The structure of liquid N-methyl pyrrolidone probed by x-ray scattering and molecular simulations

New Insights into the Structure of Glycols and Derivatives: A Comparative X-Ray Diffraction, Raman and Molecular Dynamics Study of Ethane-1,2-Diol, 2-Methoxyethan-1-ol and 1,2-Dimethoxy Ethane

In this study, we report a detailed experimental and theoretical investigation of three glycol derivatives, namely ethane-1,2-diol, 2-methoxyethan-1-ol and 1,2-dimethoxy ethane. For the first time, the X-ray spectra of the latter two liquids was measured at room temperature, and they were compared with the newly measured spectrum of ethane-1,2-diol. The experimental diffraction patterns were interpreted very satisfactorily with molecular dynamics calculations, and suggest that in liquid ethane-1,2-diol most molecules are found in gauche conformation, with intramolecular hydrogen bonds between the two hydroxyl groups. Intramolecular H-bonds are established in the mono-alkylated diol, but the interaction is weaker. The EDXD study also evidences strong intermolecular hydrogen-bond interactions, with short O···O correlations in both systems, while longer methyl-methyl interactions are found in 1,2-dimethoxy ethane. X-ray studies are complemented by micro Raman investigations at room temperature and at 80 °C, that confirm the conformational analysis predicted by X-ray experiments and simulations.

Wettability of a Poly(vinylidene fluoride) Surface by a Pure Good Solvent and a Good Solvent/Nonsolvent Mixture: All-Atom Molecular Dynamics Study

This study investigated the wettability of poly(vinylidene fluoride) (PVDF) surfaces by a good pure solvent and a good solvent/nonsolvent mixture based on all-atom molecular dynamics (MD) simulations. In particular, droplets of pure N-methyl-2-pyrrolidone (NMP) and of mixed NMP/water molecules were brought into contact with both crystalline and amorphous PVDF surfaces. The contact angles of the macroscopic droplets on the crystalline surface were higher and those on the amorphous surface were lower than the experimental values. As the PVDF sheet surface is a mixture of crystalline and amorphous phases, the experimental contact angles being between those on crystalline and amorphous surfaces is reasonable. On the crystalline surface, the decrease in the contact angle with increasing NMP concentration in the droplets can be explained by the increase in the NMP density near the solid-liquid interface. On the amorphous surface, however, the contact angle is strongly affected by the swelling of PVDF by the mixed droplets at high NMP concentrations. The solvation free energy of PVDF in NMP is greater than that in water, suggesting that this may be a driving force of the swelling of the amorphous PVDF. Furthermore, when the Cassie equation for mixed crystalline and amorphous surfaces was assumed, the calculated contact angle corresponded well with the experimental value.

All-atom molecular dynamics study on the non-solvent induced phase separation: Thermodynamics of adding water to poly(vinylidene fluoride)/N-methyl-2-pyrrolidone solution

The change in the thermodynamics when adding water in poly(vinylidene fluoride) (PVDF)/N-methyl-2-pyrrolidone (NMP) solution is studied from all atom molecular dynamics (MD) simulations. This is done by estimating the free energy of mixing of PVDF/NMP solution with increasing volume fraction of water (ϕ_w) using an appropriately chosen thermodynamic cycle and the Bennett acceptance ratio method. The MD calculations predict the thermodynamic phase separation point of water/NMP/PVDF to be at ϕ_w = 0.08, in close agreement with the experimental cloud point measurement (ϕ_w = 0.05). Examining the enthalpic and entropic components of the free energy of mixing reveals that at low concentrations of water, the enthalpy term has the most significant contribution to the miscibility of the ternary system, whereas at higher concentrations of water, the entropy term dominates. Finally, the free energy of mixing was compared with the Flory-Huggins (FH) free energy of mixing by computing the concentration-dependent interaction parameters from MD simulations. The FH model inadequately predicted the miscibility of the PVDF solution, mainly due to its negligence of the excess entropy of mixing.

Liquid-Phase Exfoliation of Phosphorene: Design Rules from Molecular Dynamics Simulations

The liquid-phase exfoliation of phosphorene, the two-dimensional derivative of black phosphorus, in the solvents dimethyl sulfoxide (DMSO), dimethylformamide (DMF), isopropyl alcohol, N-methyl-2-pyrrolidone, and N-cyclohexyl-2-pyrrolidone is investigated using three molecular-scale "computer experiments". We modeled solvent-phosphorene interactions using an atomistic force field, based on ab initio calculations and lattice dynamics, that accurately reproduces experimental mechanical properties. We probed solvent molecule ordering at phosphorene/solvent interfaces and discovered that planar molecules such as N-methyl-2-pyrrolidone preferentially orient parallel to the interface. We subsequently measured the energy required to peel a single phosphorene monolayer from a stack of black phosphorus and analyzed the role of "wedges" of solvent molecules intercalating between phosphorene sheets in initiating exfoliation. The exfoliation efficacy of a solvent is enhanced when either molecular planarity "sharpens" this molecular wedge or strong phosphorene-solvent adhesion stabilizes the newly exposed phosphorene surfaces. Finally, we examined the colloidal stability of exfoliated flakes by simulating their aggregation and showed that dispersion is favored when the cohesive energy between the molecules in the solvent monolayer confined between the phosphorene sheets is high (as with DMSO) and is hindered when the adhesion between these molecules and phosphorene is strong; the molecular planarity in solvents like DMF enhances the cohesive energy. Our results are consistent with, and provide a molecular context for, experimental exfoliation studies of phosphorene and other layered solids, and our molecular insights into the significant role of solvent molecular geometry and ordering should complement prevalent solubility-parameter-based approaches in establishing design rules for effective nanomaterial exfoliation media.

Anions, the Reporters of Structure in Ionic Liquids

In this work we compare the role that different anions play in the structure function S(q) for a set of liquids with the same cation. It is well established that because of their amphiphilic nature and their often larger size, cations play a fundamental role in the structural landscape of ionic liquids. On the other hand, it is often atoms in the anions that display the largest X-ray form factors and therefore play a very significant role as reporters of structure in small- and wide-angle X-ray scattering (SAXS/WAXS)-type experiments. For a set of liquids with similar topological landscape, how does S(q) change when the anionic scattering is deemphasized? Also, how do we computationally recover the typical length scale of important and perhaps universal ionic liquid structural features such as charge alternation when these are experimentally inaccessible from S(q) because of interference cancellations? We answer these questions by studying three different tetrapentylammonium-based liquids with the I(-), PF6(-) and N(CN)2(-) anions.