Thermodynamic study of binary mixtures of alcohols and alkoxyethanols for fuel applications: excess molar enthalpies and local composition models

Abstract

Abstract The growing demand for transportation has made the sector one of the largest sources of air pollution globally, directly impacting environmental quality and human health. To address these challenges, various mitigation strategies are being investigated, particularly those targeting emissions from vehicles using internal combustion engines. Among these, the incorporation of oxygen-rich additives, such as long-chain alcohols and alkoxyethanols, has shown promise in enhancing fuel compatibility and improving combustion behaviour, which can lead to reduced pollutant emissions. Understanding their thermodynamic behaviour, particularly excess molar enthalpy, $$H_{{\text{m}}}^{{\text{E}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>H</mml:mi> <mml:mrow> <mml:mtext>m</mml:mtext> </mml:mrow> <mml:mtext>E</mml:mtext> </mml:msubsup> </mml:math> , is crucial for optimizing fuel formulations, as hydrogen bonding and steric effects influence mixing behaviour, stability, and performance. The presence of hydroxyl (–OH) functional group in each component (alcohol or alkoxyethanol) introduces varying degrees of complexity, depending on molecular structure, steric hindrance, and the ability to form intermolecular networks. In the present work, an isothermal flow calorimeter was employed to measure the $$H_{{\text{m}}}^{{\text{E}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>H</mml:mi> <mml:mrow> <mml:mtext>m</mml:mtext> </mml:mrow> <mml:mtext>E</mml:mtext> </mml:msubsup> </mml:math> of mixtures involving alcohols and alkoxyethanols at two different temperatures: 298.15 K and 313.15 K, yielding a total of 304 experimental data points. The measured results were correlated using the Redlich–Kister equation (R–K) and modelled with various local composition models, including UNIQUAC, NRTL, and UNIFAC models. The results show that the first two models provided accurate predictions of $$H_{{\text{m}}}^{{\text{E}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>H</mml:mi> <mml:mrow> <mml:mtext>m</mml:mtext> </mml:mrow> <mml:mtext>E</mml:mtext> </mml:msubsup> </mml:math> , effectively capturing the impact of hydrogen bonding and steric hindrance on mixing behavior. Although the UNIFAC model successfully predicted the overall thermodynamic trends of the mixtures, it exhibited systematic deviations, either underestimating or overestimating $$H_{{\text{m}}}^{{\text{E}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>H</mml:mi> <mml:mrow> <mml:mtext>m</mml:mtext> </mml:mrow> <mml:mtext>E</mml:mtext> </mml:msubsup> </mml:math> , due to its limitations in describing specific molecular interactions. The analysis shows that all analysed binary mixtures exhibit endothermic behaviour at different temperatures.