3.7 Interfacial tension between oil and aqueous phases and average droplet size
Toil-aqueous phase interfacial tension was determined to understand the GMO’s surface activity in the presence of various aqueous phase additives. CO without GMO against water without any additive reported an interfacial tension of 21.23 ± 0.05 mN/m, while for MO, GMO’s interfacial tension was 46.03 ± 0.73 mN/m (data not shown). According to Tuntiwiwattanapun et al. (2013), vegetable oils with water displayed interfacial tension between 20 – 30 mN/m. With MO-water, Xaxa (2014) reported an interfacial tension of 45.65 ± 0.57 mN/m. Both the values from the present work are within the range found in the literature. Although made up of non-polar TAG, commercial vegetable oils reduced the interfacial tension due to minor polar components, such as free fatty acids, monoacylglycerols, or diacylglycerols (Gaonkar, 1989).
Interfacial tension measurement relied on the emulsifier diffusion and adsorption to the W-O interface. A higher concentration of surface-active molecules should lower interfacial tension unless the critical micelle concentration (CMC) has reached, after which no further decrease in interfacial tension would be observed. Figure 11 presents the interfacial tension of GMO in CO and MO against the various aqueous phases. All GMO interfacial tension for the CO-aqueous phase showed a non-significant difference (1.2 to 1.7 mN/m), and they were lower than all MO-aqueous phase interfacial tension (1.4 to 2.5 mN/m), except for S0, S0.125 (Figure 11A), and Ca0.045 (Figure 11B). Ghosh and Rousseau (2012) reported the interfacial tension of 4 wt% GMO in CO-water (1.6 ± 0.1 mN/m), which is similar to our data (1.2 ± 0.05 mN/m). GMO in MO-S0 system displayed a lower interfacial tension, which can be associated with GMO’s ability to pack much better at the interface against MO. Lower interfacial tension of GMO in MO-aqueous phase without any additives was also responsible for the much smaller water droplet size in W/MO emulsion (0.7 to 1.2 μm) than W/CO emulsions (4.6 to 9.2 μm) (Figure 6).
The addition of S, AA, CA or their mixtures did not significantly change W-CO interfacial tension. In the W-MO system, with the addition of sodium chloride (S0.125), a slight increase in interfacial tension was observed. Amar-Yuli and Garti (2005) reported that S creates a ”salting-out” effect, reducing the emulsifier head group hydrophilicity (due to dehydration) and promoting aggregation. The salting-out outcome may be favourable in oils with excess surface-active molecules, as in vegetable oils. However, in MO, the increase of GMO interfacial aggregation might promote some bare patches at the interface, which may not favour a decrease in interfacial tension. Besides, Amar-Yuli et al. (2007) addressed that the hydration layer of GMO polar heads decreased in size due to dehydration when GMO competes with other molecules for binding water, leading to an increase in interfacial tension. MO-aqueous phase system also observed a significant increase in interfacial tension with AA and CA compared to the aqueous phase without any additive. Therefore, increased emulsion stability against phase separation with AA and CA was not due to interfacial tension (their droplet size was also similar), but rather water droplet aggregation and network formation.
GMO in CO-water with Ca and LMP showed similar interfacial tension to the aqueous phase without additive (S0) (Figure 11B). Lutz et al. (2009) investigated the interfacial tension of limonene-water with 0.5 wt% pectin. They attributed the enhancement of GMO orientation at the interface due to the interaction of pectin carboxylic groups. Interfacial tension of the MO-aqueous phase with LMP showed an increase compared to the MO-aqueous phase without any additive or with Ca (Figure 12B). It could be due to the presence of LMP at the O/W interface (due to the lack of surface active molecule in MO) preventing GMO’s free adsorption at the interface. Many researchers have previously reported the surface activity of LMP. For example, Leroux et al. (2003) reported a decrease in interfacial tension between paraffin oil and 2 wt% pectin solution with a reduction in the degree of methylation. The addition of Ca2+ to LMP in the aqueous phase led to a steep rise in GMO’s interfacial tension for both the CO and MO emulsions. Calcium ions can promote gelation in LMP molecules due to their ionic binding with pectin carboxylic groups (Capel et al., 2006). LMP formed weak gels in the aqueous phase at room temperature with Ca2+, which might reduce GMO adsorption at the interface, leading to increased interfacial tension. Higher interfacial tension of LMP-Ca systems can also be related to their reduced emulsion stability (61 – 67%) compared to only LMP-added systems (72%) (Figure 2). Overall, the interfacial tension was influenced by the oil type and the interfacial interaction between the GMO head group and the aqueous phase additives. Interestingly, interfacial tension had less influence on emulsion stability against phase separation, which was mostly influenced by droplet aggregation and network formation.