.
On the other hand, the emulsions prepared with hexadecane and PGPR were not as stable as the soybean oil emulsions. The lower stability may be related to the linear structure of hexadecane, which did not promote a good fixation of the PGPR on the interface. With that, a weaker oil–emulsifier interaction occurred at the interface, leading to a faster destabilization.
However, Span 80 was a good emulsifier in a system composed of hexadecane at 60:40 W:O ratio. The high stability at high water content was supported by the hydrogen bonds formed between the hydroxyls of the hydrophilic moieties of Span 80 (sorbitan) and the water molecules. In addition, the emulsion at this condition showed enhanced stability due to the small droplets, as observed in the particle size distribution and the images obtained through optical microscopy. Those droplets were formed favored by the rapid migration of Span 80 to the water–hexadecane interface, as indicated by tension drop between the water–hexadecane interfacial tension (40 mN/m) to the initial tension with Span 80 in this system (6.1 mN/m) (Fig. 4). Regarding the complex modulus, although it was low (Fig. 5), it was constant with time. This indicates that the interface was stable even with the perturbation of the system. This stable elastic network could avoid the droplet coalescence. The explanation of the structured interface can be given in terms of the hydrophobic chain–chain interaction between Span 80 and hexadecane, which was more compatible than interactions between PGPR and hexadecane chains. As it is observed in Fig. 3, the hydrophobic part of Span 80 molecule has only one unsaturation in its hydrophobic chain, while PGPR hydrophobic components have one unsaturation and one hydroxyl (Fig. 2), which has a hydrophilic character. Since hexadecane is highly hydrophobic, with no unsaturation in its structure, this would create a better interaction with Span 80, which has a more hydrophobic chain. On the other hand, PGPR showed more compatibility with soybean oil, since this oil is less hydrophobic. The chain compatibility between surfactant and oil was described by Chattopadhyay, Shah, and Ghaicha (1992).