Molecular dynamics study of nanoscale organization and hydrogen bonding in binary mixtures of butylammonium nitrate...

Molecular dynamics simulations are utilized here to explore the nanoscale morphology and the nature of hydrogen bonding in the equimolar mixtures of butylammonium nitrate protic ionic liquid with ethanol, propanol, and butanol. The X-ray scattering experimental study of Greaves et al. [Phys. Chem. Chem. Phys. 13, 13 501 (2011)] has evidenced that alkylammonium nitrate plus alcohol mixtures possess nanoscale structural order which becomes more pronounced as the chain length of the alcohol increases. Our analysis carried out using simulated total and partial X-ray scattering structure functions quantifies the basis of these observations. The partial structure functions highlight the off-phase density correlations of alcohol with both cation and anion in the low-q region. We demonstrate that the chain lengthening of alcohols offers significant variation in the structuring of the polar and apolar moieties in the mixtures. The inspection based on radial distribution functions manifests the non-linear hydrogen bonds of cations with nitrate anions as well as alcohol molecules. The alcohol's hydroxyl group prefers to form linear hydrogen bonds with anions and with other alcohol molecules. Incremented chain length of alcohol improves the extent of hydrogen bonding but does not alter their geometry. Spatial distribution functions delineate similar preferences. It shows stronger directional preferences of the hydroxyl group of alcohols than cation in the vicinity of an anion. Enhanced pair correlations associated with the terminal methyl carbons suggest aggregation of butanol chains in apolar domains. Triplet correlation functions (TCFs) are also used to evaluate the orientational preferences of the present polar moieties in the mixtures. Information based on TCFs for distribution of polar head group of cations and anions unveils the dominance of equilateral configurations over the less frequent isosceles configurations in all the three mixtures.