Surface behavior of jojoba oil alone or in mixtures with soybean oil

In the present work, the behavior of jojoba oil (JO), soybean oil (SBO) and JO/SBO mixtures at the air–water interface was studied. Experiments were performed by applying the Langmuir balance method. Monomolecular layers were prepared on a water subphase, which were subjected to lateral compression in a rectangular trough, using a Wilhelmy plate as a surface pressure transducer. The results showed that JO form stable and reproducible monomolecular layers at the air–water interface. The surface pressure–area isotherms showed an extremely low collapse pressure (πc) of 2.3 mN/m, a mean molecular area of 210 Å2/molecule and a compressional modulus at πc of 23 mN/m, characteristic of liquid expanded monolayers. The compression–expansion cycle exhibited an unusual hysteresis, leading to π values higher in the expansion isotherm compared to those in the compression isotherm at the same mean molecular area. This behavior was interpreted as an increase in the hydration level of the polar groups during the lateral compression, which forced it to be immersed in the subphase. This excess hydration free energy, released to the environment during the compression process, was equivalent to ΔΔG = −94 J/molecule. SBO and JO formed non-ideal mixtures, stabilized by attractive interactions at all proportions. The values of surface tension calculated for the water/monolayer interface (γw/m = 60–70 mN/m for JO content between 0 and 100%) as well as the bending energy of this interface (700 kT units for micro-emulsion particles of 20 nm radii) were extremely high compared with those needed to obtain spontaneous emulsification (0.01 mN/m). This indicated that SBO/JO/water micro-emulsion require the addition of surfactants to become thermodynamically stable.