Interfacial Tension between Cumene and Aqueous Sodium Dodecyl Sulfate Solutions from (20 to 50) °C and pH between 4 and 9 Javad Saien* and Nafiseh Aghababaei Department of Applied Chemistry, University of Bu-Ali Sina, 65174 Hamadan, Iran Interfacial tension data obtained over the aqueous surfactant (sodium dodecyl sulfate) concentration range of (0 to 34.68 × 10 -5 ) mol‚L -1 , the pH range of 4 to 9, and the temperature range of (20 to 50) °C are presented for the recommended chemical system of cumene and water. The measurements were made using an automatic DuNou ¨ y tensiometer, and interfacial tension values ranged from (27.3 to 37.0) mN‚m -1 . The data show a nonlinear decrease with increasing surfactant concentration and pH; however, there is a linear decrease with increasing temperature. The interfacial pressure was also derived. The interfacial tension and interfacial pressure values were correlated by empirical equations. Introduction The interfacial tension is a property that plays an important role in interphase mass and heat transfer. In liquid-liquid extraction, for example, it is a key parameter affecting the hydrodynamics and contact of phases for mass transfer purposes. The liquid materials to be treated in the chemical industry, however, usually have a level of impurity or contamination that plays a central role in controlling the desired interfacial property in practical applications. The interfacial tension data for pure systems are rich in the literature; however, those for surfactant mixtures are few. A number of recent published works have investigated the modification of interfacial tension by surfactants in a more systematic way. 1,2 In this regard and for practical applica- tions, measurements of interfacial tension for organic + aqueous phases in the presence of surfactants are required. The pH of solutions in contact with organic phases also alters this property. In liquid-liquid extraction, the solute transferred from one phase to another can alter the pH values. This study presents data on the interfacial tension of the phases of cumene (isopropylbenzene) + water, a recom- mended high interfacial tension system for liquid-liquid extraction. 3,4 Using this chemical system has proven to be satisfactory in liquid-liquid extraction systems and avoids the use of highly flammable liquids, having flash points below 32 °C. 5 Solutions of surfactant sodium dodecyl sulfate (SDS), generally used for investigating the influence of contamination on operating conditions, 6-8 are used in contact with cumene, and the effects of temperature and pH are to be examined for each case. The values of interfacial pressure for this system can also be obtained from interfacial tension data of the pure chemical system and that of the surfactant solution. Experimental Section Cumene (>99%) and sodium dodecyl sulfate (g99%) were Merck products and were used as received. Laboratory- distilled water was redistilled prior to experimentation. The solubility between cumene and water is very low, which is below the limits of accuracy of the chromatographic analy- sis. 5,9 The level of purity of sodium dodecyl sulfate was investigated by obtaining its critical micelle concentration in aqueous solution using the conductometric method. 10,11 The conductivity was measured using a Genway 4020 conductometer, and the conductivity cell was calibrated with KCl solution. The measuring cell was immersed in a thermostated bath at 20 °C, keeping the temperature constant within (0.1 °C. From the variation of specific conductivity against the concentration of SDS, the critical micelle concentration obtained is 0.00833 mol‚L -1 , close to the value reported in the literature (0.00825 mol‚L -1 ) obtained with this method. 10 SDS solutions were prepared by mass using a Mettler AE-100 balance with an uncertainty of (0.1 mg. After preparing the main solution, the next desired solutions were obtained by successive dilutions. An aqueous surfac- tant (SDS) concentration range of (0 to 34.68 × 10 -5 ) mol‚L -1 was applied. The uncertainty in the concentration of SDS was estimated to be within (0.02 × 10 -5 mol‚L -1 . NaOH and HCl supplied by Merck were used as pH modifiers; in both cases, a concentration of 2 mol‚L -1 was used to reach the desired pH (between 4 and 9) in the aqueous phase, and pH measurements were performed with a Corning-M140 pH meter having an uncertainty of 0.01. The interfacial tension of the samples was measured using a PC-controlled KSV Sigma 70 tension balance that employs the DuNou ¨ y ring-detachment method. The method is reproducible and has been used by a number of investigators. 1,12-14 The platinum ring (radius, 9.545 mm; wire radius, 0.185 mm) was thoroughly cleaned by immer- sion in a concentrated solution of nitric acid. Then it was rinsed with distilled water, flame dried, washed again with distilled water, and dried before each measurement. The measurements were corrected to the actual values utilizing the corrections suggested by Huh and Mason 15 to compen- sate for the interfacial distortion. To apply the corrections introduced into the equipment software, the density of phases was required. The temperature of the system was controlled by a Multi Temp III thermostat with an uncer- * To whom correspondence may be addressed. E-mail: saien@ basu.ac.ir. 1099 J. Chem. Eng. Data 2005, 50, 1099-1102 10.1021/je050059r CCC: $30.25 © 2005 American Chemical Society Published on Web 04/12/2005