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Deep Eutectic Solvents (DESs) are a novel class of solvents with potential industrial applications in separation processes, chemical reactions, metal recovery and metal finishing processes such as electrodeposition and electropolishing. Macroscopic physical properties such as viscosity, conductivity, eutectic composition and surface tension are already available for several DESs, but the microscopic transport properties for this class of compounds are not well understood and the literature lacks experimental data that could give a better insight into the understanding of such properties. This paper presents the first pulsed field gradient nuclear magnetic resonance (PFG-NMR) study of DESs. Several choline chloride based DESs were chosen as experimental samples, each of them with a different associated hydrogen bond donor. The molecular equilibrium self-diffusion coefficient of both the choline cation and hydrogen bond donor was probed using a standard stimulated echo PFG-NMR pulse sequence. It is shown that the increasing temperature leads to a weaker interaction between the choline cation and the correspondent hydrogen bond donor. The self-diffusion coefficients of the samples obey an Arrhenius law temperature-dependence, with values of self-diffusivity in the range of [10(-10)-10(-13) m(2) s(-1)]. In addition, the results also highlight that the molecular structure of the hydrogen bond donor can greatly affect the mobility of the whole system. While for ethaline, glyceline and reline the choline cation diffuses slower than the associated hydrogen bond donor, reflecting the trend of molecular size and molecular weight, the opposite behaviour is observed for maline, in which the hydrogen bond donor, i.e. malonic acid, diffuses slower than the choline cation, with self-diffusion coefficients values of the order of 10(-13) m(2) s(-1) at room temperature, which are remarkably low values for a liquid. This is believed to be due to the formation of extensive dimer chains between malonic acid molecules, which restricts the mobility of the whole system at low temperature (<30 °C), with malonic acid and choline chloride having almost identical diffusivity values. Diffusion and viscosity data were combined together to gain insights into the diffusion mechanism, which was found to be the same as for ionic liquids with discrete anions.

The liquid state structure of the ionic liquid, 1-ethyl-3-methylimidazolium acetate ([C(2)mim][OAc]), an excellent nonderivitizing solvent for cellulosic biomass, has been investigated at 323 K by molecular dynamics (MD) simulation and by neutron diffraction using the SANDALS diffractometer at ISIS to provide experimental differential neutron scattering cross sections from H/D isotopically substituted materials. Ion-ion radial distribution functions both calculated from MD and derived from the empirical potential structure refinement (EPSR) model to the experimental data show the alternating shell structure of anions around the cation, as anticipated. Spatial probability distributions reveal the main anion-to-cation features as in-plane interactions of anions with the three imidazolium ring hydrogens and cation-cation planar stacking above/below the imidazolium rings. Interestingly, the presence of the polarized hydrogen-bond acceptor (HBA) anion (acetate) leads to an increase in anion-anion tail-tail structuring within each anion shell, an indicator of the onset of hydrophobic regions within the anion regions of the liquid. MD simulations show the importance of scaling of the effective ionic charges in the basic simulation approach to accurately reproduce both the observed experimental neutron scattering cross sections and ion self-diffusion coefficients.

The pulsed-field gradient (PFG)-NMR technique has been applied to study molecular diffusion of organic liquids within mesoporous materials used in heterogeneous catalysis, in order to assess the effect of chemical functionalities on the effective self-diffusivity of the probe molecule within the pore space. True tortuosity values of the porous matrix can be calculated from the ratio of the unrestricted free self-diffusivity to the self-diffusivity within the pore space only when the small liquid-phase probe molecules do not have any chemical functionality that interacts within the solid phase (e.g., alkanes). The use of molecules with reactive chemical functionalities gives values heavily dependent on the physical and chemical interactions within the porous medium; hence, these values cannot be defined as tortuosity. Polyols showed an interesting behavior of enhanced rate of self-diffusion within the confined pore space, and this is attributed to the ability of the porous medium to disrupt the extensive intermolecular hydrogen bonding network of polyols.

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The effect of water on the catalytic oxidation of 1,4-butanediol in methanol over Au/TiO(2) has been investigated by catalytic reaction studies and NMR diffusion and relaxation studies. The addition of water to the dry catalytic system led to a decrease of both conversion and selectivity towards dimethyl succinate. Pulsed-field gradient (PFG)-NMR spectroscopy was used to assess the effect of water addition on the effective self-diffusivity of the reactant within the catalyst. NMR relaxation studies were also carried out to probe the strength of surface interaction of the reactant in the absence and presence of water. PFG-NMR studies revealed that the addition of water to the initial system, although increasing the dilution of the system, leads to a significant decrease of effective diffusion rate of the reactant within the catalyst. From T(1) and T(2) relaxation measurements it was possible to infer the strength of surface interaction of the reactant with the catalyst surface. The addition of water was found to inhibit the adsorption of the reactant over the catalyst surface, with the T(1)/T(2) ratio of 1,4-butanediol decreasing significantly when water was added. The results overall suggest that both the decrease of diffusion rate and adsorption strength of the reactant within the catalyst, due to water addition, limits the access of reactant molecules to the catalytic sites, which results in a decrease of reaction rate and conversion.

In this work, using H and F PFG NMR, we probe the effect of temperature, ion size/type and glucose dissolution on the rate of transport in 1-ethyl-3-methylimidazolium ([EMIM] )-based ionic liquids by measuring self-diffusion coefficients. Using such data, we are able to establish the degree of ion pairing and quantify the extent of ionic aggregation during diffusion. For the neat 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) a strong degree of ion pairing is observed. The substitution of the [OAc] anion with the bis{(trifluoromethyl)sulfonyl}imide ([TFSI] ) anion reduces the pairing between the ions, which is attributed to a lower electric charge density on the [TFSI] anion, hence a weaker electric interaction with the [EMIM] cation. The effect of glucose, important for applications of ionic liquids as extracting media, on the strongly paired [EMIM][OAc] sample was also investigated and it is observed that the carbohydrate decreases the degree of ion pairing, which is attributed to the ability of glucose to disrupt inter-ionic interactions by forming hydrogen bonding, particularly with the [OAc] anion. Calculations of aggregation number from diffusion data show that the [OAc] anion diffuses as a part of larger aggregates compared to the [EMIM] cation. The results and analysis presented here show the usefulness of PFG NMR in studies of ionic liquids, giving new insights into ion pairing and aggregation and the factors affecting these parameters.

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