## 3 Results and Discussion
### Influence of Ester Gunn Addition on Ostwald Ripening
Under fixed homogenization conditions, the efficiency of droplet disruption usually decreases with an increase in the viscosity of the oil phase or the oil-water interfacial tension [26]. In our study, the viscosity of the oil phase increased as the ester gum concentration increased (Table 1). Moreover, the interfacial tension increased as the amount of ester gum added to the orange oil increased. For instance, the interfacial tension was 0.205 (100%), 0.237 (116%), 0.231 (113%), 0.240 (117%), and 0.280 (139%) mN/m for P10 SE-stabilized emulsions containing 0, 2.5, 5, 10, and 20% ester gum, respectively. Similarly, it was 0.589 (100%), 0.593 (101%), 0.620 (105%), 0.666 (113%), and 0.797 (135%) mN/m for the P10 SE-stabilized emulsions containing 0, 2.5, 5, 10, and 20% ester gum, respectively. Consequently, one would expect that the initial droplet size should increase with the increasing ester gum level. Therefore, our results suggest that the reduction of droplet size with the increasing ester gum concentration was more likely due to its ability to inhibit droplet growth during homogenization, than its ability to facilitate droplet breakup. Figure 3: Molecular structures of the main components in commercially available ester gum. Ostwald ripening was not observed in any of the emulsions when the ester gum concentration in the oil phase was 20% (Figure 4), which can be attributed to an \"entropy of mixing\" phenomenon [1]. When the oil phase of an emulsion consists of a mixture of a water-soluble oil (such as essential oil) and a water-insoluble oil (such as ester gum), the water-soluble oils diffuse from the small to the large droplets much faster than the water-insoluble ones. Eventually, the concentration of water-soluble oil is higher in the smaller droplets than in the larger ones (and vice versa), leading to a concentration gradient in the system. A concentration gradient between the oil droplets is thermodynamically unfavorable due to the entropy of mixing effects, which favors a net movement of the water-soluble oil from the larger to the smaller droplets. Eventually, the driving force for the entropy of mixing is the  
Figure 4: Influence of the addition of ester gum into the oil phase on the mean droplet diameter (volume-length mean diameter, \(d_{43}\)) of orange oil emulsions stabilized with ethylene glycol alkyl ether emulsifiers. (**A**): P10 SE-stabilized emulsion, (**B**): P20 SE-stabilized emulsion, (**C**): P23 LE-stabilized emulsion, (**D**): P100 SE-stabilized emulsion, (**E**): droplet size in emulsions after 14-day storage. For changes in the mean droplet diameter (area-volume mean diameter, \(d_{32}\)) of orange oil emulsions, see Supplementary Figure S1. same as the driving force for Ostwald ripening, after which the droplet size and composition remain constant. Our results suggest that 20% of ester gum was sufficient to completely inhibit Ostwald ripening in the orange oil emulsions during or just after homogenization, independent of the surfactant type or concentration used. At lower ester gum levels (e.g., 2.5, 5 or 10%), a steep increase in droplet size was observed in all the emulsions during the first day of storage, after which it remained relatively constant in most systems. This result suggests that these levels of ester gum were not sufficient to inhibit Ostwald ripening. Nevertheless, the final extent of droplet growth did decrease as the ester gum level was raised, which suggests that Ostwald ripening was arrested sooner. In general, it has been shown that the rate and extent of Ostwald ripening in an emulsion containing mixed oils depends on the volume fraction and water solubility of the oils [27]. Our results also showed that the ability of the ester gum to inhibit Ostwald ripening was influenced by the nature of the surfactant used to coat the oil droplets (Figure 4).