Elsevier

Fluid Phase Equilibria

Volume 358, 25 November 2013, Pages 161-165
Fluid Phase Equilibria

Viscosity of liquid systems involving hydrogenated and fluorinated substances: Liquid mixtures of (hexane + perfluorohexane)

https://doi.org/10.1016/j.fluid.2013.07.060Get rights and content

Abstract

The viscosity of (hexane + perfluorohexane) mixtures has been measured at 298 K, 303 K and 308 K, over the whole composition range. The results show large negative deviations from the arithmetic mean of the viscosities of the pure components, reaching −17% at x(perfluorohexane) = 0.7. To obtain molecular level insight into the behaviour of the system, all-atom molecular dynamics simulations have been performed and used to calculate the viscosities, radial distribution functions, and rotational relaxation times of the studied (hexane + perfluorohexane) mixtures. This is the first effort to assess the effect of mixing hydrogenated and fluorinated molecules in the viscosity.

Introduction

It is well known that alkane + perfluoroalkane mixtures display large deviations from ideal behaviour (liquid–liquid immiscibility, large positive excess volumes and enthalpies, positive deviations to Raoult's law, etc.). In the case of transport properties, even though the concept of ideality is not clearly defined, atypical behaviour could also be expected due to the alleged weak cross interactions between hydrogenated and perfluorinated chains. However, despite its obvious importance, the subject has not been addressed in the literature. The only measurements of viscosity reported for mixtures of alkanes and perfluoroalkanes are those of McLure and Clements [1], who focused on the discontinuity in viscosity near a liquid–liquid critical temperature. The authors measured the viscosity of a single (hexane (H6) + perfluorohexane (F6)) mixture, at the critical composition (x(F6) = 0.370), at several temperatures above the liquid–liquid upper critical solution temperature (UCST). Below the UCST, the viscosity of both phases in equilibrium was also measured at several temperatures.

The knowledge of the viscosity behaviour of alkane + perfluoroalkane mixtures is of special importance in the field of biphasic synthesis and catalysis known as “fluorous chemistry”, since the solvents used are (partially miscible) mixtures of hydro- and fluorocarbons. Additionally, this knowledge is of great benefit to the interpretation of perfluoroalkylalkane (PFAA) viscosities, recently measured [2] in our group. PFAA are diblock compounds made of alkyl and perfluoroalkyl segments covalently bonded to form a single chain. They can be pictured as chemical mixtures of two mutually phobic segments that in most cases would otherwise phase separate.

Structurally, the substitution of hydrogen for the larger and heavier fluorine atom results in a larger cross-sectional area for perfluorinated chains [3] (0.283 nm2 compared to 0.185 nm2 for n-alkanes) and also in higher densities and molar volumes for n-perfluoroalkanes, when compared to n-alkanes with the same number of carbon atoms. Consequently, in a mixture of a n-alkane with the corresponding perfluoroalkane, the volume fraction of perfluoroalkane is always considerably higher than its mole fraction and a probe immersed in such a mixture would “experience” a much more fluorinated environment than expected from the molar composition. Another important difference between fluorinated and hydrogenated chains is conformational. While the n-alkanes tend to be in an all-trans planar form, n-perfluoroalkanes display a helical conformation [4]; also, the flexible character of hydrogenated chains contrasts with the rigidity of their perfluorinated counterparts. It is believed that one of the consequences of chain stiffness in liquid fluorocarbons is a less efficient molecular packing and the existence of “holes” in the liquid. This can explain (at least in part) the enhanced solubility of simple gases (oxygen, nitrogen, etc.) in liquid perfluoroalkanes. On the other hand, perfluoroalkanes tend to be more volatile than the corresponding n-alkanes due to their weaker cohesive forces, showing however higher viscosity, which can also be attributed to the known rigidity of their molecular structure.

