Dielectric and refractive index measurements for the systems 1-pentanol + 2,5,8,11,14-pentaoxapentadecane, or for 2,5,8,11,14-pentaoxapentadecane + octane at (293.15–303.15) K

doi:10.1016/j.tca.2012.10.024

Thermochimica Acta

Volume 551, 10 January 2013, Pages 70-77

https://doi.org/10.1016/j.tca.2012.10.024 Get rights and content

Abstract

Relative permittivities, ɛ_r, and refractive indices, n_D, have been measured at (293.15–303.15) K, for the mixtures 1-pentanol + 2,5,8,11,14-pentaoxapentadecane (TEGDME) or TEGDME + octane. These data have been used, together with density measurements available in the literature, to calculate the correlation factors, g_K, according to the Kirkwood–Fröhlich equations. The curves of the deviations of ɛ_r from the ideal behaviour, Δɛ_r, of the 1-pentanol + TEGDME system are shifted to high mole fractions of the alcohol, and show a rather large minimum. The shape of the Δɛ_r curves of methanol + TEGDME or +polyethylene glycol dimethyl ether 250 (PEG-250) mixtures is similar. This reveals that polyethers can break the alcohol self-association even at high alkanol concentrations. The higher Δɛ_r values of the methanol solutions are ascribed to large self-association of this alcohol. The Δɛ_r curves of the 1-pentanol + dibutyl ether (DBE) system are nearly symmetrical, and the Δɛ_r values are higher than those of the corresponding TEGDME mixture. This indicates that effects related to the alcohol self-association are much more relevant in the DBE system. These findings are supported by g_K and molar polarization data. Values of molar refraction of 1-pentanol systems reveal that dispersion forces become more important in the sequence: octane < DBE < TEGDME. Such forces are also more relevant in the 1-pentanol + TEGDME system than in the methanol + TEGDME solution. The comparison of ɛ_r and g_K data for TEGDME or DBE + octane mixtures shows than the polyether is a more structured liquid.

Graphical abstract

Highlights

► ɛ_r and n_D have been measured at (293.15–303.15) K for 1-pentanol + TEGDME or TEGDME + n-C₈. ► Correlation factors have been calculated according to the Kirkwood–Fröhlich equations. ► Polyethers can break alcohol self-association at high alkanol concentrations. ► Effects due to alcohol self-association are much relevant in systems with monoethers. ► Conclusions are supported by g_K and molar polarization data.

Introduction

1-Alkanol + ether mixtures are of high interest from both practical and theoretical points of view. For example, mixtures containing oxygenated compounds, such as ethers and/or alkanols are of great importance because they are increasingly used as additives to gasoline owing to their octane-enhancing and pollution-reducing properties [1], [2]. Polyethers are important solvents in many chemical reactions such as Grignard reduction, or alkylation or organo-metallic reactions [3]. On the other hand, in a series of recent articles [4], [5], [6], [7], we have shown some crucial features of 1-alkanol + ether systems. A short summary follows. (i) Effects related to self-association of 1-alkanols are determinant when considering the thermodynamic properties of mixtures with linear monoethers. (ii) Such effects are weakened if a linear monoether is replaced by a cyclic monoether, in such way that, from 1-hexanol, 1-alkanol + tetrahydrofuran, or +tetrahydropyran mixtures show a structure close to that of random mixing. (iii) The replacement of a linear monoether by a linear polyether also leads to a weakening of the mentioned effects relative to self-association of 1-alkanols, while dipolar interactions are increased. These results have been obtained from measurements on phase equilibria, molar excess enthalpies, molar excess heat capacities at constant pressure or from volumetric measurements and from the application of different theories such ERAS [8], DISQUAC [9], Flory [10], or the Kirkwood–Buff integrals formalism [11]. The investigation of the mixture structure can be also carried out on the basis of ɛ_r, and n_D, data. These magnitudes together with density data make possible the determination of the Kirkwood's correlation factor, g_K [12], [13], [14], [15], which provides useful information on the mixture structure. As a part of a general systematic study in which liquid solutions are investigated using g_K, we report here ɛ_r and n_D measurements for the mixtures 1-pentanol + TEGDME, or TEGDME + octane at (293.15–303.15) K. Previously, we have investigated 1-pentanol + DBE, or +octane mixtures, or the DBE + octane system [16]. ɛ_r and n_D data at different temperatures for the methanol + TEGDME, or +PEG-250 systems [17], [18], [19], or for glyme + alkane mixtures [20], [21], [22] are available in the literature.

