Solvation and Ion-Pairing Effects of Choline Acetate Electrolyte in Protic and Aprotic Solvents Studied by NMR Titrations

Choline-based electrolytes have been proposed as environmentally friendly and low-cost alternatives for secondary zinc air batteries. Choline acetate [Ch](+)[OAc](-) in protic (D2O) and aprotic (DMSO-d(6)) solvents has been studied by means of concentration-dependent H-1 NMR, viscosity, and density measurements. The viscosities have been calculated on the basis of the Jones-Dole equation and showed that the dominant contribution originates from short-range ion-solvent interactions. Site-specific association affinities were assigned from NMR chemical shift titrations. In DMSO-d(6), the hydroxyl group of choline was found to have the smallest dissociation constant followed by the methyl group of acetate. The corresponding Gibbs energies at low concentration were found to be in agreement with a solvent-separated ion pair (2SIP) configuration, whereas at concentrations above 300 mM, a solvent-shared ion pair (SIP) configuration was assigned. For [Ch](+)[OAc](-) in D2O, association effects were found to be weaker, attributed to the high dielectric constant of the solvent. On time scales on the order of 100 ms, NMR linewidth perturbations indicated a change in the local rotational dynamics of the ions, attributed to short-range cation-solvent interactions and not to solvent viscosity. At 184 mM, similar to 40 % of the cations in DMSO-d(6) and similar to 10 % in D2O were found to exhibit short-range interactions, as indicated by the linewidth perturbations. It was found that at about 300 mM, the ions in DMSO-d(6) exhibit a transition from free to collective translational dynamics on time scales on the order of 400 ms. In DMSO-d(6), both ions were found to be almost equally solvated, whereas in D2O solvation of acetate was stronger, as indicated by the obtained effective hydrodynamic radii. For [Ch](+)[OAc](-) in DMSO-d(6), the results suggest a solvent-shared ion association with weak H-bonding interactions for concentrations between 0.3-1 M. Overall, the extent of ion association in solvents such as DMSO is not expected to significantly limit charge transport and hinder the performance of choline-based electrolytes.