Interfacial Tension and Density of Water + Branched Hydrocarbon Binary Systems in the Range 303-343 K Carlos Gilberto Aranda-Bravo, † Ascencio ´n Romero-Martı ´nez,* Arturo Trejo,* and Jacinto A ´ guila-Herna ´ndez Instituto Mexicano del Petro ´leo, Programa de Ingenierı ´a Molecular, A ´ rea de InVestigacio ´n en Termofı ´sica, Eje Central La ´zaro Ca ´rdenas Norte 152, 07730, Me ´xico, D.F., Me ´xico Experimental results for the liquid-liquid interfacial tension of water + branched hydrocarbon binary systems were obtained using the pendant drop method. The branched hydrocarbons included in this study were 2-methylpentane and 3-methylpentane as structural isomers of C 6 H 14 ; 2,3-dimethylpentane as isomer of C 7 H 16 ; and 2,2,4-trimethylpentane and 2,3,4-trimethylpentane as isomers of C 8 H 18 . The temperatures at which the experiments were carried out were 303.15, 313.15, 323.15, 333.15, and 343.15 K. Density values of both saturated liquid phases were also experimentally determined. An experimental apparatus for achieving the liquid-liquid equilibrium with online sampling and density measurement was developed. Density values obtained for both saturated liquid phases follow the expected behavior with temperature: they decrease with increasing temperature, whereas the effect of the structural isomerism on density was mainly observed on the results for the hydrocarbon-rich liquid phase. Also, the interfacial tension values decrease with increasing temperature for a given water + hydrocarbon binary system and as the size of the hydrocarbon increases. Estimated values for the interfacial tension of the systems with the different branched isomers were obtained using a method developed in previous work. 1. Introduction Interfacial tension is an important thermophysical property that has high relevance for both scientiﬁc and practical applica- tions. It reﬂects the type of molecular interactions that take place during the liquid-liquid contact in systems conformed by molecules with marked difference in chemical nature and molecular interactions. These are some of the characteristics that make a given system to present liquid-liquid partial miscibility, which indicates high nonideality of the liquid phases of the system under study. Interfacial tension governs the mass and heat transfer between two liquid phases and also the stability of emulsions, the mobility of a liquid trough oriﬁces, wetting, and miscibility. 1 In the oil industry, interfacial tension is important because it plays a key role in different processes, such as liquid-liquid extraction and enhanced oil recovery. 2 It is also an important property to consider for the study of the liquid contact between liqueﬁed petroleum gas (LPG) and the aqueous solvent used to remove acid gases during the sweetening of this type of hydrocarbon-rich stream. 3 The physical manifestation of the interfacial tension, either liquid-liquid, solid-liquid, or liquid-gas, is one of the characteristics that has been widely studied to apply a large family of chemical products, known generically as surfactants. These compounds are found in many different applications, such as managing emulsion formation or rupture, enhancing the solubility of components (gases or liquids) in gas-liquid or liquid-liquid systems, modifying the formation of clusters of hydrates to inﬂuence the formation or rupture of foams, and so forth. Also, in the distribution of particles of immiscible metals in solid solutions, the effect of interfacial energy is crucial in the solidiﬁcation processes of monotectic alloys. 4 The systematic work carried out by our research group on the understanding of the inﬂuence of interfacial properties in different applications of industrial interest includes the evalu- ation and selection of solvents for the sweetening of liquid hydrocarbon streams, 5 the study of solvents to enhance the liquid extraction capacity and selectivity for aromatic components in a given hydrocarbon stream, 6 and the study of interfaces to establish the characterization of defoamers or foaming inhibitors in sweetening processes. 7 These efforts have taken us to develop some contributions, experimental, 8 correlation, 9 and prediction, 10,11 on the surface properties of systems of interest for the oil and natural gas production, transport, and processing industries. The experimental results obtained in this work correspond to the interfacial tension of binary systems formed by water + a hydrocarbon, where the hydrocarbon is an isomer with the following structural molecular formulas: C 6 H 14 (hexane, 2-me- thylpentane, and 3-methypentane), C 7 H 16 (heptane, 2,3-dimeth- ylpentane), and C 8 H 18 (octane, 2,2,4-trimethylpentane, and 2,3,4- trimethylpentane). The temperatures at which this study was conducted were 303.15, 313.15, 323.15, 333.15, and 343.15 K. For these measurements, the pendant drop method was used. The experimental results were obtained with an average uncertainty of (0.07 mN m -1 . Also, as part of this study, density values of the saturated liquid phases formed by the partially miscible binary systems studied here were obtained using a vibrating tube densimeter. Density values were obtained for the saturated liquid (water- rich and hydrocarbon-rich) phases with an uncertainty of (0.005 kg m -3 at 303, 313, and 323.15 K, (0.010 kg m -3 at 333.15 K, and (0.050 kg m -3 at 343.15 K. 2. Experimental Section The materials used in this study were as follows: nitrogen (AGA Mexico, 99.5 mol %), hexane (Aldrich, lot 06723EQ, 99.1 mol %), 2-methylpentane (Aldrich, lot 07622HS, 99.7 mol %), 3-methylpentane (Aldrich, lot 01016KX, 99.9 mol %), * To whom correspondence should be addressed. Tel.: +5255 9175 8366; +5255 91758373. E-mail: aromero@imp.mx; atrejo@imp.mx. † Postgraduate bursar of the Mexican Petroleum Institute. Ind. Eng. Chem. Res. 2009, 48, 1476–1483 1476 10.1021/ie801101r CCC: $40.75  2009 American Chemical Society Published on Web 12/18/2008