Interfacial studies of essential oil—water systems

Abstract

The surface tension of mint, lemon and orange oils and the interfacial tension of these oils against water were investigated by the Wilhelmy plate method. Variation of these tensions as well as variation of the surface tension of waters after prolonged contact with the oils were measured as a function of time and temperature. Spreading coefficients S ow calculated from these data show that the spreading tendency of these oils, in the temperature range 30–50°C, is in the order: orange oil > lemon oil > mint oil. Combined gas chromatography and mass spectroscopy analytical data reveal a high concentration of mint oil constituents in water equilibrated with this oil. Adsorption of these surface-active species at the air/water interface considerably lowers its surface tension. Positive temperature coefficients of the surface tension of this water and of the mint oil/water interfacial tension are attributed to the change in chemical composition occurring at these interfaces during heating. Lowering of the interfacial tension at orange oil and lemon oil/water interfaces was observed when these interfaces were heated. This may be attributed to the interfacial adsorption of 8- p-menthene-1,2-diol present in the waters equilibrated with orange and lemon oils. The data obtained may be indicative of phenomena observed during encapsulation of these essential oils by the complex coacervation procedure.