The Effects of Sulfonate Molecular Weight and Salt Concentration on the Interfacial Tension of Oil-Brine-Surfactant Systems
Authors: Wilson Peggy M. , Murphy Charles L. , Foster William R. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
This paper was prepared for the Improved Oil Recovery Symposium of the Society of Petroleum Engineers of AIME, to be held in Tulsa, Okla., March 22–24, 1976. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
In support of a program in low tension waterflooding, the interfacial activities of a large number of experimental and commercial petroleum sulfonates and synthetic alkylaryl petroleum sulfonates and synthetic alkylaryl sulfonates were studied. Interfacial tensions against crude oils of the order of 10-4 dyne per centimeter can be obtained with aqueous per centimeter can be obtained with aqueous solutions of these compounds. Interfacial tensions are found to be a function of the average molecular weight and concentration of the sulfonate, the nature and concentration of the electrolyte, and the crude oil. Against Loma Novia crude oil, lowest tensions are produced when the average molecular weight of the produced when the average molecular weight of the gas oil aromatic fraction, prior to sulfonation, is about 305. This is also the optimum weight of sulfonates that produce lowest tensions at lowest sulfonate concentration levels. Within the range studied, variations in the structure of the alkyl side chains and the aromatic rings of synthetic alkylaryl sulfonates do not grossly affect interfacial activity. The optimum salinity declines as sulfonate molecular weight increases.
Molecular compositions were examined for several sulfonates. Molecular weight distribution data are shown for sulfonate compositions that produce low tensions and for one that does not. Also, chromatographic transport through a column packed with Sephadex LH-20 results in separation of a petroleum sulfonate into interfacially active and inactive components. Original behavior is restored by recombination of components.

Introduction
Several authors have reported the need for low interfacial tensions between the crude oil and the aqueous phase in tertiary oil recovery by immiscible displacement processes. However, there is no adequate processes. However, there is no adequate molecular theory of liquid-liquid interfacial tension with which to correlate data and develop predictive methods. New studies toward this goal have begun in several universities. Meanwhile, statistical mechanical theories of surface tension between a pure liquid and its vapor can provide some insight into liquid-liquid systems.