A combined approach of molecular dynamics simulations, wide angle X-ray scattering experiments, and density measurements was employed to study the structural properties of N-methyl-2-pyrrolidone (NMP) + water mixtures over the whole concentration range. Remarkably, a very good agreement between computed and experimental densities and diffraction patterns was achieved, especially if the effect of the mixture composition on NMP charges is taken into account. Analysis of the intermolecular organization, as revealed by the radial and spatial distribution functions of relevant solvent atoms, nicely explained the density maximum observed experimentally.

N-Methyl-2-pyrrolidone (NMP) is a solvent with applications in different industrial fields. Although largely employed in aqueous mixtures, little is known on the structural and dynamic properties of this system. In order to improve the knowledge on NMP aqueous solutions, useful to the development of their applications, NMR spectroscopy, calorimetric titration, and puckering analysis of molecular dynamics (MD) simulations were employed in this work. Our calorimetric study evidenced the presence of strong interactions between NMP and water and revealed that, under comparable conditions, the solvation of NMP by water results in an interaction stronger than the solvation of water by NMP. Overall, the changes of (1)H and (13)C chemical shifts and 2D ROESY spectra upon dilution suggested a preferential location of water nearby the carbonyl group of NMP and the formation of hydrogen bonding between these two molecules. In parallel, observation of correlation times by (13)C NMR spectroscopy evidenced a different dynamic behavior moving from the NMP-rich region to the water-rich region, characterized by a maximum value at about 0.7 water mole fraction. MD simulations showed that the NMP conformation remains the same over the whole concentration range. Our results were discussed in terms of changes in the NMP assembling upon dilution.

A systematic study of a series of room-temperature ionic liquids, belonging to the ethylammonium alkanoate family (EAX), was carried out at 298.15 K and 0.1 MPa with the aim of investigating the effect of the anionic chain length on some thermophysical properties and their behaviour in water (W), over the whole mole fraction range. The determination of Gutmann acceptor numbers for the pure EAX by using 31 P NMR spectroscopy allowed us to obtain a quantitative measure of Lewis acidity. Experimental densities, q, were used to calculate molar volumes, V m , and excess molar volumes, V E . Complementary information was obtained by isothermal titration calorimetry that provided the values of the heat of mixing, H E , and the excess partial molar enthalpies of each component, H E 1 and H E 2 . The density values of pure EAX samples decrease as the alkyl chain length of the anion increases. Moderate negative V E and H E values were found for each EAX ? W system, indicating the presence of attracting interactions between the constituents.

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In this paper, small angle X-ray scattering has been used to study a series of ionic liquids, alkylammonium alkanoates ([N0 0 0 n][CmCO2]), with varying alkyl chain lengths in the cation and the anion. We investigate the behaviour and the structure of such ionic liquids in their neat state, and in binary mixtures with water. To the best of our knowledge, this is the first structural study dealing with the behaviour of propylammonium propanoate [N0 0 0 3][C2CO2], butylammonium propanoate [N0 0 0 4][C2CO2], propylammonium butanoate [N0 0 0 3][C3CO2] and butylammonium butanoate [N0 0 0 4][C3CO2] when mixed in water. Generally, in the ionic liquids containing alkyl chains on both the cation and the anion, the correlation distance and the resulting scattering peak, which signal intermediate range order, are affected equally by both of the chains. The interesting result obtained regarding these systems is that the shift of the pre- and principal peaks with the addition of water depends on the overall molar concentration of the chains and is generally cumulative. Although, for a given sum of cation and anion chain lengths, the shift is reliant on the cation-anion combination in the neat state, it is not the case in their mixtures with water. In some recent papers, it has been reported that with addition of water, the pre-peak position remains constant, but our results show a shift in both pre- and principal Q peaks.