Thermodynamics of mixtures containing aromatic nitriles

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Highlights

  • LLE curves are provided for C6H5–(CH2)n−1CN + alkane mixtures (n = 2,3).

  • Aromatic nitrile mixtures are investigated using SLE, VLE, LLE, HmE, UVmE, CpmE, VmE and Kirkwood’s correlation factor.

  • Dipolar interactions between nitrile molecules become stronger in the order: linear < aromatic.

  • This is due to proximity effects between C6H5– and CN groups. These effects become stronger when n increases.

  • DISQUAC and ERAS models are applied.

Abstract

The coexistence curves of liquid-liquid equilibrium (LLE) for the mixtures: phenylacetonitrile + heptane, + octane, + nonane, + cyclooctane, or + 2,2,4-trimethylpentane and for 3-phenylpropionitrile + heptane, or + octane are reported. Aromatic nitrile + alkane, + aromatic hydrocarbon or + 1 alkanol systems are investigated using a set of thermophysical properties: phase equilibria (solid-liquid, SLE, vapour-liquid, VLE and LLE), excess molar functions, enthalpies (HmE), isochoric internal energies (UVmE), isobaric heat capacities (CpmE) and volumes (VmE), and the Kirkwood’s correlation factor. Due to proximity effects between the phenyl and the CN groups, dipolar interactions between molecules of aromatic nitriles are stronger than those between molecules of isomeric linear nitriles. Dipolar interactions become weaker in the order: 3-phenylpropionitrile > phenylacetonitrile > benzonitrile. Benzonitrile + aromatic hydrocarbon mixtures are characterized by dispersive interactions and structural effects. The latter are more important in systems with phenylacetonitrile. Structural effects are also present in benzonitrile + n-alkane, or + 1-alkanol + mixtures. The systems mentioned above have been studied using DISQUAC. Interaction parameters for contacts where the CN group in aromatic nitriles participates are given. DISQUAC describes correctly any type of phase equilibria, CpmE of benzonitrile + hydrocarbon mixtures and HmE of benzonitrile + cyclohexane, or 1-alkanol systems. Large differences encountered between theoretical HmE values and experimental data for some solutions are discussed. 1-Alkanol + benzonitrile mixtures are also investigated by means of the ERAS model. ERAS represents well HmE of these systems. The VmE curves of solutions with longer 1-alkanols are more poorly described, which has been explained in terms of the existence of structural effects.

Introduction

Aromatic nitriles, C6H5–(CH2)n−1CN (n = 1 (benzonitrile); n = 2 (phenylacetonitrile); n = 3 (3-phenylpropionitrile)) are rather polar compounds as it is indicated by their high dipole moments (μ) [1]: 3.87 D (n = 1); 3.50 D (n = 2); 3.29 D (n = 3). They are good solvents and are used as starting materials in the synthesis of fungicides, fragrances and pharmaceuticals, as analgesics or antihistamines. Recently, the mixture formed by 3-phenylpropionitrile and supercritical CO2 has received attention as the miscibility of these type of systems is a relevant condition for polymer processes [2].

It is well known that the existence of two groups (X, Y) of the same or different nature within the same molecule leads to an enhancement of the dipolar interactions between the mentioned molecules. For example, the 1-butanol + heptane mixture is miscible at any concentration at 298.15 K and, at equimolar composition, its excess molar enthalpy, HmE, is 575 J·mol−1 [3]. In contrast, the 2-methoxyethanol (isomeric molecule of 1-butanol) + heptane system is characterized by a moderately high UCST (319.74 K [4]), what has been typically ascribed to proximity effects between the –O– and –OH groups within the alkoxyethanol [5]. It is also known, that the application of group contribution methods to systems characterized by proximity effects between two given groups X,Y leads to erroneous results when one uses interaction parameters for the (X/Y) contacts obtained from solutions with X and Y belonging to different molecules [6]. In fact, proximity effects can drastically change the interaction parameters of the applied model. Thus, in terms of UNIFAC (Dortmund version) specific main groups have been defined for aniline or phenol for a better representation of the thermodynamic properties of mixtures involving these compounds [7]. We are engaged in a systematic research on proximity effects between the phenyl group C6H5– and a polar group as carbonyl, aldehyde, alkanoate, amine, alkanol or oxygen. At this end, we have provided LLE measurements for systems formed by one alkane and a polar aromatic compound such as: C6H5–(CH2)n-1COCH3 (n = 1,2,3) [8], [9], [10]; C6H5–CHO [11]; C6H5–CH2OCOCH3 [9], [10] C6H5–CH2NH2 [12]; C6H5–CH2OH [13], or C6H5–O–CH2CH2OH [14]. These data together with measurements available in the literature on VLE or HmE for these or related compounds, aniline or phenol, e.g., have been employed for the characterization of aromatic polar compound + organic solvent mixtures [8], [9], [10], [11], [12], [13], [14], [15], [16] in terms of the DISQUAC group contribution model [17]. Here, we continue with this research line and report LLE data for the systems: C6H5–CH2CN + heptane, or + octane, or + nonane, or + cyclooctane, or + 2,2,4-trimethylpentane, and for C6H5–CH2CH2CN + heptane, or + octane. In addition, aromatic nitrile + organic solvent mixtures are characterized using DISQUAC and the interaction parameters for a number of contacts, CN/aliphatic; CN/aromatic and CN/OH, are provided. UNIFAC (Dortmund) and DISQUAC interaction parameters for the contacts CN/aliphatic and CN/OH in systems involving linear nitriles are available in the literature [7], [18], [19]. The mixtures 1-alkanol + linear nitrile or + benzonitrile have been investigated by means of the Flory theory [20], and the ERAS model [21] has been applied to 1-alkanol + benzonitrile systems [22]. On the other hand, due to high μ values of aromatic nitriles, one may expect the existence of strong dipolar interactions between nitrile molecules in solutions where these compounds participate. An interesting question arises from the concentration dependence of HmE of benzonitrile + benzene, or + toluene systems. Some authors have reported M-shaped HmE curves for these mixtures [23], [24]. Such anomalous concentration dependence of HmE has been explained by invoking charge-transfer complex formation between benzonitrile and the aromatic hydrocarbon [23]. However, it must be remarked that other researchers have not observed the mentioned trends for benzonitrile mixtures [25]. This matter will also be considered along the work.

