Thermodynamics of mixtures containing amines. XI. Liquid + liquid equilibria and molar excess enthalpies at 298.15 K for N-methylaniline + hydrocarbon systems. Characterization in terms of DISQUAC and ERAS models

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Abstract

Liquid + liquid equilibrium (LLE) temperatures have been determined for the N-methylaniline + tetradecane, or +hexadecane systems by the method of the critical opalescence using a laser scattering technique. The coexistence curves have an upper critical solution temperature (UCST) and, due to size effects, are skewed towards high mole fractions of the amine. Excess molar enthalpies, HmE, at 298.15 K and atmospheric pressure, have been also measured over the entire mole fraction range, using a Tian–Calvet microcalorimeter, for the mixtures N-methylamine + heptane, +octane, +decane, +cyclohexane, or +toluene. The HmE curves of alkane solutions are characterized by a large maximum and a rather flattened top, which are typical features of systems at temperature close to the UCST. N-methylamine + alkane, +benzene, +toluene or +1-alkanol mixtures have been investigated in terms of the DISQUAC and ERAS models. The corresponding interaction parameters are reported. From the analysis of the experimental data and of the theoretical results, it is shown that: (i) HmE of the studied N-methylamine solutions is mainly determined by the disruption, upon mixing, of the interactions between like molecules; (ii) interactions between isomeric aromatic amines become weaker in the sequence: primary > secondary > tertiary; (iii) for isomeric molecules, interactions between aromatic amines are stronger than between linear amines; (iv) in aromatic amine + aromatic hydrocarbon or +1-alcohol systems, amine-solvent interactions are stronger in the order tertiary < secondary < primary; (v) physical interactions play a dominant role in the investigated mixtures; (vi) DISQUAC and ERAS models provide similar HmE results for systems including alkanes. In the case of mixtures with aromatic hydrocarbons or 1-alkanols, where interactions between unlike molecules are relevant, HmE is better described by DISQUAC; and (vii) the quasichemical interchange coefficients (l = 1,3) for the contacts amine/aliphatic; amine/aromatic; amine/cyclic and amine/hydroxyl are the same for systems with aniline, 2-methylaniline, N-methylaniline, or N,N-dimethylaniline.

Highlights

► We report LLE for N-methylaniline + n-C14 or +n-C16. ► We report HmE at 298.15 K for N-methylaniline + n-C7, +n-C8, +n-C10, +C6H12, +C7H8. ► DISQUAC and ERAS interaction parameters are given for the studied mixtures. ► Physical interactions are more relevant than those of association/solvation effects.

Introduction

The study of mixtures with amines makes possible to examine the influence of some interesting effects on their thermodynamic properties. For example, the treatment of systems with primary or secondary linear amines allows the study of the size and steric effects produced by the alkyl groups attached to the amine group. Effects related to the quasi globular shape of small N,N,N-trialkylamines can be analyzed through solutions containing these amines. On the other hand, effects related to the ring strain or to the influence of an aromatic ring may be investigated through systems containing cyclic or aromatic amines, respectively.

N-methylaniline is an aromatic secondary amine, which is weakly self-associated, as its Trouton’s constant indicates (91.1 J · mol−1 · K−1; enthalpy of vaporization, 42.8 J · mol−1 [1] at the boiling point, 469.6 K [2]). In fact, the Trouton’s constant of non-associated species is 92.05 J · mol−1 · K−1 [3], while that for 1-alkanols is 110.88 J · mol−1 · K−1 [3]. N-methylaniline is an important intermediate for dyes or agrochemical manufacturing. Interestingly, aniline polymers are widely studied due their high stability and wide range of conductivity. They are used in transistors, solar cells or light emitting diodes [4]. N-substituted polymers (Poly (N-methylaniline), e.g.) provide better solubility and processability, which is of importance in printable electronics. Poly (N-methylaniline) has been studied as cathode active material in aqueous rechargeable batteries [5].

