Characterizing Petroleum Sulfonates By Phase Behavior
Authors: Sitton D.M. 
Published: December 2007

Abstract
A novel technique has been developed that is a fast, reproducible, and accurate method for determining the effective average equivalent weight of a petroleum sulfonate. The technique simultaneously provides information on the phase behavior of provides information on the phase behavior of a formulation of the sulfonate, similar to formulations used for tertiary oil recovery, with respect to hydrocarbons having different equivalent alkane carbon numbers (EACN). The method is based on the phase behavior obtained when 3 parts by volume of pure hydrocarbon are equilibrated at 25 degrees C with 5 pure hydrocarbon are equilibrated at 25 degrees C with 5 parts by volume of a sulfonate composition containing parts by volume of a sulfonate composition containing 3.6 weight percent active sulfonate, 3.0 weight percent isobutyl alcohol, 1.5 weight percent sodium percent isobutyl alcohol, 1.5 weight percent sodium chloride and distilled water.
The sulfonate systems exhibit one of three different types of multiphase regions when equilibrated with oils of different values for the EACN. One region consists of two phases with the sulfonate in the lower phase (gamma type), another region consists of three phases with the sulfonate in the middle phase (beta type), and the third region consists of two phases with the sulfonate in the top phase (alpha type). The sulfonate appears in the phase (alpha type). The sulfonate appears in the upper phase for low EACN oils, in the middle phase for higher EACN oils, and in the lower phase for the oils having the highest EACN values. If the volumes of the phases axe plotted against EACN (a phase-volume diagram), then a linear relationship exist the EACN at the midpoint of the three-phase region and the average equivalent weight of the sulfonate.

Introduction
Characterizing petroleum sulfonates by the average equivalent weight method described in ASTM Procedure D-855-56 is usable only for a sodium Procedure D-855-56 is usable only for a sodium sulfonate and is very time-consuming. Analytical characterization methods based on anionic surfactant dye complexes are subject to various problems from impurities in the dye, and salt effects and the interference of the unreacted oil in the sulfonate. The results of numerous investigations of the phase behavior of sulfonate systems have suggested phase behavior of sulfonate systems have suggested that sulfonates can be characterized by the phase behavior obtained in an oil-sulfonate-alcohol-brine system.
It has been shown that systems consisting of oil, sulfonate, alcohol, and brine exhibit three different types of multiphase regions:a microemulsion in equilibrium with an oil phase (herein-after referred to as a gamma-type system);a microemulsion in equilibrium with both an oil phase and a water phase (a beta-type system); anda microemulsion in equilibrium with a water phase (an alpha-type system).
Among the variables that affect the region in which a particular system will appear are salinity, oil type, sulfonate average equivalent weight,) alcohol type, and temperature. With all variables fixed except salinity, the system will shift from a gamma type to a beta type to an alpha type as the salinity increases from zero.
In this study all variables were fixed except oil type, and it was found that the system goes from an alpha type to a beta type to a gamma type as the oil changes from one with a low carbon number to one with a high carbon number. Systematic trends are observed when the phase-volumes are plotted vs oil type (a phase-volume diagram) for different sulfonates. It was found, for example, that for a low average equivalent weight sulfonate, such as Witco TRS-40 (340 equivalent weight), the beta region occurs at low carbon numbers (EAGN's), while for a higher average equivalent weight sulfonate, such as Witco TRS 10-410, the beta region occurred at higher EACN's.
As we shall see, the carbon number at the midpoint of the beta region of the phase-volume diagram (denoted by m) is related linearly to the average equivalent weight of three Witco sulfonates and 10 mixtures of Witco sulfonate. It will be seen, however, that the beta region will not appear necessarily at any carbon number in the range C1 to C16 for all sulfonates.