Liquid–liquid equilibria for benzaldehyde + n-alkane mixtures and characterization of benzaldehyde + hydrocarbon systems in terms of DISQUAC

doi:10.1016/j.fluid.2014.01.013

Fluid Phase Equilibria

Volume 366, 25 March 2014, Pages 61-68

https://doi.org/10.1016/j.fluid.2014.01.013 Get rights and content

Highlights

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LLE coexistence curves for benzaldehyde + n-C₁₀, n-C₁₂, n-C₁₄; n-C₁₆ are reported.
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All the systems show an UCST.
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Benzaldehyde + hydrocarbon mixtures are characterized using DISQUAC.
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CHO/aromatic contacts are dispersive. CHO/alkane contacts are dispersive and quasichemical.
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DISQUAC correctly describes $H_{m}^{E}$ and VLE, SLE, LLE of the studied systems.

Abstract

Liquid–liquid equilibrium (LLE) temperatures have been determined for benzaldehyde + CH₃(CH₂)_nCH₃ (n = 8, 10,12,14) mixtures by means of the opalescence method using a laser scattering technique. All the solutions show an upper critical solution temperature (UCST), which increases almost linearly with n. Benzaldehyde + benzene, or + alkyl-benzene, or + alkane mixtures have been treated in terms of the group contribution model DISQUAC. The corresponding interaction parameters for the contacts formyl/aromatic, formyl/aliphatic, formyl/c-CH₂ are reported. The former are entirely dispersive; the remainder contacts are characterized by both dispersive and quasichemical interaction parameters. The model correctly describes excess molar enthalpies, $H_{m}^{E}$ , and the phase equilibrium diagrams, liquid–liquid, vapour–liquid (VLE) and solid–liquid (SLE) of the investigated solutions, over a wide range of temperature using the same set of interaction parameters. Complex phase diagrams including simultaneous LLE and SLE are also well represented by DISQUAC. It is shown that interactions between benzaldehyde and an aromatic hydrocarbon are mainly dispersive, and that those between benzaldehyde molecules in alkane solutions are of dipolar type.

Graphical abstract

Introduction

Benzaldehyde is a compound widely used for the manufacture of odorants and flavouring chemicals (e.g., cinnamaldehyde). It is also employed as starting material for some pharmaceuticals, such as ampicillin and pesticides. On the other hand, the study of mixtures containing an aromatic compound with a polar functional group allows investigate a number of effects such as the so-called n–π interactions, i.e., the intramolecular interactions between the phenyl group and a polar functional group. In the present work, we report LLE data for the benzaldehyde + decane, + dodecane, + tetradecane, or + hexadecane mixtures, and we extend the DISQUAC group contribution model [1] to systems containing this aldehyde (polar group: formyl, CHO) and one hydrocarbon, namely, alkane, benzene or alkyl-benzene. LLE measurements for the dodecane system are available in the literature [2]. Aliphatic aldehyde + alkane mixtures have been previously treated in terms of DISQUAC, firstly using the quasichemical approximation [3], [4], [5], and later taken into consideration both dispersive and quasichemical interaction parameters [6]. The distinction between aliphatic and aromatic polar compounds, in the framework of any theoretical model, is necessary for the improvement of predictions of thermodynamic properties of solutions involving such compounds [7].

Section snippets

Materials

Table 1 shows information on source, purity, water contents, determined by the Karl–Fischer method, and density, ρ, of the pure compounds used in this work. The chemicals were used without further purification. Densities were measured using a vibrating-tube densimeter and a sound analyser, Anton Paar model DSA-5000. The resolution in density is |Δρ/ρ| = ±6 × 10⁻⁶, while the corresponding accuracy is estimated to be ±2 × 10⁻² kg m⁻³. The ρ values of the pure liquids are in good agreement with those from

Experimental results

Table 2 lists the directly measured liquid–liquid equilibrium temperatures, T, vs. the mole fraction of the aldehyde, x₁, for benzaldehyde + decane, + dodecane, + tetradecane, or + hexadecane mixtures.

All the systems show an UCST. Note that the LLE curves have a flat maximum, and that their symmetry depends on the alkane size (Fig. 1). The UCST increases with the chain length of the alkane. LLE phase diagrams of many other systems as those containing linear organic carbonate [10], acetic anhydride [11]

Model

DISQUAC is a group contribution model based on the rigid lattice theory developed by Guggenheim [27]. The main features of DISQUAC are as follows. (i) The total molecular volumes, r_i, surfaces, q_i, and the molecular surface fractions, α_i, of the compounds present in the mixture are calculated additively on the basis of the group volumes R_G and surfaces Q_G recommended by Bondi [28]. As volume and surface units, the volume $R_{{CH}_{4}}$ and surface $Q_{{CH}_{4}}$ of methane are taken arbitrarily [29]. For the

Adjustment of DISQUAC interaction parameters

In the framework of DISQUAC, benzaldehyde + hydrocarbon mixtures are regarded as possessing the following three types of surface: (i) type a, aliphatic (CH₃, CH₂, in n-alkanes or alkyl-benzenes); (ii) type k (CHO in benzaldehyde); (iii) type s (s = b, C₆H₆, C₆H₅ in benzaldehyde or alkyl-benzenes; s = c, c-CH₂ in cyclohexane).

The general procedure applied in the estimation of the interaction parameters has been explained in detail elsewhere [32], [36]. Final values of the fitted parameters in this

Theoretical results

Results from the DISQUAC model for coordinates of the critical points, VLE, $G_{m}^{E}$ (molar excess Gibbs energies), solid–liquid equilibria and $H_{m}^{E}$ are shown in Table 3, Table 5, Table 6, Table 7 and in Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6. For the sake of clarity, relative standard deviations for pressure, P, and for SLE temperatures, T, defined as $σ_{r} (F) = {\{\frac{1}{N} \sum {[\frac{F_{\exp} - F_{calc}}{F_{\exp}}]}^{2}\}}^{1 / 2}$ where F = P or T are given in Table 5, Table 6, respectively. Table 6 also contains absolute mean deviation for SLE

Discussion

Hereafter, we are referring to values of the excess functions at 298.15 K and equimolar composition.

As indicated above, the benzaldehyde/benzene interactions are essentially dispersive. This is also supported by decreasing $H_{m}^{E}$ values of the benzaldehyde + benzene mixture with the increasing of temperature. Thus, ${(Δ H_{m}^{E} / Δ T)}_{P} = - 2.3 {J mol}^{- 1} K^{- 1}$ [7], [37]. Note that the molar excess isobaric heat capacity is −3.3 J mol⁻¹ K⁻¹ for the benzene + heptane system [43].

The existence of strong dipole–dipole

Conclusions

LLE coexistence curves for the mixtures benzaldehyde + decane, + dodecane, + tetradecane, or + hexadecane have been obtained. All the systems show an UCST, which increases with the chain length of the alkane. Benzaldehyde + hydrocarbon mixtures have been treated in terms of DISQUAC. The model correctly describes $H_{m}^{E}$ and phase diagrams LLE, VLE, SLE of the studied solutions using the same set of interaction parameters.

Acknowledgement

The authors gratefully acknowledge the financial support received from the Ministerio de Ciencia e Innovación, under the Project FIS2010-16957.

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