Water-in-Oil Emulsions

 When water phase and organic phase mix together with the presence of surfactant (the emulsifying agent), emulsion or even microemulsion is formed. For used lube oil sample, organic phase volume is much larger than water phase volume, so it is a water- 75 in-oil (W/O) emulsion case. Oil is the continuous phase as water droplets are dispersed in oil. By definition, micellar aggregates often occur when amphiphilic molecules (amphiphiles) dissolve in solution and form micelles. The spherical form of micelle (structure of micellar aggregates) is now generally accepted as approximating the actual structure (Moroi, 1992), outcomes the lamellar form (proposed by McBain) or rod-like form (proposed by Debye). According to SARA analyses (by TLC-FID method), the constituents of unused lube oil or used lube oil are very similar, with approximately > 98% aromatics, <1% resins, <1% asphaltenes, and no saturates. A Hartley micelle (i.e., polar-external spherical micelle) will form when the surfactant molecules migrate into the reversed (oil external-water internal) micelle and disrupt the polar structure. Micelle reversion caused by ultrasonic irradiation is illustrated in Figure 3-3. When water is the dispersed phase in organic phase, the center of the reversed micelle can be metals, trace water, or other heteroatoms (e.g., S, N compounds). Sonicationinduced cavitation causes the metals, polar and non-polar components to reorganize into a continuous, single-phase micelle in polar external form (Hartley micelle). When polar ends face towards the outside, attached molecules, such as metals and heteroatoms, can easily be removed by effective contact when they are brought outside as the micelle is reversed. The bondings between surfaces are greatly reduced (chelation for metals and hydrogen bonding for heteroatoms) so that these molecules can be removed. Metals are expected to separate from organic phase and stay in the water phase while heteroatoms may not only separate from the oil phase, but may possibly break down into smaller molecules by free radical attack.