Thermophysical Behavior of n-Alkane + Alkylbenzoate Mixed Solvents. Measurements and Properties Modeling Santiago Aparicio, Rafael Alcalde, Begon˜ a Garcı´a, and Jose´ M. Leal* Universidad de Burgos, Departamento de Quı´mica, 09001 Burgos, Spain This work contributes to an improved understanding of the structure and dynamics of ester- containing solvents. A broad experimental study has been performed on the thermophysical properties of (C 6 -C 14 ) n-alkane + (C 1 -C 4 ) alkylbenzoate binary solvents over the whole composition range and within the 278.15-318.15 K temperature range. The excess and mixing derived properties were analyzed in terms of structural effects and intermolecular forces. To put forward reliable models that may describe the behavior of these complex mixtures, the cubic equations of state by Soave and Peng-Robinson were combined with a set of 10 different mixing rules to correlate the volumetric and viscometric properties. Likewise, to gain a deeper insight into the structure of these solvents the Prigogine-Flory-Patterson and extended real associated solution models were applied to correlate the excess molar volumes. The different contributions to the excess and mixing properties were analyzed as well as their variation with the nature and size of the components involved. Introduction The experimental study and proper interpretation of the thermophysical properties of pure liquids and mixed solvents play a key role in solution chemistry; physical properties and correlation methods may be regarded as the raw material for chemical processes design. 1 The modeling of processes is critically dependent on an accurate knowledge of the thermodynamic behavior of the solvents involved; hence, potentially unreliable results arising from such operations may incur either unnecessary costs or fail to achieve production targets. 2 Most commercial process simulators are equipped with a set of thermodynamic models and databases; however, these models can neither reliably cover the temperature, pressure, and composition ranges needed nor can they reflect new and innovative compounds. Moreover, to assess the validity for a particular solvent, such models must be probed with experimental data; hence, a combination of experimental data and theoretical mod- eling provides an optimal approach to study pure and mixed solvents. Benzoic acid esters are a very interesting class of solvents. In addition to their dipolar and hydrophobic nature, the easily polarizable π electron system confers a highly selective ability and converts these solvents into ideally suited compounds for a number of valuable applications. Alkyl benzoates are used to control thick- ening, flow, and viscosity properties of cellulose ethers, as fragrances and antibacterial agents in cosmetic formulations, as plasticizers to produce poly(vinyl chlo- ride) (PVC) polymers, and as textile dye carriers for the treatment of synthetic fibers, among others. However, despite these applications and promising properties, there is only scant literature available on these sol- vents. 3-6 On the other hand alkanes, the simplest class of organic compounds, contain no functional group, and the smaller alkanes play an important role as model molecules for the behavior of larger compounds. This work reports a broad study on the (C 6 -C 14 ) n-alkane + (C 1 -C 4 ) alkylbenzoate (methylbenzoate (MB), ethylbenzoate (EB), propylbenzoate (PB), and butylbenzoate (BB)) binary mixtures over the 278.15- 318.15 K temperature range and pursues providing a deeper insight into the intermolecular interactions between alkanes and aromatic esters by extending the database of these systems. 7-10 The measured and derived thermophysical properties were treated accord- ing to different approaches; the volumetric and visco- metric measurements were correlated with the cubic equations of state (EOS) by Soave (SRK) 11 and Peng- Robinson (PR). 12 Due to their accuracy, reliability, simplicity, and speed of computation these EOS are widely used in industry and are contained in the modern process simulation packages. 13-15 Extending the EOS to multicomponent mixtures requires use of suitable mixing rules. In this work two different approaches were used. A reliable method to extend the pure-fluid equa- tion of state to mixtures, the one-fluid van der Waals model, required seven quadratic and nonquadratic meaningful mixing rules. Another plausible approach consists of combining an EOS with an excess Gibbs energy model; hence, pursuant to the Wong-Sandler model, 16 three different mixing rules were also applied. Although cubic EOS are quite valuable tools for correlation and/or prediction purposes, in practice they are not most useful for learning fluid properties at a microscopic level; 13 hence, the Prigogine-Flory-Patter- son (PFP) 17 and modified extended real associated solution (ERAS) 18 models were used to correlate the volumetric properties; to obtain meaningful results a global optimization procedure was applied, and thus the parameters and contributions evaluated were analyzed in terms of intermolecular forces. Experimental Section Reagents. Pure solvents, of the highest purity com- mercially available, were purchased from Fluka and Aldrich. They were degassed with ultrasound and kept * Corresponding author. E-mail: jmleal@ubu.es. 7575 Ind. Eng. Chem. Res. 2005, 44, 7575-7583 10.1021/ie0502281 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/20/2005