pure surfactant, pure oil, and simple brine, provided the basic know-how on the physico-chemical formulation effect on surfactant–oil–water systems. Improving the understanding by using systems which do not exhibit complex behavior, i.e. This is what is presented in this second part of the review, but the techniques to study experimentally the performance improvement through mixtures will be discussed in the third part. Thanks to the clearer understanding of the performance variations through the analysis of the simple system cases, an organized review to the more complex practical system can be proposed, with some ideas for potential future improvement. Since Winsor’s premise in the 1950s, many studies have reported the performance of a huge variety of surfactants and co-surfactants, with different head and tail groups, and depending on the other variables like oil (E)ACN, brine salinity, and temperature. But when the limit cannot be displaced, another way is necessary to generate a minimum within the feasible range, which is carried out by so-called synergistic effects, which often take place with mixtures of components. As a consequence displacing a limit could be a way to attain a better performance. It also shows that the characteristics of the iso-performance contours do not involve any minimum in such very simple system, but an improvement in some direction of change until a restriction or limit is attained. It shows for the first time that the tension performance is in effect simply related with the four variables, which are actually the main ones from the physicochemical point of view. Such an analysis is reported in this article based on a published study of very pure systems. Since this correlation for the attainment of a tension minimum in simple cases systems involves only four variables, it has been thought that the scrutiny of such a simple situation could improve the understanding and that some general tendencies could be found. Some possible trends have been found, but not as the effect of each formulation variable, and with some discrepancies probably due to a very large number of variables in most practical cases. At optimum formulation, a minimum tension is attained, but the value of the minimum, which is a measure of the performance for enhanced oil recovery, has not been still clearly related to the formulation. It was reported that for simple systems containing pure components, the optimum formulation takes place when a simple linear correlation is satisfied by four variables representing the oil, water, and surfactant nature, as well as temperature. Hence, a minimum tension occurrence should be sought under such an optimum condition. In the first part of this review it is shown that the minimum tension in a formulation scan is attained at the so-called optimum formulation, in which the affinity of the amphiphile(s) at interface is exactly the same for the oil and water phases at the given temperature. 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. 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 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. This relation is probably too simple when the number of variables is increased as in practical cases. 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. 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 minimum interfacial tension occurrence along a formulation scan at the so-called optimum formulation is discussed to be related to the interfacial curvature.
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