Winsors basic premise

The first hint about boosting the quality of a microemulsion formulation was proposed by Winsor 50 years ago. In a formulation scan, the best solubilisation is attained when R = N/D = 1, i.e. when the numerator (N) and denominator (D) are equal. Consequently, the comparison should always be carried out between two optimum formulations, i.e. two cases in which R = 1. But R = 1 may be attained in different ways, for instance as ratios such as 2/2 or 5/5, i.e. with equal interactions of the surfactant for both phases, but with different magnitudes of interactions. In order to compare a case R = 1 =2/2 and a case R= 1 = 5/5, two compensating changes are required. For instance, a salinity change would alter the denominator, whereas an ACN change would alter the numerator. 

The way to carry out quality testing experiments is thus to select two variables, one as the scan variable and the other as the perturbation variable, for instance salinity S and oil ACN. For a given ACNi oil, a salinity scan is carried out and an optimum salinity S is found. Then, the oil nature is changed to ACN2, and a new salinity scan is carried 

Both changes lead to an increase in the solubilisation capacity, while the overall formulation is still the same [41]. Consequently, surfactants with bulkier groups on both sides are likely to be more efficient. For instance, in a system containing equal volumes of water and oil and a polyethoxylated alkylphenols an optimum formulation is found at EON = 5.1 for the nonyl species, and at EON = 8.3 for the dodecyl one at ambient temperature (solubilisation of 8 and 21 mL of oil and water per gram of surfactant). The value of the optimum solubility parameter changes with the alcohol content, as will be discussed next, but the trend is the same for all alcohols. 

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