Density measurements under pressure for the binary system (ethanol + methylcyclohexane) Claus K. Ze ´berg-Mikkelsen a,b , Luis Lugo b , Josefa Ferna ´ndez b, * a Center for Phase Equilibria and Separation Processes (IVCSEP), Department of Chemical Engineering, Technical University of Denmark, Building 229, 2800 Lyngby, Denmark b Laboratorio de Propiedades Termofı ´sicas, Departamento de Fı ´sica Aplicada, Facultad de Fı ´sica, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain Received 3 December 2004; revised 10 March 2005; accepted 16 March 2005 Available online 11 May 2005 Abstract The density of the asymmetrical binary system composed of ethanol and methylcyclohexane has been measured under pressure using a vibrating tube densimeter. The measurements have been performed for eight different compositions as well as the pure com- pounds at eight temperatures in the range 283.15 K to 353.15 K and ten isobars up to 45 MPa. The uncertainty for the reported densities is estimated to be 0.1 kg Æ m 3 . A non-monotonical behavior involving a minimum in the density versus composition has been found for this binary system. The measured data have been used to study the behavior and influence of temperature, pres- sure and composition on the excess molar volume, the isothermal compressibility, and the isobaric thermal expansion, revealing that a volume expansion occurs for this binary system. The results have been interpreted as due to changes in the intermolecular distances or free-volume, disruption of the order molecular structure and the breaking of hydrogen bonds within the self-associating alcohol. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Alcohol; Density; High pressure; Hydrocarbon; Measurements 1. Introduction Within the petrochemical industry alcohols are used in a wide range of important applications, such as addi- tives to gasoline, cosurfactants in oil-in-water emulsions, or in the removal of gas hydrates in pipelines, etc. With the addition of alcohols to hydrocarbon and petroleum fluids, complex fluid behaviors can be developed. In or- der to study and understand the behavior of these sys- tems under various operating conditions, experimental thermophysical property measurements of simplified mixtures can provide valuable information about their behavior under operating conditions both from a funda- mental as well as an applied point of view. An important fluid property required in various engineering and scien- tific disciplines as well as to obtain other thermophysical properties is the density. Despite of this and although naphthenic hydrocarbons are important constituents of petroleum fluids, only a few studies, primarily reporting the excess molar volume, have been performed on bin- ary mixtures composed of an alcohol and a naphthenic hydrocarbon at atmospheric pressure and in the vicinity of ambient temperature [1–8]. But Tu et al. [9] measured the density at 0.1 MPa for several systems involving dif- ferent alcohols and a synthetic C 6+ fraction with a fixed composition of n-hexane, n-heptane, 3-methylpentane, methylcyclohexane, and p-xylene. Studies under pressure have only been done by Ozawa et al. [10], who measured the specific volume of binary (ethanol + methylcyclopentane) mixtures up to 196 MPa. At atmo- spheric pressure, a non-monotonical behavior involving a minimum in the density versus the composition has 0021-9614/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2005.03.016 * Corresponding author. Tel.: +34 981 563100Ext.14046; fax: +34 981 520 676. E-mail address: fajferna@usc.es (J. Ferna ´ndez). www.elsevier.com/locate/jct J. Chem. Thermodynamics 37 (2005) 1294–1304