posterH2OinPFCs

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Published on January 1, 2008

Author: Dabby

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SOLUBILITY OF WATER IN LIQUID PERFLUOROCARBONS M. G. Freire1, L. M. N. B. F. Santos2, I.M. Marrucho1 and J.A.P. Coutinho1 1CICECO, Departamento de Química, Universidade de Aveiro, Aveiro, Portugal 2Centro de Investigação em Química, Departamento de Química, Faculdade de Ciências, Universidade do Porto, Porto, Portugal :  SOLUBILITY OF WATER IN LIQUID PERFLUOROCARBONS M. G. Freire1, L. M. N. B. F. Santos2, I.M. Marrucho1 and J.A.P. Coutinho1 1CICECO, Departamento de Química, Universidade de Aveiro, Aveiro, Portugal 2Centro de Investigação em Química, Departamento de Química, Faculdade de Ciências, Universidade do Porto, Porto, Portugal Fluorinated molecules such as perfluorocarbons (PFCs) and their derivates represent a very interesting and stimulating class of chemicals in the physical chemistry and polymer science due to their specific and unusual properties. It is however in the biomedical field where most of the relevant applications of PFCs are found. They can be used in tissue oxygenation as blood substitutes, anti-tumural agents and perfusates for isolated organs, gas-carriers in eye surgery, diagnostic imaging agents, drug delivery and in the aeration of biological reactors [1]. The aim of this work is the study of water solubility in liquid perfluorocarbons at several temperatures since data of this property are extremely scarce in the literature. A 737 model Karl Fisher titrator was used and it has been adapted for measuring the low solubility of water in the perfluorocarbons solvents. The study of pure perfluorocarbons is based in measurements of the solubility of water in linear, cyclic and aromatic perfluorocarbons in the temperature range between 295.7 and 309.6 K and at atmospheric pressure. Since to our knowledge there are no known data in literature for the PFCs, the method was validated with the determination of the n-heptane solubility in water in the same temperature range comparing the measures with literature data. Experimental values of water solubility in PFCs, expressed in mol fractions, are presented in Table 1. Table 1. Solubility of water in liquid PFCs From the Gibbs-Duhem equation, assuming that for such a diluted solution both the activity coefficient for water in the water phase and that for the organic component in the organic phase are equal to one, it’s possible to obtain the following approximation [2]: The linear dependence of ln(x) with temperature for an individual case (C6F6) and for the linear compounds studied is shown in Figures 1 and 2, respectively. Experimental data is well described by the linear representation of ln(x) with temperature for all the studied cases. The solubility of water in the linear PFCs is fairly insensitive to the carbon number (CN), rising just slightly with increasing CN because it seems to be primarily influenced by the breaking of hydrogen bonds in water. The water solubility in PFCs is strongly dependent on the temperature. The determination of water solubility has a particular significance since it may be a way to access to thermodynamic properties of these systems. Solubility measurements at different temperatures allow the determination of thermodynamic parameters of the solution that are difficult to obtain using direct methods, such as ∆Hº. The thermodynamic parameters are presented in Table 2. Table 2. Thermodynamic parameters at 298.2 K for the water dissolution in PFCs The heat of solution for water in PFCs is always positive and is nearly the same for all the PFC compounds and slightly higher than the heat of solution for the n-alkanes. This increase may be due to the larger size of the PFC molecules increasing the difficulty of the dissolution of water. Typical values for the energy of a “normal” hydrogen bond range between 20 and 40 KJ.mol-1 suggests that the dissolution of n water molecules leads to the breaking of n hydrogen bonds, which to a good approximation is independent of both carbon number and temperature. The lone pair electrons of oxygen interaction with the  system of the hexafluorobenzene ring reduces the heat of solution increasing the solubility of water on this compound.

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