D.H. Anjum, S. Medina, A.R. Behzad, S.A. Tabatabai, T.O. Leiknes
Microscopy and Microanalysis, volume 23, issue S1, pp. 2158-2159, (2017)
Oil in water emulsion, Produced water, Cryo electron microscopy, Scanning electron microscopy, Transmission electron microscopy
Emulsions are very important to several key industries such as water treatment (i.e. municipal/industrial wastewater), pharmaceutical, and petroleum [1, 2]. Crude oil is typically extracted from oil-wells by injecting water into the reservoir resulting in an extracted flow of water-oil-gas mixture. The water from the extraction process is referred to as produced water (PW). During the separation process both waterin-oil (W/O) and oil-in-water (O/W) emulsions are formed. The O/W emulsions are not readily removed during the oil and gas separation processes and remain in the PW, making discharge or reuse of the PW a challenge. Thus, a more efficient removal of the O/W emulsions is necessary to minimize their unwanted effects on the environment. However, the removal of oil from water demands a full understanding of oil and water mixing in these emulsions. In this regard, cryo scanning electron microscopy (cryoSEM) and cryo transmission electron microscopy (cryoTEM) can be utilized to obtain more detailed information about the emulsions with nanometer range resolutions . There is still a need to address the effects of specimen preparation methods on the successful imaging of emulsions with these techniques. This study presents an investigation of sample preparation strategies for improved SEM and TEM analysis to characterize O/W emulsions in PW. Emulsions were synthesized by mixing ~1 % crude oil in water and then stabilizing them with sodium dodecyl benzene sulfonate (SDBS) surfactant. The specimens for cryoSEM were prepared by using plunge and high-pressure freezing (HPF) methods. Whereas the cryoTEM specimens were prepared with an automated plunge freezing system (VitrobotTM from FEI Company). SEM and TEM analyses were performed by utilizing Nova nanoSEM and Titan Krios instruments from FEI Company, respectively.
CryoSEM analysis of specimens frozen with both plunge and HPF methods yielded similar results (Figure 1A). From those results, it was observed that most of the oil had left the specimens during the water sublimation process (a pre-requisite for high quality cryoSEM analysis). As a result, only few oil droplets along with some smudges were left behind to image (e.g. the circled area in Figure 1A). To get around this problem, the emulsions were mixed in agarose in the hope that it would help retain their morphology. Plunge frozen specimens of agarose only were first analyzed to get an understanding of the agarose morphology. As seen from the image presented in Figure 1B, the agarose formed a filament-like morphology with irregular-size pores. The plunge freezing of agarose with emulsions was then conducted and a representative image of their specimens is shown in Figure 1C. Basically, a similar smudges-like structures (enclosed by dotted circle in Figure 1C) of oil were observed along with agarose. The structures observed were very similar to the ones presented in Figure 1A, thus the plunge freezing of agarose with emulsions did not allow a successful SEM analysis. Samples were then frozen with the HPF method and a SEM image of one of the specimens is shown in Figure 1D. Results show that the quality of HPF specimens was excellent since the spherical morphology of oil droplets was nicely retained.