Counterion-Regulated Dynamics of Water Confined in Lyotropic Liquid Crystalline Morphologies

Abstract

The dynamics of confined water is of fundamental and long-standing interest. In technologically important forms of confinement, such as proton-exchange membranes, electrostatic interactions with the confining matrix and counterions play significant roles on the properties of water. There has been recent interest on the dynamics of water confined to the lyotropic liquid crystalline (LLC) morphologies of Gemini dicarboxylate surfactants. These systems are exciting because the nature of confinement, for example, size and curvature of channels and surface functionality is dictated by the chemistry of the self-assembling surfactant molecules. Quasielastic neutron scattering experiments have shown an interesting dependence of the water self-diffusion constant, D, on the identity (denoted α) of the counterion: at high hydration, the magnitude of the water self-diffusion constant is in the order D < D < D, where TMA, Na, and K refer to tetramethyl ammonium, sodium, and potassium counterions, respectively. This sequence is similar to what is seen in bulk electrolyte solutions. At low hydrations, however, the order of water self-diffusion is different, that is, D < D < D. In this work, we present molecular dynamics simulations for the dynamics of water in the LLC phases of dicarboxylate Gemini surfactants. The simulations reproduce the trends seen in experiments. From an analysis of the trajectories, we hypothesize that two competing factors play a role: the volume accessible to the water molecules and the correlations between the water and the counterion. The excluded volume effect is the largest with TMA, and the electrostatic correlation is the strongest with Na. The observed trend is a result of which of these two effects is dominant at a given water to surfactant ratio.

Consequences of Convex Nanopore Chemistry on Confined Water Dynamics

“…Adding another δ-function to the fitting routine did not significantly improve the fit quality. We note that a similar QENS data analysis approach for structurally homologous alkylcarboxylate LLCs was well-corroborated by MD simulations, indicating that strong coherent scattering does not prevent extraction of water self-diffusion coefficients. In view of these caveats, we emphasize that the present work focuses on qualitative trends in water dynamics.…”

“…The balance between these three effects leads to D H 2 O,K + > D H 2 O,Na + > D H 2 O,NMe 4 + at w 0 = 15. Observations in aqueous salt solutions suggest this trend to be hydration independent, however QENS analyses revealed the trend D H 2 O,K + > D H 2 O,NMe 4 + > D H 2 O,Na + at w 0 = 6 in these carboxylate LLCs. Detailed SAXS analyses of the LLCs demonstrated localization of the NMe 4 + ions at the aqueous channel center, which decreases their excluded volume and increases water diffusion compared to the more homogeneously distributed Na + ions.…”

“…+ at w 0 = 15. Observations in aqueous salt solutions 45 suggest this trend to be hydration independent, however QENS analyses revealed the trend D . Plots of 1/⟨τ β ⟩ versus q 2 (vertical gray line indicates the data cut off at q 2 = 1.44 Å −2 ), from which the average water self-diffusion coefficient ⟨D⟩ is calculated as the slope of a linear least-squares regression fit (see Table S3 for R 2 values).…”

“…Previous simulations 45 and QENS experiments 30 explored the roles of hydration level and surfactant counterion identity (Na + , K + , NMe 4 + ) on water dynamics in CO 2 M-74d carboxylate LLCs (Figure 6B). Three related factors were invoked to account for the observed counterion-specific water dynamics: (1) water−cation electrostatic attractions, (2) counterion excluded volume, and (3) water-headgroup attractions.…”

“…They observed somewhat slower water diffusion in sulfonate-lined pores than in their carboxylated counterparts, although their simulations did not consider differences in headgroup−counterion association and their consequences for counterion distribution within the pores. Subsequent MD simulations 45 and experiments 30 explored the roles of surfactant hydration and counterion identity (Na + , K + , NMe 4 + ) on water dynamics in the convex nanopores of gemini dicarboxylate LLCs. These studies established that counterion identity and their spatial distribution within an ionic nanochannel strongly impact water diffusion.…”