3.1. Interfacial tension
The interfacial tension measured at the water–pure soybean oil interface was about 22 mN/m, while at water–hexadecane interface was 40 mN/m. When the emulsifiers were added, all of them showed a considerable reduction of the interfacial tension (Fig. 4). At water–soybean oil interface, the initial interfacial tension was around 6.7 mN/m with PGPR and 2.9 mN/m with Span 80. In the latter system, an opaque film formed at the interface during the tension measurement. The rigid interface of water–soybean oil stabilized with Span 80 could have contributed to lower the interfacial tension. The interfacial tension of both systems decreased with time, reaching values of 2.7 mN/m with PGPR and 2.3 mN/m with Span 80 in the stability. The initial interfacial tension at water–hexadecane interface was approximately 5.9 mN/m with PGPR and 6.1 mN/m with Span 80 and after 1 h decreased to 3.2 and 3.6, respectively. Lecithin showed low interfacial tension at water–soybean oil interface (1.2 mN/m) at stability, although the initial tension was higher (9.2 mN/m) than in other emulsifiers systems. At water–hexadecane interface with lecithin, the initial interfacial tension was even higher (24.1 mN/m), thus the mobility of lecithin was much lower. The interfacial tension of that system at the stability was 2.8 mN/m. Comparing all the systems, the interfacial tension of water–hexadecane in the stability condition was higher than that of water–soybean interface, due to the less polar character of hexadecane