On the micro-heterogeneous structure of neat and aqueous propylamine mixtures: A computer simulation study

Abstract

The micro-heterogeneous structure of neat and aqueous propylamine is examined through computer simulations. Neat propylamine is found to have a pre-peak in the nitrogen-nitrogen structure factor, due to the presence of branched chain-like aminogen clusters. Aqueous propylamine mixtures are found to be micro-segregated at all amine concentrations. Both the waterwater and the amine nitrogen-nitrogen structure factors show the caracteristic domain pre-peaks in their respective moderate to high contents. The amine cluster pre-peak, still visible at high amine content, disappears into the domain pre-peak at lower amine contents. Interestingly, water domains forms linears clusters for water concentrations below equimolar. We discuss the specificity the amine brings to the nature of the water clustering as compared with other type of solutes. In particular, we find that the Kirkwood-Buff integrals are quasi-ideal in the amine mole fraction range 0.3 < x < 1, which we interpret as being consistent with the linear water clusters observed in this range, and which act as individual supra-molecular entities. We conjecture that such cluster shapes are consistent with the existence of a lower critical solution temperature for this system as well as other aqueous amines.

Microscopic origin of the scattering pre-peak in aqueous propylamine mixtures: X-ray and neutron experiments versus simulations

“…Fig.2a and 2b indicate that water and propylamine form segregated domains, much like what has been reported in several of our earlier papers, for aqueous mono-ols 37 and other mix- tures 38,39 . For higher propylamine concentrations, we have previously reported that water tends to form chain-like aggregates 27 . In order to show this more specifically, we have explicitly shown separately water molecules (lower parts) and propylamine nitrogen groups (upper parts) for the cases of 50% and 80%.…”

“…One sees very clearly water chains, and less clearly the nitrogen dimers or trimer short chains. These snapshots would suggest that the pre-peak observed for x > 0.1 could originate from water chain clusters, much like in neat alcohols 10,11 , but also in neat propylamine 27 . But this conclusion does not apply to the 20% case, where it is clearly seen that water forms globular domains.…”

“…The computer simulation data used in this work has been previously reported by some of us 27 , and we briefly give some details. All simulations were performed with the program package Gromacs 28 , with the forcefield Gromos53a6 29 for propylamine and the SPC/e 30 water.…”

Universal features in lifetime distribution of clusters in hydrogen bonding liquids

“…However, the case of the propylamine (the upper most curve in purple) is very interesting. In previous studies [44], we have compared the clustering of neat 1propanol to that of 1-propylamine, and shown that the clustering of the amine group was not so important, both in size and Figure 9: Influence of alkyl tails on lifetime distribution for methanol (red), ethanol (green), propanol(blue), octanol (gold) and propylamine (purple). The black curve is for water and serves as a reference.…”

“…The present work aims at revealing surprising repeatability of this transient part across several H-bonding liquids. It is initially motivated by the fact that H-bonding liquids other than water, such as alcohols and amines, are known to form short chain-like clusters, both from scattering experiments [20,21,22,23,24,25,26,27,28,29,30,31,11,32], spectroscopy investigations [33,34,35,36,37] and computer simulations [38,39,40,41,42,43,44]. Exploring this transient regime, essentially in the sub 0.5 ps region, we have uncovered a universal dynamical behaviour common to all H-bonding liquids, which is unexpected in this time domain, where the differences in the molecular interactions play an important role.…”