Section snippets

Materials

Table 1 shows some properties of the pure chemicals used along the experimental part of this investigation: source, purity, water contents, determined by the Karl-Fischer method, and density (ρ). The reagents were employed without further purification. Density values were determined by means of a vibrating-tube densimeter and a sound analyser, Anton Paar model DSA-5000. The repeatability of the ρ measurements is 5·10−3 kg·m−3, while their relative standard uncertainty is 0.0012. From the values

Experimental results

Table 2 lists the directly measured liquid-liquid equilibrium temperatures, T, vs. x1, the mole fraction of the aromatic nitrile, for the systems: phenylacetonitrile + n-C7, or + n-C8, or + n-C9, or + cyclooctane, or + 2,3,4-trimethylpentane, and for 3-phenylpropionitrile + n-C7, or + n-C8. A literature survey shows that there are no data available for comparison. Some experimental results are also represented in Fig. 1.

For the considered systems, the LLE curves are characterized by a rather

DISQUAC

DISQUAC is a group contribution model based on the rigid lattice theory developed by Guggenheim [34]. Some important features of DISQUAC are the following. (i) The total molecular volumes, ri, surfaces, qi, and the molecular surface fractions, αsi, of the compounds present in the mixture are calculated additively on the basis of the group volumes RG and surfaces QG recommended by Bondi [35]. The volume RCH4 and surface QCH4 of methane are taken arbitrarily as equal to 1 [36]. The geometrical

DISQUAC interaction parameters

In the framework of DISQUAC, the systems under study are regarded as possessing the following types of surfaces: (i) type a, aliphatic (CH3, CH2, in n-alkanes, or aromatic hydrocarbon, or aromatic nitrile, or 1-alkanols); (ii) type n (CN in aromatic nitrile); (iii) type s (s = b, C6H6, or C6H5 in aromatic hydrocarbons or nitriles; s = c–CH2 in cycloalkanes; s = h, OH in 1-alkanols).

The general procedure applied in the estimation of the interaction parameters have been explained in detail in earlier

Theoretical results

Results from the DISQUAC model on phase equilibria, HmE and CpmE are shown in Table 6, Table 7, Table 8, Table 9 and in Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7 (see also Table S2 and Figs. S1 and S2 of supplementary material). Table 6, Table 7, Table 9 contain relative deviations for pressure, temperature and HmE, respectively, defined asσr(F)=1NFexp-FcalcFexp21/2(F=P,T)dev(HmE)=1NHm,expE-Hm,calcEHm,expE(x1=0.5)21/2

ERAS results on HmE and VmE for 1-alkanol + benzonitrile systems

Discussion

Below, we are referring to values of the excess molar functions at 298.15 K and equimolar composition. We also refer to aromatic polar compounds of general formula C6H5–(CH2)n−1X (X = Cl, CN, NH2, OH) or C6H5–(CH2)n−1XCH3 (X = CO, CHO, COO, O).

We start remarking that the UCST values, ranged between (280,360) K for systems formed by one aromatic nitrile and one alkane reveal the existence of strong dipolar interactions between nitrile molecules. Accordingly, the HmE and TSmE(=HmE-GmE) values for

Conclusions

LLE data for the systems phenylacetonitrile + n-C7, + n-C8,+ n-C9, + c-C8, or + i-C8 and for 3-phenylpropionitrile + n-C7, or + n-C8, have been reported. Dipolar interactions between aromatic nitrile molecules become weaker in the sequence: 3-phenylpropionitrile > phenylacetonitrile > benzonitrile. Aromatic nitrile + alkane, + aromatic hydrocarbon, or + 1-alkanol mixtures have been studied using DISQUAC. The interchange coefficients for contacts involving the aromatic nitrile group have been

Acknowledgements

The authors gratefully acknowledge the financial support received from the Consejería de Educación y Cultura of Junta de Castilla y León, under Project BU034U16. F. Hevia gratefully acknowledges the grant received from the program ‘Ayudas para la Formación de Profesorado Universitario (convocatoria 2014), de los subprogramas de Formación y de Movilidad incluidos en el Programa Estatal de Promoción del Talento y su Empleabilidad, en el marco del Plan Estatal de Investigación Científica y Técnica

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