Considerable changes in volume occur when hydrogenated chains are mixed with fluorinated chains. We have recently reported partial molar volumes at infinite dilution for a series of n-perfluoroalkanes in n-octane [5], [6]. For example, when a molecule of F6 is immersed in n-octane, at infinite dilution, its molar volume increases from 202.4 cm3 mol−1 to 229.3 cm3 mol−1, i.e., 13%. Literally, a layer of empty space is created around the F6 molecule. The effect is even more pronounced when n-alkanes are dissolved in n-perfluoroalkanes: their molar volumes increase by 20% [7]! It should be emphasized that these are infinite dilution data, i.e., no packing effects are included that at higher concentrations would contribute to the excess volume. In view of these results, it can be anticipated that the viscosity of mixtures of alkanes and perfluoroalkanes might be lower than that expected from the viscosity of the pure components.

In this work we have measured the viscosity of mixtures of (H6 + F6) at three temperatures, over the whole composition range. Additionally, all-atom molecular dynamics simulations have been performed and used to calculate viscosity and rotational relaxation times for the studied mixtures, as well as to examine the molecular level structure via radial distribution functions. We believe this is the first study of the transport properties of alkane + perfluoroalkane mixtures.

Section snippets

Experimental

H6 and F6, both with 99% claimed purity, were purchased from Sigma–Aldrich and used as received. The mixtures were prepared by weight in 20 ml screw-cap vials. To ensure internal consistency of the results, the viscosity of H6 was measured in the same experimental conditions as the mixtures. The viscosity of F6 was recently measured by our group [2] using the same method, briefly outlined below.

The viscosities were measured using a Schott-Geräte Type 545–00/0 Ubbelhode viscometer, with the AVS

Simulations

To compare with experiment, atomistic molecular dynamics (MD) simulations have been performed. The optimized potentials for liquid simulations all-atom (OPLS-AA) force field, originally developed for alkanes [8] and extended to perfluoroalkanes [9], was used to model the molecules studied. The OPLS-AA force field was developed to accurately reproduce experimental properties of liquids, and recent studies [2], [10] demonstrate excellent agreement with experiment for pressure vs. density

Results

The experimentally measured dynamic viscosity of H6 is presented in Table 2. The results compare favourably with literature data [15] (deviations are less than 0.5%). Table 3 reports the experimental viscosities of the H6 + F6 mixtures, as well as the calculated deviations from the arithmetic mean of pure component viscosities:Δη=ηexp(ηF6xF6+ηH6xH6)

The experimental viscosity of the pure compounds at each temperature was interpolated using the Andrade equation [17]. Fig. 2a plots the viscosities

Discussion

It is known that binary mixtures often show negative viscosity deviations, as defined in Eq. (4). Hence, most predictive methods for the viscosity of mixtures propose the use of equations similar to Eq. (4), but in terms of the logarithm of viscosity, using subsequent adjustable parameters which are fitted to each binary mixture [17].

To provide additional insight into the mixing behaviour, Fig. 3 compares the experimental viscosities of H6 + F6 mixtures at 298 K with literature results [18] for H6 +

Conclusions

The effect of mixing hydrogenated and fluorinated molecules on the viscosity of the mixture has been assessed for the first time. The viscosity of H6 + F6 has been measured over the whole composition range, at three temperatures and shows large negative deviations from the arithmetic mean of the viscosities of the pure components, reaching −17% at x(F6) = 0.7. The results obtained from molecular dynamics simulations follow the trend exhibited by the experiments, and give additional insight into the

Acknowledgements

PM acknowledges funding from Fundação para a Ciência e Tecnologia, in the form of a Post-Doctoral Grant (Ref.: SFRH/BPD/81748/2011). CMC, CRI, JBL and JB acknowledge partial support from the National Science Foundation under grant number OCI-1047828. JB also acknowledges support from the Department of Education for a Graduate Assistance in Areas of National Need (GAANN) Fellowship under grant number P200A090323.

References (18)

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