DIELECTRIC AND REFRACTIVE INDEX MEASUREMENTS OF 1-ALKANOL + TRIETHYLAMINE SYSTEMS. APPLICATION OF THE KIRKWOOD-FRÖHLICH MODEL F. Hevia1,*, A. Cobos1, J.A. González1 , I. García de la Fuente1, J.C. 1 1 2 1 Cobos , V. Alonso , C. Alonso-Tristán , L.F. Sanz 1
2
G.E.T.E.F., Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Valladolid. Paseo de Belén, 7, 47011 Valladolid, Spain
Unidad de Investigación Consolidada UIC-011, JCyL. Dpto. Ing. Electromecánica, Universidad de Burgos. Avda. Cantabria s/n. 09006, Burgos, Spain. *
[email protected]
EXPERIMENTAL
General aim: Experimental analysis of thermophysical properties of mixtures containing alcohols and amines. Particular purposes: (i) Report the relative permittivities at 1 MHz (εr) and refractive indices at the sodium D-line (nD) for binary mixtures 1-alkanol + triethylamine (TEA) at (293.15–303.15) K and 0.1 MPa. The 1-alkanols considered are: methanol (1OH), 1-propanol (3OH), 1-butanol (4OH), 1-pentanol (5OH), and 1-heptanol (7OH). (ii) Apply the Kirkwood-Fröhlich model to these mixtures.
RELATIVE PERMITTIVITY
REFRACTIVE INDEX
εr measurement: 4294A precision impedance analyser; 16452A cell (paralell-plate capacitor); 16048G test lead (Agilent). Temperature control: LAUDA RE304 thermostatic bath (±0.02 K). Precission of the method: Relative standard uncertainty, ur(εr) = 0.003. Relative combined standard uncertainty, Urc(εrE) = 0.03.
nD measurement: Bellingham+Stanley RFM970 refractometer. Temperature control: Peltier modules (±0.02 K). Precission of the method: Standard uncertainty, u(nD) = 0.00008.
The excess relative permittivities [1], εrE (Fig. 1), and their derivative with respect to temperature at constant pressure, (∂εrE /∂T)p (Fig. 2), have been fitted to unweighted linear least squares Redlich-Kister regressions. The number of coefficients of the polynomial is determined by an F-test of additional term at 99.5% confidence level.
Properties of 1-alkanol + TEA systems at temperature T = 298.15 K and pressure p = 0.1 MPa Notation
RESULTS
Symbol
1 m r 1Vm Bm 0E 1 Sm m T E E p T , p 0
CONCLUSIONS
9kBTVm 0 ( r r )(2 r r ) gK N A 2 r ( r 2) 2
Meaning and comments
Vm
Molar volume. Data taken from literature [3].
ϕ1
Volume fraction of 1-alkanol.
χm
Molar dielectric susceptibility.
Bm
Molar macroscopic dipole moment.
Sm
Molar entropy.
E
Electric field intensity.
gK
Kirkwood correlation factor.
μ
Gas-phase molecular dipole moment.
εr∞
High-frequency relative permittivity. Estimated from nD.
gK
E
Excess Kirkwood correlation factor.
The εrE curve of the methanol system shows positive and negative values. The former are encountered at higher concentrations of methanol and reveal that, in that region, the contribution to εrE from methanol-TEA interactions is dominant. The minima of the εrE curves follow the pattern: 1OH > 3OH > 4OH < 5OH < 7OH. The increase of the chain length of the 1-alkanol leads to εrE curves which are skewed to higher ϕ1 (volume fraction of 1-alkanol) values. Such a behaviour is also encountered in 1-alkanol + hexylamine, + dipropylamine or + cyclohexylamine [2] systems and can be explained in terms of a lower and weaker self-association of longer 1-alkanols. This also explains why, at low TEA concentrations, the difficulty for a change in the electric field to decrease the entropy by orientating the dipoles is lower for longer 1-alkanols (Fig. 4). Calculations on gk (Kirkwood correlation factor [4], Fig. 5) support this conclusion, indicating that the structure of the mixture changes rapidly with the TEA concentration for 3OH, 4OH, 5OH, 7OH. In contrast, for 1OH the gK curve varies very little for ϕ1 > 0.7. The minima of the gKE curves (Fig. 6) occurs at lower ϕ1 than in the εrE curves. Therefore, according to the Kirkwood-Fröhlich model, the destruction of dipole correlations is not the only responsible for the εrE minima, but there are other effects involved.
REFERENCES [1] J.C.R. Reis, T.P. Iglesias, G. Douhéret, M.I. Davis; Phys. Chem. Chem. Phys. 11 (2009) 3977–3986. [2] J.A. González, L.F. Sanz, I. García de la Fuente, J.C. Cobos; J. Chem. Thermodyn. 91 (2015) 267-278. [3] S. Villa, N. Riesco, I.G. de la Fuente, J.A. González and J.C.Cobos, Fluid Phase Equilib. 216 (2004) 123-133. [4] A. Chelkowski, Dielectric Physics, Elsevier, Amsterdam, 1980.
ACKNOWLEDGEMENTS This work receives financial support from Junta de Castilla y León (Project BU034U16). F. Hevia and A. Cobos are grateful to Ministerio de Educación, Cultura y Deporte for the grants FPU-14/04104 and FPU-15/05456 respectively.
