Local Structure of DMF–Water Mixtures, as Seen from Computer Simulations and Voronoi Analysis

Abstract

Molecular dynamics simulations of mixtures of N,N-dimethylformamide (DMF) with water of various compositions, covering the entire composition range, are performed on the canonical (N,V,T) ensemble. The local structure of the mixtures is analyzed in terms of radial distribution functions and the contributions of the first five neighbors to them, various order parameters of the water molecules around each other, and properties of the Voronoi polyhedra of the molecules. The analyses lead to the following main conclusions. The two molecules are mixing with each other even on the molecular scale; however, small self-aggregates of both components persist even at their small mole fraction values. In particular, water–water H-bonds exist in the entire composition range, while water clusters larger than 3 and 2 molecules disappear above the DMF mole fraction values of about 0.7 and 0.9, respectively. The O atoms of the DMF molecules can well replace water O atoms in the hydrogen-bonding network. Further, the H-bonding structure is enhanced by the presence of the hydrophobic CH3 groups of the DMF molecules. On the other hand, the H-bonding network of the molecules gradually breaks down upon the addition of DMF to the system due to the lack of H-donating groups of the DMF molecules. Finally, in neat DMF, the molecules form weak, CH-donated H-bonds with each other; however, these H-bonds disappear upon the addition of water due to the increasing competition with the considerably stronger OH-donated H-bonds DMF can form with the water molecules.

Surface Properties of N,N-Dimethylformamide–Water Mixtures, As Seen from Computer Simulations

“…Not surprisingly, similar monolayer adsorption was found earlier in the aqueous solutions of other solutes of H-bonding ability that contain also at least one CH 3 group, such as methanol, 69 DMSO, 70 acetone, 71 and methylamine. 72 Apart from this enrichment of DMF in the surface layer, the two molecules are found, similarly to the bulk liquid phase, 3 to mix with each other even on the molecular scale. Thus, no marked self-association of the like molecules is observed even at the liquid surface.…”

“…The obtained distributions exhibit rather similar behavior to what has previously been seen in the bulk liquid phase of these systems. 3 Thus, the peak of the P(A) distribution shifts smoothly from the position characteristic to neat water to that of neat DMF with increasing x DMF , indicating that the two molecules mix with each other on the molecular scale even at the liquid surface. More importantly, the distributions obtained by disregarding one of the components follow very well the gamma distribution in almost every case.…”

“…This fact is also reflected in a rather low dielectric constant value of DMF of about 40. 4 However, adding water to DMF immediately turns on the H-bond formation, 3 which makes DMF−water mixtures highly non-ideal. 5,6 This nonideal behavior is in a clear contrast with that of aqueous mixtures of formamide, i.e., the double demethylated analogue of DMF, which is regarded to be nearly ideal.…”

“…This fact is also reflected in a rather low dielectric constant value of DMF of about 40 . However, adding water to DMF immediately turns on the H-bond formation, which makes DMF–water mixtures highly non-ideal. , This non-ideal behavior is in a clear contrast with that of aqueous mixtures of formamide, i.e., the double demethylated analogue of DMF, which is regarded to be nearly ideal. − Thus, the properties of aqueous DMF mixtures are governed by the delicate interplay of hydrophobic, dipolar, and H-bonding interactions. Furthermore, DMF is the smallest aprotic molecule that bears a peptide bond, enabling its aqueous solutions to be used as biomimetic models of hydrated proteins. − …”

“…3. DMF mole fraction in the first (red), second (green), third (magenta), and fourth (blue) molecular layer beneath the liquid surface, as a function of the DMF mole fraction in the bulk liquid phase.…”