Diffusion Coefficients of 2,2,2-trifluoroethanol/water mixtures
Luís A. M. Pereira1,2, João P. C. A. P. Ramalho1, Alfredo J. P. Carvalho1, Pedro Morgado2, Eduardo J. M. Filipe2, Luís F.G. Martins1
1CQE-Centro de Química de Évora, University of Évora, Rua Romão Ramalho, 59, 7000-671 Évora, Portugal,2Centro de Química Estrutural, Instituto Superior Técnico, Av. Rovisco Pais 1049-001 Lisbon, Portugal
Keywords: 2,2,2-trifluoroethanol, diffusion coefficient, water, molecular dynamics
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1. Introduction
Aqueous mixtures of 2,2,2-trifluoroethanol have received significant attention in the last years because their applications, such asworking fluid in Rankine cycle thermal engines or as solvent in studies of protein stability (protein folding). From the fundamental point of view, fluoroalcohols have an amphyphile character, due to the simultaneous presence of a hydrophobic (and alkane-phobic) fluorinated surface and a hydroxyl group. Mixtures of 2,2,2-trifluoroethanol with hydrogenated alcohols were studied recently in our research group (experimental determination of pVT surfaces as well as thermal, volumetric and structural properties by computer simulation) [1]. In the case of 2,2,2-trifluoroethanol/water this study suggested a preference of cross hydrogen bond. Hydrogen bonds play a determinant role in the structure of aqueous mixtures of alcohols, and should influence the dynamic properties of binary systems of fluorinated alcohols with water. The goal of this work is to study a dynamic property (diffusion coefficient) of binary mixtures 2,2,2-trifluoroethanol/water, comparing the results with equilibrium and structural properties already known for this and related systems.
2. Results
Mutual diffusion coefficients of binary mixtures of 2,2,2-trifluoroethanol with water were measured by Taylor dispersion method as a function molar fraction, for dilute solutions atfour different temperatures. Mutual diffusion coefficients at infinite dilution as a function of temperature were obtained by extrapolation. In parallel, intra-diffusion coefficients of 2,2,2-trifluoroethanol in water were obtained by molecular dynamics also for dilute solutions (infinite dilution limit) at several temperatures. Mutual diffusion coefficients were estimated and compared with experimental ones. The results were also compared with intra-diffusion coefficients from literature [2] and rationalized in terms of equilibrium properties of this binary system and other related systems (such as hydrogenated alcohols/water) [3].
3. Conclusion
The conclusions can be summarized as follows: 1) Experimental and simulated results of mutual diffusion coefficients of 2,2,2-trifluoroethanol/water mixtures compare favourably. 2) Mutual diffusion coefficients for the system 2,2,2-trifluoroethanol/water were consistently lower than those for ethanol/water system. The higher molecular weight of 2,2,2-trifluoroethanol and a stronger hydrogen bond between 2,2,2-trifluoroethanol and water in comparison with ethanol could be two possible reasons for this tendency.
References
[1]Duarte, P., Rodrigues, D., Silva, M., Morgado, P., Martins, L.F.G., Eduardo J. M. Filipe, E.J.M. (submitted to publication)
[2] Harris, K.R., Newitt, P.J.,Derlacki, Z.J., J. Chem. Soc., Faraday Trans.,94(14), 1963-1970 (1998)
[3] Harris, K.R., Goscinska, T., Lam, H. N., J. Chem. Soc., Faraday Trans.,89 (12), 1969-1974 (1993)