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Surfactant–oil–water systems exhibit a low interfacial tension minimum when the interactions between the adsorbed surfactant with water exactly coincide with its interactions with oil. This occurrence takes place at the so-called optimum formulation, which was conceptually derived by Winsor in the 1950s and rendered by numerical correlations for enhanced oil recovery in the 1970s. The actual low value of the interfacial tension minimum has been found to increase or decrease with formulation variables and though some hints are available, no general relationship has been reported up to now, probably because too many variables are involved in complex interactions. It is shown in the present article that a linear relationship between low-tension performance and formulation variables can be found for very simple ternary systems containing a pure ethoxylated alcohol, n-alkane and water at variable temperature, i.e., when there are only four degrees of freedom. In such a case the iso-performance contours studied in bi-dimensional spaces are reported to be almost straight lines and as a consequence the path to lower the tension through formulation adjustments is easy to find as being perpendicular as possible to the contours. On the other hand, it is shown that displacing the limit of restrictions like the surfactant precipitation boundary is a priority issue, thus justifying many trends which have been proposed on empirical grounds in the past years. The reported simple guidelines for a simple surfactant–oil–water ternary is likely to considerably facilitate the formulator's work in a real system with a score of formulation variables.

The minimum interfacial tension occurrence along a formulation scan at the so-called optimum formulation is discussed to be related to the interfacial curvature. The attained minimum tension is inversely proportional to the domain size of the bicontinuous microemulsion and to the interfacial layer rigidity, but no accurate prediction is available. The data from a very simple ternary system made of pure products accurately follows the correlation for optimum formulation, and exhibit a linear relationship between the performance index as the logarithm of the minimum tension at optimum, and the formulation variables. This relation is probably too simple when the number of variables is increased as in practical cases. The review of published data for more realistic systems proposed for enhanced oil recovery over the past 30 years indicates a general guidelines following Winsor’s basic studies concerning the surfactant–oil–water interfacial interactions. It is well known that the major performance benefits are achieved by blending amphiphilic species at the interface as intermolecular or intramolecular mixtures, sometimes in extremely complex formulations. The complexity is such that a good knowledge of the possible trends and an experienced practical know-how to avoid trial and error are important for the practitioner in enhanced oil recovery.