The Determination Of Mass And Equivalent Weight Of Petroleum Sulfonates By HPLC
Authors: McQuigg K.W. , Willhite G.P. , Michnick M.J. 
Published: December 2007

Abstract
A method for analysis of petroleum sulfonates using high pressure liquid chromatography was extended to systems where fractionation of the sulfonate occurs. Response factors in terms of mass and equivalent weight were determined from systems containing Witco TRS 10-80 Petronate in which mono to disulfonate ratios were altered. A correlation was developed relating the equivalent weight of the fractionated petroleum sulfonate to the portion of the chromatogram petroleum sulfonate to the portion of the chromatogram area represented in the disulfonate peak. The correlation gives some information on average molecular weights of the major constituents in the sulfonate.
Application to other petroleum sulfonates was demonstrated using Stepan Petrostep-500. Response factors and equivalent weight correlations were developed from the analysis of five equilibrated systems.

Introduction
Petroleum sulfonates are a class of surfactants which can reduce interfacial forces to the levels required to attain near miscible displacement of residual oil by a carefully formulated surfactant-solutian. Quantitative analysis of petroleum sulfonates is important in studies of displacement mechanisms as well as analysis of field tests to infer process efficiency. process efficiency. Several wet laboratory techniques for sulfonate analysis such as the Epton method, have been developed but no technique is completely satisfactory. Analysis is difficult because the surfactants are mixtures with a range of molecular weight and degree of mono, di and polysulfonation. Major difficulties in these methods include the development of a meaningful calibration curve for fractionated sulfonates, and incomplete analysis of low molecular weight sulfonates.
High pressure liquid chromatography using an ion exchange column has been investigated as a tool to overcome these limitations as well as permit automation of the analytical technique. Suffridge and Zornes et al have shown the method is satisfactory for unfractionated petroleum sulfonates. Analysis of low equivalent weight sulfonates and solutions believed to contain polysulfonated sulfonates has been demonstrated. This method also requires development of a calibration curve to relate the chromatograph areas to concentration and equivalent weight when the sulfonate has been fractioned between phases.
Zornes et al included a few examples of fractionated samples which indicated some adjustment of average response factors was necessary. A method was proposed to account for fractionation by altering average response factors for change in equivalent weight.
The purpose of the research described in this paper was to extend the HPLC technique for analysis of paper was to extend the HPLC technique for analysis of phases containing fractionated petroleum sulfonates. phases containing fractionated petroleum sulfonates. This was done by analysis of a series of equilibrated systems in which a wide range of fractionation was indicated by equivalent weight analysis and chromatogram areas.

THEORY

DETERMINATION OF HPLC RESPONSE FACTORS
The analysis of solutions containing fractionated petroleum sulfonates by HPLC is based on separation of petroleum sulfonates by HPLC is based on separation of a sulfonate sample into two principal peals which are detected in the UV range. Figure 1 is a chromatogram (254 nm) of a sample containing fractionated TRS 10-80 Petronate. The second peak represents about 81% of Petronate. The second peak represents about 81% of the total chromatograph area. An unfractionated sample of TRS 10-80 using the same operating conditions has about 23% of the total area in the second peak. Conversion of absorbance measurements to concentrations requires development of response factors which relate the absorbances measured in area units for each peak on a chromatogram to concentration. The fundamental relationship describing the effect of concentration on UV absorbance is Beer's law. Beer's law has been shown to be valid for mixtures of petroleum sulfonates where the same molecular species are present in the same proportion at each concentration.