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Energy & Fuels 2005, 19, 1330-1336
Asphaltene Monolayers at a Toluene/Water Interface† Li Yan Zhang, Robert Lopetinsky, Zhenghe Xu, and Jacob H. Masliyah* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada Received September 14, 2004. Revised Manuscript Received January 26, 2005
Asphaltene films at a toluene/water interface were characterized using a Langmuir interfacial trough. An asphaltene monolayer was determined to remain at the toluene/water interface, and asphaltenes do not migrate from the monolayer to the bulk toluene or water phase. When the hydrocarbon phase is removed and replaced with a fresh toluene phase (dilution experiment), the pressure-area isotherm obtained was almost identical to that from the original asphaltene monolayer at the interface. Addition of 2-propanol into the oil phase (toluene) caused the interfacial asphaltene monolayer to become more flexible. However, 2-propanol molecules did not displace asphaltene molecules. Atomic force microscopy (AFM) images of the transferred LangmuirBlodgett (LB) asphaltene films from toluene-water and a mixture of toluene and 2-propanol/ water interface show similar aggregated structures. Fourier transform infrared (FTIR) spectra of the deposited LB asphaltene films show the presence of bonded water in asphaltene films.
Introduction Removal of emulsified water from crude oil or bitumen has been a challenge to oil industry worldwide.1-4 Undesirable water-in-oil emulsions are formed during the bitumen recovery (the Clark Water Extraction) process, where water is used as a medium to separate bitumen from the sand grains. Emulsified water remains in the solvent-diluted bitumen as dispersed water droplets on the order of micrometers in size. Dewatering efforts have been hindered by the unusual stability of the emulsions,2 because of the adsorption of stabilizing agents such as asphaltenes and fine solids at the water/ hydrocarbon interface. The surface of the dispersed water droplets is coated with a protective film that prevents water droplets from coalescence. Asphaltenes, by definition, is a solubility class of material that is soluble in toluene but insoluble in alkanes such as n-heptane. Asphaltenes are known to stabilize water-in-oil (W/O) emulsions by forming interfacial films surrounding the dispersed water droplets. Using rheological measurements5,6 and a micropipet technique,2,7 “skins” were noted at an oil/water interface. † Presented at the 5th International Conference on Petroleum Phase Behavior and Fouling. * Author to whom correspondence should be addressed. Telephone: 1-780-492-4673. Fax: 1-780-492-2881. E-mail address: Jacob.Masliyah@ ualberta.ca. (1) McLean, J. D.; Kilpatrick, P. K. Effects of Asphaltene Solvency on Stability of Water-in-Crude-Oil Emulsions. J. Colloid Interface Sci. 1997, 189 (2), 242-253. (2) Yeung, A.; Dabros, T.; Czarnecki, J.; Masliyah, J. On the Interfacial Properties of Micrometer-Sized Water Droplets in Crude Oil. Proc. R. Soc. London, Ser. A 1999, 455 (1990), 3709-3723. (3) Gu, G.; Xu, Z.; Nandakumar, K.; Masliyah, J. H. Influence of Water-Soluble and Water-Insoluble Natural Surface-Active Components on the Stability of Water-in-Toluene-Diluted Bitumen Emulsion. Fuel 2002, 81 (14), 1859-1869. (4) Spiecker, P. M.; Gawrys, K. L.; Kilpatrick, P. K. Aggregation and Solubility Behavior of Asphaltenes and Their Subfractions. J. Colloid Interface Sci. 2003, 267 (1), 178-193. (5) Dodd, C. G. The Rheological Properties of Films at Crude Petroleum-Water Interfaces. J. Phys. Chem. 1960, 64 (5), 544-550.
Skins were also observed using a pendant drop apparatus. Reisberg and Doscher,8 and Strassner,9 showed that asphaltenes form rigid films at an oil/water interface. Pictures taken from (W/O) emulsions clearly showed the presence of crumpled interfacial films. Jeribi et al.10 reported that a rigid skin can be visually observed at the water/asphaltene/toluene interface if the asphaltene concentration in toluene is >10% by mass. At lower asphaltene concentrations in toluene (
