactions in water, [60][61][62] we devised a two-step procedure to reconstruct the free energy surface of water dissociation on titania. First, we estimated the free energy barrier to transfer a proton from water to a surface O 2c atom using umbrella sampling.…”

Characterization of transport through polymers for fracking fluid treatment and organic acid concentration in extractive membrane bioreactors

“…Partitioning behaviour of ionizable organic compounds such as carboxylic acids have variable solubility within organic solvents and polymers depending on whether the pH is above or below the acid's pKa. The dissociated (ionic) form at a pH above the pKa has increased solubility in aqueous buffers due to ionic interactions, whereas the undissociated (organic form) below the pKa of the acid, has limited solubility in aqueous buffers and higher solvent/polymer affinity because the main interactions are not ionic but are due to functional group interactions with the polymer/solvent . By contrast, comparison of acetic acid [Fig.…”