Densities and interfacial tensions for fatty acid methyl esters (from methyl formate to methyl heptanoate) + water demixed mixtures at atmospheric pressure conditions

Experimental determination of mass densities and interfacial tensions (IFT) at liquid-liquid equilibrium (LLE) for methyl ester (FAME) demixed aqueous family (from methyl formate to methyl heptanoate) are reported as a function of temperature at atmospheric pressure (101.3 kPa). Mass density determinations are carried out in a vibrating tube densimeter, whereas a spinning drop tensiometer is used for performing IFT measurements. Furthermore, the IFT data have been correlated by using a simplified version of the Square Gradient Theory (SSGT) as applied to the Non-Random Two Liquids (NRTL) activity coefficient model. Based on the experimental results, the mass densities of both aqueous and methyl ester bulk phase decrease with temperature, whereas the IFT increases with the temperature and reaches a maximum value, which is related to the maximum tie line in the LLE. For higher temperatures, IFT starts to decrease with temperature. The correlation approach of IFT (SSGT + NRTL) was found to be suitable for correlating the IFT of the mixtures as a function of temperature and also provides a route to explore other interfacial properties, such as interfacial concentration profiles, which are difficult to measure. Considering the results from the calculated interfacial profiles along the interfacial region, the components (FAMEs and water) neither exhibit surface activity nor adsorption activity and the interfacial width decreases with temperature, as it was expected.