Ion and solvent dynamics in aqueous tetramethylammonium chloride solutions

Abstract

It has been demonstrated using isotopic substitution that the distinct inelastic and quasielastic incoherent neutron scattering spectra from N(CH,)i ions and the water in liquid and frozen solutions of tetramethylamm,onium chloride can be identified. The salt is weakly 'structure-making', as may be seen from the small changes in water spectrum with concentration. The intraionic torsional modes persist in solution. Their response and that of the highly overdamped but persistent whole-ion librations as well as the translational and rotational diffusion constants allow this tendency to be quantified The observed effects show that this ion is a useful probe for 'structure-making' conditions, e.g. at charged interfaces.

Ion-Specific Confined Water Dynamics in Convex Nanopores of Gemini Surfactant Lyotropic Liquid Crystals

“…In LLCs at w 0 = 15, we observe ⟨ D H 2 O,K + ⟩ > ⟨ D H 2 O,Na + ⟩ > ⟨ D H 2 O,NMe4 + ⟩ (Table ). This counterion-dependent trend is exactly that reported for bulk electrolyte solutions, , supporting the idea that both counterion–water attractions and counterion-excluded volume influence confined water diffusion. Because the difference in excluded volume between Na + and K + is negligible, we reason that the higher charge density of Na + drives increased counterion–water attractions that slow water diffusion in Na-74d LLCs (Figure A).…”

“…At high hydrations of w 0 = 15 (∼3.7 m salt in H 2 O), D H2O,MCl measured by NMR in aqueous monovalent electrolyte solutions of NaCl, KCl, or NMe 4 Cl is similar to that of bulk water, presumably because of the relatively low ion concentration. However, D H 2 O,MCl within those aqueous electrolytes is still faster than that in water confined in H I LLCs with w 0 = 15 by factors of 3, 3, and 5, respectively. , The magnitude of this divergence increases with LLC pore diameter reduction: water diffusion in NMe 4 Cl at w 0 = 4.5 is 10 times faster than in the NMe 4 -74d G I LLC at w 0 = 6. While NaCl and KCl are insoluble in water at hydrations comparable to our QENS experiments, MD simulations of such solutions at w 0 = 6 confirm this trend …”

“…Brown et al demonstrated that QENS and 1 H NMR yield similar water diffusion coefficients for concentrated NMe 4 Cl solutions. 61 At high hydrations of w 0 = 15 (∼3.7 m salt in H 2 O), D H2O,MCl measured by NMR in aqueous monovalent electrolyte solutions of NaCl, KCl, or NMe 4 Cl is similar to that of bulk water, presumably because of the relatively low ion concentration. However, D H 2 O,MCl within those aqueous electrolytes is still faster than that in water confined in H I LLCs with w 0 = 15 by factors of 3, 3, and 5, respectively.…”

“…In agreement with previous experimental studies of confined water, , the magnitudes of the water self-diffusion coefficients in LLCs markedly differ from those observed in aqueous ionic solutions. Brown et al demonstrated that QENS and 1 H NMR yield similar water diffusion coefficients for concentrated NMe 4 Cl solutions . At high hydrations of w 0 = 15 (∼3.7 m salt in H 2 O), D H2O,MCl measured by NMR in aqueous monovalent electrolyte solutions of NaCl, KCl, or NMe 4 Cl is similar to that of bulk water, presumably because of the relatively low ion concentration.…”

“…49,50 excluded volume alone, one would expect water diffusivity in these aqueous electrolytes to decrease in the order Na + > K + > NMe 4 + . Experimental 61,62 and MD simulation investigations 34 have shown that water diffusivity in aqueous MCl solutions actually decreases in a different order: K + > Na + > NMe 4 + . Thus, the large excluded volume of the NMe 4 + ion outweighs its low charge density and cation−water attractions induce slower water diffusion for Na + than for the more polarizable K + .…”