Section snippets

Materials

1-Pentanol, octane and TEGDME were supplied by Fluka, and used without further purification. Their purity in mass fractions was ≥0.99; ≥0.99 and ≥0.98, respectively. Values of the physical properties of pure compounds, density, ρ (measured with an Anton Paar DMA 602 vibrating-tube densimeter (uncertainty 5 g cm⁻³), thermostated within ±0.01 K); ɛ_r, and n_D are listed in Table 1. They are in good agreement with the literature values (Table 1).

Apparatus and procedures

Binary mixtures were prepared by mass in small flasks of

Results

Table 2 lists, in the temperature range (293.15–303.15) K, values of ɛ_r and of deviations of this magnitude from the ideal state, Δɛ_r, vs. x₁, the mole fraction of the first component for 1-pentanol + TEGDME or for TEGDME + octane systems. For an ideal mixture at the same temperature and pressure than the system under study, the $ε_{r}^{id}$ values are calculated from the equation [22]: $ε_{r}^{id} = ϕ_{1} ε_{r 1} + ϕ_{2} ε_{r 2}$ where ϕ_i = x_iV_i/V^id. Deviations from the ideal behaviour are calculated from the expression: $Δ ε_{r} = ε_{r} - ϕ_{1} ε_{r 1} - ϕ_{2} ε_{r 2}$

Discussion

Inspection of the Δɛ_r curves allows state some interesting features. (i) The curves of the 1-pentanol + TEGDME system are skewed to high x₁ values, and show a rather large minimum (Fig. 1). This indicates that the polyether is a very active breaker of the alcohol structure, in such way that the total number of parallel aligned effective dipoles of 1-pentanol, that contribute to the dielectric polarization of the system, decreases upon mixing [26]. A similar behaviour is observed for methanol +

Conclusions

The properties ɛ_r and n_D have been measured at (293.15–303.15) K for the systems 1-pentanol + TEGDME, or TEGMDE + octane. Values of Δɛ_r and g_K for 1-alkanol + TEGDME, or +PEG-250 show that polyethers can break the alcohol structure even at high alcohol concentrations, and that DBE is a less active compound when breaking the alcohol self-association. This is supported by P_m values and by the temperature dependence of ɛ_r. R_m values of 1-pentanol systems reveal that dispersion forces become more

Acknowledgements

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

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Citation Excerpt :
The stated purity of the alcohols was checked using density and refractive index measurements at 298.15 K. The measured refractive index and density data of the chemicals used in this study along with the literature values [31–33] are given in table 1. A three-terminal cylindrical cell with 27 pF of empty capacitance was used for the capacitance measurements.
This paper presents relative permittivities, excess permittivities, effective dipole moments, and excess Kirkwood correlation factors of binary mixtures of 1,4-butanediol with two primary pentanol isomers [1-pentanol (amyl alcohol) + 3-methyl-1-butanol (isoamyl alcohol)] from T = (298.15 to 318.15) K at p = 101.3 kPa over the entire composition range. Experimental permittivity values for polar–non-polar binary systems of (1,4-dioxane + amyl alcohol or isoamyl alcohol) were also obtained as a function of composition at the same range of temperatures. The experimental permittivity data were fitted using Redlich–Kister equation to evaluate the adjustable parameters and the standard errors. From the experimental data, the excess parameters were calculated. In this work, variations of effective dipole moment and correlation factor were investigated using Kirkwood−Frohlich equation. The experimental data of measurements were used in the analysis of the homo- and hetero interactions occurring in these binary solutions.

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