This work is part of a series devoted to the experimental and theoretical characterization of mixtures containing amines. We have provided excess molar volumes of N,N,N-trialkylamine [6], [7], or methyl butyl amine [8] +alkane mixtures, as well as liquid + liquid and/or solid + liquid equilibrium temperatures for 2-methylaniline [9], imidazoles [10] or quinoline [11] +hydrocarbon systems. Mixtures with aniline [12], 2-methylaniline [9], pyridines [13], [14], [15], [16], quinoline [11] or imidazoles [10] or cyclic amines [17] with various solvents have been studied using different models: DISQUAC [18], [19], ERAS [20], Kirkwood–Buff integrals [21], or the formalism of the concentration-concentration structure factor [16]. As continuation of these works, we report here liquid + liquid equilibrium temperatures for N-methylaniline + tetradecane, or +hexadecane systems and molar excess enthalpies at 298.15 K and at atmospheric pressure for the mixtures N-methylaniline + heptane, +octane, +decane, +cyclohexane, or +toluene. HmE data for systems with toluene [22] or cyclohexane [23] at T  298.15 K are available in the literature. In order to gain insight into the interactions and structure of systems formed by N-methylaniline and alkane, aromatic compound or 1-alkanol, they are studied through the application of the DISQUAC [18], [19] and ERAS [20] models. In the framework of UNIFAC (Dortmund version) [24], aniline, is a main group by itself [25], but no specific group has been defined for N-methylaniline [25], [26], [27], [28].

Section snippets

Materials

Prior to the measurements, the chemicals were stored over molecular sieves (Union Carbide Type 4 Å from Fluka). All the chemicals were used without further purification. Table 1 contains information about the source, purity, water content, determined by Karl-Fischer method, and densities, ρ, of the pure compounds, measured using a vibrating-tube densimeter and a sound analyser, Anton Paar model DSA-5000. The uncertainty for the ρ values is ±1·10−2 kg · m−3, while the corresponding precision is ±1·10

Experimental results

TABLE 2, TABLE 3 list the direct experimental results of TLLE and of HmE, respectively, vs. the mole fraction of the amine, x1, for the investigated mixtures (see FIGURE 1, FIGURE 2, FIGURE 3, FIGURE 4). At equimolar composition, the values HmE (toluene, 303.15 K) = 520 J · mol−1 [22] and HmE (cyclohexane, 323.15 K) = 1780 J · mol−1 [23] are more or less consistent with those determined in this work at 298.15 K (497 and 1617 J · mol−1). The LLE coexistence curves show an UCST and have a rather flat horizontal

DISQUAC

This is a group contribution model based on the rigid lattice theory developed by Guggenheim [43]. A brief summary of the main features of DISQUAC follows. (i) The total molecular volumes, ri, surfaces, qi, and the molecular surface fractions, αi, of the compounds present in the mixture are calculated additively on the basis of the group volumes RG and surfaces QG recommended by Bondi [44]. As volume and surface units, the volume RCH4 and surface QCH4 of methane are taken arbitrarily [45]. The

Adjustment of DISQUAC interaction parameters

In the framework of DISQUAC, N-methylaniline + organic solvent mixtures are regarded as possessing the following five types of surface: (i) type a, aliphatic (CH3, CH2, in N-methylaniline, n-alkanes, toluene, or 1-alcohols); (ii) type b, aromatic (C6H6 in benzene; C6H5 in toluene or N-methylaniline); (iii) type c, c-CH2 in cyclohexane (iv) type h, OH in 1-alcohols; (v) type n, NH in N-methylaniline.

The general procedure applied in the estimation of the interaction parameters has been explained in

Theoretical results

Results from DISQUAC for LLE are compared with experimental data in figure 1. As, usually [12], [53], the coordinates of the critical points are represented by DISQUAC in the correct range of composition and temperature (table 4). Note that calculations are developed under the assumption that the excess functions are analytical close to the critical points, while the thermodynamic properties are, really, expressed in terms of scaling laws with universal critical exponents and universal scaling

Discussion

Below, we are referring to values of the excess functions at 298.15 K and equimolar composition. Inspection of table 9 shows that the HmE values of the N-methylaniline solutions are positive. Consequently, the main contribution to HmE comes from the disruption, upon mixing, of the interactions between like molecules.

The UCST of mixtures containing an aromatic amine and heptane decreases in the sequence: aniline (343.11 K [59]) >2-methylaniline (292.86 K [9]) >N-methylaniline (273.15 K [60]) >255.2

Conclusions

The liquid + liquid equilibrium curves have been reported for N-methylaniline + tetradecane, or +hexadecane systems. HmE values at 298.15 K have been measured for the N-methylaniline + heptane, +octane, +decane, +cyclohexane, or +toluene mixtures. These systems together with those including 1-alkanols or benzene have been investigated by means of the DISQUAC and ERAS models. The corresponding interaction parameters have been determined. The Csn,lQUAC (l = 1, 3; s = a, b, c, h) interchange coefficients are

Acknowledgements

The authors gratefully acknowledge the financial support received from the Ministerio de Ciencia e Innovación, under the Project FIS2010-16957. I.A. acknowledges the grant financed jointly by the Junta de Castilla y León and Fondo Social Europeo.

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