Theoretical and Experimental Study of the Excess Thermodynamic Properties of Highly Nonideal Liquid Mixtures of Butanol Isomers + DBE

Leon De Villiers Engelbrecht,Riccardo Farris,Alessandra Piras,Silvia Porcedda,Aatto Laaksonen,Tudor Vasiliu,Flaminia Cesare Marincola,Francesca Mocci,Monica Demurtas

doi:10.1021/acs.jpcb.0c10076

Abstract

Binary alcohol + ether liquid mixtures are of significant importance as potential biofuels or additives for internal combustion engines and attract considerable fundamental interest as model systems containing one strongly H-bonded self-associating component (alcohol) and one that is unable to do so (ether), but that can interact strongly as a H-bond acceptor. In this context, the excess thermodynamic properties of these mixtures, specifically the excess molar enthalpies and volumes (HE and VE), have been extensively measured. Butanol isomer + di-n-butyl ether (DBE) mixtures received significant attention because of interesting differences in their VE, changing from negative (1- and isobutanol) to positive (2- and tert-butanol) with increasing alkyl group branching. With the aim of shedding light on the differences in alcohol self-association and cross-species H-bonding, considered responsible for the observed differences, we studied representative 1- and 2-butanol + DBE mixtures by molecular dynamics simulations and experimental excess property measurements. The simulations reveal marked differences in the self-association of the two isomers and, while supporting the existing interpretations of the HE and VE in a general sense, our results suggest, for the first time, that subtle changes in H-bonded topologies may contribute significantly to the anomalous volumetric properties of these mixtures.

Highlights

Measurements of the molar enthalpies of mixing, ΔmixH, a property coinciding with the excess molar enthalpy, HE, and the excess molar volumes, VE, of binary mixtures of selected butanol isomers and di-n-butyl ether (DBE) have been reported by several groups over the past 3 decades.[2−4,6] Both excess thermodynamic properties have been found to increase in magnitude in the order 1-butanol < 2-methyl-1-propanol < 2-butanol < 2-methyl-2-propanol, with the VE values showing an interesting change in sign from negative at all compositions for 1-butanol and isobutanol, to predominantly positive for 2-butanol and positive at all compositions for tert-butyl alcohol + DBE mixtures
The computed HE for 2-butanol mixtures is very close to the experimental values, while for 1-butanol mixtures they have a trend similar to the experimental values showing a similar asymmetry with both maxima around xDBE = 0.7, but are generally lower than experiment
It should be noted at this stage that the magnitude of deviations from experiment observed for the simulated 1-butanol + DBE mixtures is not uncommon among computer simulation studies using standard classical force fields, e.g., optimized potentials for liquid simulations-all atoms (OPLS-AA),[37] often reporting larger deviations,[38] and will be further discussed below

Summary

Introduction

The physical properties of binary liquid mixtures of alcohols with either alkanes or ethers have been the subject of sustained research interest over the past few decades due to both their practical importance as fuel additives in the internal combustion engines and fundamental interest (a mixture of self-associating and non-self-associating components).[1−7]In fuel applications, oxygenated compounds, e.g., alcohols and ethers, are used and investigated as gasoline/fuel additives with the aim of reducing pollutants from vehicle exhaust emissions by altering combustion characteristics.[7−9] Higher alcohols, and butanol isomers in particular, are increasingly studied for applications as biofuels since they may offer advantages over the traditional biofuel, ethanol, including better compatibility with the existing engine designs at 85% per volume compared with the corresponding ethanol-based fuel (E85), and higher energy content per unit volume.[8,10] butanol isomers are included in the international regulation on the promotion of the use of energy from renewable sources.[11]. Measurements of the molar enthalpies of mixing, ΔmixH, a property coinciding with the excess molar enthalpy, HE, and the excess molar volumes, VE, of binary mixtures of selected butanol isomers and DBE have been reported by several groups over the past 3 decades.[2−4,6] Both excess thermodynamic properties have been found to increase in magnitude in the order 1-butanol (nbutanol) < 2-methyl-1-propanol (isobutanol) < 2-butanol (secbutanol) < 2-methyl-2-propanol (tert-butanol; tert-butyl alcohol), with the VE values showing an interesting change in sign from negative at all compositions for 1-butanol and isobutanol, to predominantly positive for 2-butanol and positive at all compositions for tert-butyl alcohol + DBE mixtures These excess property trends have been interpreted as due to the differences in the balance of intermolecular interaction contributions arising from the breakage of alcohol−alcohol hydrogen bonds (H-bonds) upon mixing, the formation of Received: November 8, 2020 Revised: December 16, 2020 Published: January 11, 2021

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Results

Conclusion