The Opposite Effect of Water and N-Methyl-2-Pyrrolidone Cosolvents on the Nanostructural Organization of Ethylammonium Butanoate Ionic Liquid A Small- and Wide-Angle X-Ray Scattering and Molecular Dynamics Simulations Study

Two series of mixtures of ethylammoniumbutanoate (EAB, [N_{0 0 0 2}][C₃CO₂]) in water and N-methyl-2-pyrrolidone (NMP) have been prepared at different molar fractions to assess the effect of these two polar solvents on the nanostructural order present in [N_{0 0 0 2}][C₃CO₂]. The small- and wide-angle X-ray scattering (SWAXS) pattern of the liquid in neat state shows a prepeak at Q = 0.513 Å^-1, which is associated with the aggregation of nonpolar alkyl chains of both cations and anions. Interestingly, the two solvents affect the nanostructure of [N_{0 0 0 2}][C₃CO₂] differently, though both are polar. In the case of water addition to the mixture, the prepeak shifts to lower Q values, while in NMP, it moves toward higher values. Also, the principal peaks move in opposite direction in both solvents. The underlying expansion (water) or contraction (NMP) of the solutions observed by the scattering experiments is discussed in terms of molecular dynamics (MD) simulations, which are in very good agreement with the observed patterns.