Thermodynamics of amide + amine mixtures. 5. Excess molar enthalpies of N,N-dimethylformamide or N,N-dimethylacetamide + N-propylpropan-1-amine, + N-butylbutan-1-amine, + butan-1-amine, or + hexan-1-amine systems at 298.15 K. Application of the ERAS model
Graphical abstract
Introduction
It is well-known that a suitable approach for the investigation of the highly complex chemical environment of proteins is to study small organic molecules whose functional groups are similar to those present in the biomolecule [1]. The systematic physical and chemical characterization of such molecules and of their mixtures in terms of thermodynamic, transport and dielectric properties is necessary in this framework. The study of amide + amine systems is relevant, as it allows to gain insight into the behavior of the amide group when it is surrounded by different environments. In fact, the hydrogen-bonded structures where the amide group is involved can show very different biological activities depending on the mentioned environments [2]. On the other hand, the strong polarity of amides, which in the case of tertiary amide leads to the creation of a certain local order [3,4], together with their high solvating capability and liquid state range –due to their ability to form hydrogen bonds– [5], makes them a very important kind of organic solvents. Similarly, amines are also an important class of substances since many biological relevant molecules contain the amine group [[6], [7], [8]]. In addition, the low vapor pressure of amines makes them useful in green chemistry. Thus, mixtures containing amines are being investigated to be used in CO2 capture [9] and, interestingly, many of the ions of the technically important ionic liquids are related to amine groups [10].
In previous works, we have measured densities, speeds of sound and refractive indices of N,N-dimethylformamide (DMF) [11], or N,N-dimethylacetamide (DMA) [12] + N-propylpropan-1-amine (DPA) or + butan-1-amine (BA) at (293.15–303.15) K, and + N-butylbutan-1-amine (DBA) or + hexan-1-amine (HxA) at 298.15 K. In addition, we have reported low-frequency permittivity measurements of the mentioned systems and of the DMF + aniline mixture at (293.15–303.15) K [13,14]. This database has been interpreted in terms of solute-solvent interactions and structural effects. We have also applied the ERAS [15] and the Kirkwood-Fröhlich models [[16], [17], [18], [19]] to the study of amine + amide mixtures. The latter is useful for the calculation of the Balankina relative excess Kirkwood correlation factors [20], which provide information on the dipole correlations present in the considered systems. Calorimetric data are essential for the study of the type and strength of interactions present in liquid mixtures. As the data available in the literature on excess molar enthalpies, , for amine + amide mixtures is scarce [[21], [22], [23]], we continue this series of works reporting values for DMF or DMA + DPA, or + DBA, or + BA or + HxA systems at 298.15 K. Finally, the systems are characterized in terms of the ERAS model, revisiting the previously reported parameters which were determined using volumetric data only [14].
Section snippets
Materials
Information about the purity and source of the pure compounds used along the experiments is collected in Table 1. They were used without further purification. It also shows their densities () at 0.1 MPa and at 298.15 K. These results agree well with literature data.
Apparatus and procedure
Molar quantities were calculated using the relative atomic mass Table of 2015 issued by the Commission on Isotopic Abundances and Atomic Weights (IUPAC) [24].
Densities were obtained using a vibrating-tube densimeter DMA HPM from
Results
Data on are listed in Table 2. They were fitted to a Redlich-Kister equation [27] by an unweighted linear least-squares regression. The Redlich-Kister equation for the excess property is given by:
The number, , of necessary coefficients for this regression has been determined, for each system, by applying an F-test of additional term [28] at 99.5% confidence level. The standard deviations, , are defined by:where the
ERAS model
The Extended Real Associated Solution (ERAS) model [15,30] combines the Real Association Solution Model [[31], [32], [33], [34]] with Flory's thermal equation of state [[35], [36], [37], [38], [39]]. Some important features of this model are now given. (i) The excess molar functions of enthalpy and volume () are calculated as the sum of two contributions. The chemical contribution, arises from hydrogen bonding; the physical contribution, , is related to nonpolar Van
Discussion
We are referring throughout this section to values of the excess functions and of the thermophysical properties at 298.15 K and at = 0.5, except otherwise specified.
As previously mentioned, DMF and DMA are very polar substances, since their dipole moment is 3.7 D [48,49]. Consequently, their alkane mixtures show immiscibility gaps up to rather high temperatures. Thus, systems formed by DMF and heptane or hexadecane have upper critical solution temperatures (UCST) of 342.55 K [50] and
Conclusions
Excess molar enthalpies of amide (DMF or DMA) + linear primary or secondary amine (BA, HxA, DPA or DBA) have been reported at T = 298.15 K and p = 0.1 MPa. The positive values arise from the dominant contribution from the rupture of amide-amide and amine-amine interactions along mixing. Dipolar interactions are stronger in DMF systems. DMA mixtures show lower values for a fixed amine, suggesting that the variation of the rupture of amide-amide interactions is the predominant effect.
Acknowledgements
F. Hevia is grateful to J.-Y. Coxam and K. Ballerat-Busserolles for the opportunity to do the experimental part of this work at their laboratory at Institut de Chimie de Clermont-Ferrand, and also acknowledges Ministerio de Educación, Cultura y Deporte for the grant FPU14/04104 and for the complementary grants EST16/00824 and EST17/00292. In addition, the authors FH, JAG, IGF and JCC gratefully acknowledge the financial support received from the Consejería de Educación de Castilla y León, under
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