Microstructures of negative and positive azeotropes

Abstract

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Azeotropes famously impose fundamental restrictions on distillation processes, yet their special thermodynamic properties make them highly desirable for a diverse range of industrial and technological applications. Using neutron diffraction, we investigate the structures of two prototypical azeotropes, the negative acetone-chloroform and the positive benzenemethanol azeotrope. C-H···O hydrogen bonding is the dominating interaction in the negative azeotrope but C-Cl···O halogen bonding contributes as well. Hydrogen-bonded chains of methanol molecules, which are on average longer than in pure methanol, are the defining structural feature of the positive azeotrope illustrating the fundamentally different local mixing in the two kinds of azeotropes. The emerging trend for both azeotropes is that the more volatile components experience the more pronounced structural changes in their local environments as the azeotropes form. The mixing of the acetonechloroform azeotrope is essentially random above 20 Å where the running Kirkwood-Buff integrals of our structural model converge closely to the ones expected from thermodynamic data. The benzene-methanol azeotrope on the other hand displays extended methanol-rich regions and consequently, the running Kirkwood-Buff integrals oscillate up to at least 60 Å. Our study provides first insights into the microstructures of azeotropes and a direct link with their thermodynamic properties. Ultimately, this will provide a route for creating tailored molecular environments in azeotropes to improve and fine-tune their performances.

On the structure of prilocaine in aqueous and amphiphilic solutions

“…Neutron diffraction with isotopic substitution (NDIS) is a technique by which the structure of hydrogen containing liquids and solutions can be measured directly. 10,15,[17][18][19][20][21][22][23][24][25][26][27][28][29] The use of neutrons as a probe for hydrogen containing solutions exploits the different neutron scattering intensities for H versus D. 30 Neutron diffraction measurements give the average structure in solution in reciprocal space as a structure factor F(Q), where…”

“…Empirical potential structure refinement is a Monte Carlo based simulation specifically designed to model the structure of liquids and other disordered materials. 32 EPSR is a technique which has been used in combination with NDIS measurements to determine the local structure of a variety of molecules in solution, 10,15,[20][21][22][23][24][26][27][28]33 and is unique as it uses a set of measured diffraction data as a constraint for resultant computational model. By beginning with a set of starting potentials, Lennard-Jones and appropriate charges in the present work, these potentials are iteratively refined until the molecular structure is consistent with the measured F(Q) data.…”

On the structure of an aqueous propylene glycol solution

“…EPSR has been specifically designed in order to produce a computational model constrained to fit the experimental neutron diffraction data. 34,35 The use of EPSR to analyse neutron diffraction data has been frequently exploited to study the solution properties of a range of organic liquids, 11,12,14,[36][37][38][39][40][41][42][43][44][45][46] biological and drug molecules [47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62] and ionic species. [63][64][65][66] A set of reference potentials are used to start the EPSR process and these potentials are subsequently modified to provide a fit to the neutron diffraction data through a reverse Monte Carlo process.…”

Local structure and orientational ordering in liquid bromoform

“…[18] In the chloroform acetone azeotrope, the polar aggregates were found to be disrupted to some extent by the presence of acetone. [19] Liquid CHBr3 has received much less attention so far. [1] Pothoczki et al…”