Vapor-Liquid Equilibrium of Ferrocene in Some Organic Solvents Using Spectroscopic Methods M. Aslam Siddiqi* and Burak Atakan Thermodynamics, IVG, Faculty of Engineering, University of Duisburg Essen, Campus Duisburg, Lotharstrasse 1, 47048 Duisburg, Germany Isobaric vapor-liquid equilibrium (VLE) data were measured at p ) 1.01325 bar for the mixtures of ferrocene in ethanol, toluene, hexane, and 2,2,4-trimethylpentane (isooctane). A dynamical equilibrium apparatus has been used to establish the equilibrium between the phases. The spectrophotometric method has been used to measure the compositions of vapor and liquid phases. Ferrocene is only weakly soluble in these solvents. The experimental binary data were correlated with the NRTL and UNIQUAC models, and the parameters were determined. Introduction Chemical vapor deposition (CVD) processes are widely used to deposit thin, highly uniform solid films that are used as mechanical or optical coatings for various purposes. In general, a reactive gas mixture flows continuously through the controlled environment of a reactor process chamber, which contains the substrates on which deposition takes place. Chemical reactions in the gas phase and at the surfaces are activated thermally, by light, or by a glow discharge plasma and lead to the deposition of the bulk material. Solutions of metalorganic compounds are sometimes used to evaporate the metal containing substances () precursors) in order to produce nanoparticles or thin solid films in chemical vapor deposition (CVD). 1 The precursors used are mostly organometallic compounds. 2 The thermodynamic properties of such compounds are scarce, even for such long- known molecules as ferrocene (Fe(C 5 H 5 ) 2 ). We have, therefore, started a program to study the vapor pressures, thermal stability, and phase equilibrium properties of these compounds. Ferrocene is also used as a catalyst in many chemical process industries; hence, its vapor-liquid equilibrium (VLE) behavior in different solvents is also interesting. In the present paper, the VLE of ferrocene in some organic solvents is reported, which should allow the calculation of the vapor composition in a simple way. Both UV/VIS and FTIR spectroscopy have been used successfully for several years in our department to study vapor-liquid and chemical equilibrium of diverse systems. 3-5 For this study, a dynamical equilibrium apparatus has been used to establish the equilibrium between the phases. The samples of the vapor phase and the liquid phase are then taken and analyzed by using UV/VIS spectroscopic methods. Ferrocene absorbs in the range of 300 nm to 650 nm. If any decomposition takes place in the boiling process, the spectra will clearly show this. The VLE of ferrocene in ethanol, toluene, hexane, and 2,2,4- trimethylpentane (isooctane) has been measured under atmo- spheric conditions. Ferrocene is only weakly soluble in these solvents. After the equilibrium is established, small samples of the liquid phase and the vapor phase are taken and analyzed by UV/VIS spectroscopy. The spectra did not indicate any decom- position or complex formation for the concentrations studied here. So the distribution of ferrocene in the liquid and the vapor phase were measured, and activity coefficients for the liquid phase were determined. Since ferrocene is a solid at the investigated temperatures, one can either use the hypothetical liquid reference state or the easily measurable solid reference state. 6 The two reference states for the activity coefficients are discussed. The activity coefficients for ferrocene in solution were much higher throughout than expected from ideal solution behavior (i.e., 1), even if the hypothetical liquid ferrocene reference state is used for calculations. The deviations from ideal behavior are very strong in ethanol and hexane (activity coefficients between 10 and 20) but are also obvious in toluene (γ ) 1.5 to 2). The data have been fitted to some common activity coefficient models (NRTL and UNIQUAC), and the corresponding parameters were determined. Experimental Section Materials. Ferrocene (purity > 98 %) purchased from Fluka was used without further purification as the measured sublima- tion pressures at different temperatures were the same as those obtained with the sample purified by sublimation. Ethanol (purity > 99.8 %), hexane (purity > 99 %), and toluene (purity > 99.7 %) were supplied by Riedel de Haen, and 2,2,4- trimethylpentane (purity > 99.5 %) was obtained from Merck. The densities and the normal boiling points of the solvents were checked and found to match with established literature values. Apparatus and Procedure. The VLE measurements were performed using a dynamic equilibrium still shown in Figure 1. The heating coil (4) immersed into the still (1) (boiling chamber) takes care for the uniform vapor formation. The rising vapor bubbles stir up effectively the contents of the still and convey a considerable quantity of liquid when going through the Cotrell tube (2) so that a vapor-liquid mixture is discharged over the temperature measurement device (Pt-100 thermometer) inserted through the opening (6). The liquid and the vapor phases are separated. The liquid flows back to the still while the vapor is condensed in the cooler and then flows back into the still. The volume of the boiling chamber is about 100 cm 3 , 80 cm 3 of which is occupied by the liquid. Hence the liquid composition is constant during the experiment. * Corresponding author. Fax: +49 203 379 1594. E-mail: siddiqi@ uni-duisburg.de. 1092 J. Chem. Eng. Data 2006, 51, 1092-1096 10.1021/je060002k CCC: $33.50 © 2006 American Chemical Society Published on Web 04/01/2006