On Interfacial Tensions Measured With Alkyl Aryl Sulfonate Surfactants
Authors: Puig J.E. , Franses E.I. , Davis H.T. , Miller W.G. , Scriven L.E. 
Published: December 2007

Abstract
Tensions of aqueous solutions and dispersions of sodium p (1-heptylnonyl/benzene sulfonate against decane are not ultralow in the absence of salt, but can fall below 0.01 dyne/cm (0.01 mN/m) with 0.3 wt% (3 g/kg) NaCl, depending on order of mixing, preparation age, and decane drop size. A dispersed preparation age, and decane drop size. A dispersed liquid crystalline phase revealed by spectroturbidimetry and microscopy appears responsible for ultralow tensions. Results with petroleum sulfonate Witco TRS 10-80 support this conclusion.

Introduction
The importance of ultralow [less than 0.01 dyne/ cm (0.01 mN/m)] interfacial tensions between oil and water as the means of achieving high capillary-number displacement in one mechanism of enhancing oil recovery is well recognized. Low tension is related closely to the phase behavior of suitable surfactant with oil, water, and often an alcohol, as well as other additives. Information in the technical and patent literature concentrates on commercial sulfonate surfactant systems. Few studies of the relationship between tension and phase behavior with pure surfactants have appeared. Since the pioneering work of Hartley on molecular structure dependence of interfacial tension, the influences of WOR, salinity, temperature, surfactant molecular-weight distribution, and other factors have been investigated. However, the origins of ultralow interfacial tensions are not yet known.This paper deals with the low-tension regime associated with low surfactant concentrations, i.e., no more than 1 wt% (10 g/kg). In contrast to the second regime, in which microemulsion plays the central role, this regime does not require the presence of a suitable alcohol or other cosurfactant. presence of a suitable alcohol or other cosurfactant. In this regime it is not clear whether ultralow tensions are equilibrium or nonequilibrium properties. Even with compositions capable of producing ultralow tensions, the measured tensions depend on the way the system is prepared, and time effects are seen. Nevertheless, reproducible measurements can be made by observing a fixed experimental protocol. Tensions measured this way with a given surfactant correlate strongly with semiempirical assignment of carbon numbers to a wide range of hydrocarbons. The correlation extends across related sulfonate surfactants and is a potentially useful means of characterizing low-tension formulations. Understanding this correlation may shed light on the mechanism of ultralow tension in the low-surfactant concentration regime and thus may aid process studies. Understanding the factors responsible for nonequilibrium effects is crucial to interpret laboratory and field data and to design surfactant flooding processes rationally. Commercial surfactants are complex mixtures and are characterized inadequately for basic physicochemical study of order-of-mixing effects physicochemical study of order-of-mixing effects and time-varying interfacial tensions. Experience suggested that a representative pure surfactant would display much of the behavior of commercial mixtures, thereby exposing the practically important aspects to systematic scientific study. The U. of Texas group synthesized an agreed-on alkyl aryl sulfonate that is the subject of extensive studies in this laboratory and elsewhere. Results on its phase behavior with brine and with decane are phase behavior with brine and with decane are reported elsewhere. Parallel studies are under way with a petroleum sulfonate surfactant to establish the extent to which a pure surfactant can mimic the behavior of a commercial mixture. Companion investigations are aimed at explaining the unknown microstructures of ultralow tension interfaces.
SPEJ
p. 71