Classification of ester oils according to their Equivalent Alkane Carbon Number (EACN) and asymmetry of fish diagrams of C10E4/ester oil/water systems

• “Fish-tail” temperatures determined for surfactant/ester oils/water systems.
• Equivalent Alkane Carbon Numbers (EACNs) of fifteen ester oils computed.
• EACNs of monoesters expressed in terms of carbon number and position of ester group.
• High solubility of C10E4 in glycerol trioctanoate induces asymmetry of fish diagram.

The phase behavior of well-defined C10E4/ester oil/water systems versus temperature was investigated. Fifteen ester oils were studied and their Equivalent Alkane Carbon Numbers (EACNs) were determined from the so-called fish-tail temperature T* of the fish diagrams obtained with an equal weight amount of oil and water (fw = 0.5). The influence of the chemical structure of linear monoester on EACN was quantitatively rationalized in terms of ester bonds position and total carbon number, and explained by the influence of these polar oils on the “effective” packing parameter of the interfacial surfactant, which takes into account its entire physicochemical environment. In order to compare the behaviors of typical mono-, di-, and triester oils, three fish diagrams were entirely plotted with isopropyl myristate, bis (2-ethylhexyl) adipate, and glycerol trioctanoate. When the number of ester bonds increases, a more pronounced asymmetry of the three-phase body of the fish diagram with respect to T* is observed. In this case, T* is much closer to the upper limit temperature Tu than to the lower limit temperature Tl of the three-phase zone. This asymmetry is suggested to be linked to an increased solubility of the surfactant in the oil phase, which decreases the surfactant availability for the interfacial pseudo-phase. As a consequence, the asymmetry depends on the water–oil ratio, and a method is proposed to determine the fw value at which T* is located at the mean value of Tu and Tl.

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