Structural properties of hydrogen-bond network in liquid formamide-water mixtures

Abstract

Molecular dynamics simulations have been performed to investigate the structural properties of the extended hydrogen-bond (H-bond) network of liquid formamide(FA)-water(W) mixtures over the whole concentration range. Instruments of complex network analysis have been utilized to identify and quantify the structural aspects of the H-bond networks involving FA-FA, W-W, and FA-W H-bonding interactions. These include the degree distributions, the distributions of network neighborhood numbers up to the second order, and the geodesic distributions. Collectively, they provide information regarding not only the local H-bonding environment of the species but also the H-bonding environment beyond their immediate neighbors. These analyses revealed that although the connectivity patterns within the H-bond network of the mixtures are altered significantly with the composition, the degree of connectivity of the networks are well maintained. Neighborhood analysis revealed that in the water rich solution nearly all FA molecules form the H-bonded clusters of the form FA⋯W⋯W and that the H-bonded cluster [W⋯W⋯FA⋯FA⋯W⋯W ] is the dominant structural motif involving FA dimers. According to the geodesic analysis, population of the chain-like H-bond pathways composed of one type of molecular species increases with the increasing mole fraction of the other type of species, until the mixture reaches the equimolar composition.