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45097-G5
Investigating the Stability of Interfacial Films in Crude Oil Emulsions by Light Transmission Through Gold Nanoholes

Di Gao, University of Pittsburgh

The stability of interfacial films between crude oil and water plays an important role in the demulsification process and determines the efficiency of oil recovery. We have developed a method to form water-oil interfacial films in gold nanoholes (120 nm in diameter) and to monitor the film thinning and rupture process by measuring the light transmission spectrum through the gold holes. The monitoring principle is based on the fact that changes in the refractive index of the medium inside the gold holes cause shifts of the localized surface plasmon resonance peak in the transmission spectrum. Using this technique, we have investigated the influence of the natural surface-active materials in crude oil, such as petroleum acids and asphaltenes, on the stability of the interfacial films in alkali and brine systems, as well as the effectiveness of chemical demulsifiers in destabilizing the asphaltene films formed at the water-oil interface. In addition, we will study the mechanism of the rupture of the interfacial films with or without the demulsifiers. Three mechanisms are hypothesized: 1) the film keeps thinning until a critical film thickness and then ruptures; 2) the film keeps a certain thickness and instantly ruptures due to the formation and quick development of local structural defects; and 3) the film ruptures in steps by forming long-range ordered structures. We expect that this technique could be developed into a simple, robust, and inexpensive platform for investigation of the stability of interfacial films for a variety of applications from petroleum industry to high-tech field of biotechnologies.

 We have used our method to monitor the rupturing process of water-diesel oil interfacial films. The Au nanoholes on microscope glass slides were fabricated using a simple, and inexpensive colloidal lithography technique. Briefly, negatively charged polystyrene nanoparticles were dispensed in an aqueous solution onto positively charged microscope glass slides pre-modified with an aminosilane layer. This produced a sub-monolayer of polystyrene particles on glass slides. Next, a ~40 nm thick Au film was evaporated onto the glass slides. The particles were then stripped, which left nanometric holes in the Au films. These holes were about 120 nm in diameter and randomly placed on the glass slide. The transmission spectrum of the Au holes in air exhibited a resonance peak around wavelength of 600 nm. The peak shifted to the longer wavelength region as the refractive index of the medium around the holes increased, with almost a linear correlation at a rate of approximately 110 nm per refractive index unit.
To form water-diesel oil interfacial films inside the Au holes in aqueous solutions, the Au surface was modified with a self-assembled monolayer (SAM) from the precursor 1-dodecanethiol (CH₃(CH₂)₁₀CH₂SH). The SAM-coated Au surface had a high water contact angle (hydrophobic) and a low oil contact angle (oleophilic). The diesel oil was then introduced to the Au surface using a pipette while keeping the glass slide vertically immersed in water. A thin oil film was formed on the Au surface and inside the holes after draining the oil by gravity. The glass slide was then encapsulated by a PDMS mold to form a flow cell. Next, petroleum sulfonates LH, a surfactant used as a demulsifier in oil industry, was loaded to the system at the concentration of 5 ppm. Transmission spectra and the position of the LSPR peak in the spectra were recorded continuously. It was observed that the LSPR peak instantly shifted to the left (shorter wavelength) upon introduction of the surfactant, and kept shifting gradually to the left for about 160 seconds until another sudden shift further to the left.
A tentative explanation to the observed phenomenon was developed as the following. The introduced surfactant adsorbed onto the interfacial film between the oil phase and the aqueous phase, which resulted in a reduction of the surface tension as well as a change in the refractive index of the interfacial film. The latter directly led to a blue-shift of the LSPR peak on the spectrum. Simultaneously, the film inside the holes started the thinning process because of the reduced surface tension, which caused gradually change of the refractive index of the medium inside the Au holes, and hence the shift of the LSPR peak. The film thinning process continued until a critical film thickness was reached and the film ruptured, upon which the Au holes were filled with aqueous solution, a medium with a lower refractive index than the oil. Thus, the LSPR peak experienced a sudden shift to the left.

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