Petroleum & Coal ISSN 1337-7027 Available online at www.vurup.sk/ petroleum-coal Petroleum & Coal 54 (2) 110-113, 2012 THE EFFECT OF NON-IONIC SURFACTANTS ON THE INTERFACIAL TENSION BETWEEN CRUDE OIL AND WATER Amir Mosayeb, Reza Abedini , * Department of Petroleum Engineering, Mahallat Branch, Islamic Azad University, Mahallat, Iran, *corresponding author reza_abedini20@yahoo.com Received November 19, 2011, Accepted April 15, 2012 Abstract In this study, the effect of non-ionic surfactants on the interfacial tension between crude oil and water were investigated. The interfacial tension was measured by rising drop technique. The interfacial tensions decreased significantly in concentration range of (0-1.25 g.l -1 ). By increasing of temperature, the time needed for an interfacial tension to reach equilibrium reduced. The effect of mixed surfactants on the decreasing of the interfacial tension was more to contrast with individual surfactant, because of the synergism. Keywords: Surfactant; Enhanced oil recovery; Interfacial tension; Critical micelle concentration. 1. I ntroduction Surfactants are special classes of molecules that are both hydrophobic and hydrophilic; hence the most stable configuration for these molecules is at the crude oil and water interface. These molecules are generally injected in order to reduce the interfacial tension between crude oil and water. The interfacial tension plays a fundamental role in conventional and enhanced oil recovery (EOR) methods. The use of surfactants in enhancing oil recovery has generally based on the reduction of the interfacial tension between the crude oil and the flooding phase. This procedure can be performed by reducing the capillary forces to improve the microscopic displacement efficiency [1] . Various parameters such as purity of surfactant, class of surfactant, mineral salt, pH and temperature are effective on the performance of surfactant. Purity of the surfactant has a huge impact on the surface tension and interfacial tension. According to previous investigations, if there is a small amount of impurity in the surfactants, interfacial tension has the different treatment against the surfactant concentration and it can be seen minimum values of interfacial tension vicinity to critical micelle concentration [2] . Among the classes of surfactants, non-ionic and anionic surfactants have considerably more effect on the interfacial tension compared to other surfactants. The temperature increases the diffusion and adsorption velocities of surfactants at the interface. Therefore, temperature has remarkable effect on the interfacial rheological properties. Effect of ethoxylated nonyl phenols as non-ionic surfactants with ethoxy group numbers 4 and 9 (nonyl phenol 4 and nonyl phenol 9) on the interfacial tension between crude oil and water were investigated, in this study. These surfactants can be classified as good interfacial tension reducer and wetting agent. These classes of surfactants are very suitable for reducing of interfacial tension when used in conjunction or with other surfactants in specially designed formulations. Bedő et al. [3] extensively studied micellization, structure of micelles and the thermodynamic parameters of micellization of the ethoxylated nonyl phenols. 2. Experimental The crude oil applied as model in this study was obtained from the Refinery of Tehran. Before measurement of interfacial tension crude oil was dehydrated by centrifugation. Specifications and some physical and chemical properties of this crude oil are listed in table 1.