(168c) Synergy of Crude Oil Amphiphiles on the Interfacial Film Elasticity

Asphaltenes and resins comprise the most polar components and a portion of the most interfacially active compounds in crude oil. Crude oil asphaltenes represent a solubility class formed by a series of different amphiphilic species. Asphaltene constitutes the fraction of the crude oil composed of brown and black composites, which are insoluble in low boiling n-alkanes and quite soluble in toluene, benzene, carbon disulfide, chloroform and other chlorinated hydrocarbon solvents.

In solution, asphaltenes reduce the interface tension of organic solvents, promoting foam and emulsion stabilization. In addition, asphaltenes display a self-association behavior and adsorb at interfaces similar to conventional surfactants. Asphaltene deposition is a major problem in oil production. The deposition can result in the reduction of reservoir-rock permeability and a decrease in the pipe effective diameter. Despite the variety of techniques and methodologies available to evaluate asphaltene deposition inhibition, the addition of dispersants and inhibitors has been the preferred method. Asphaltenes adsorb at the air-water and oil-water interfaces, forming monomolecular layers as a Langmuir film. The properties of the interfacial film enclosing dispersed droplets in petroleum emulsions are mainly dependent on the concentration of polar molecules in the oil, the composition and pH of the aqueous phase, the temperature and film compressibility. Strong films, which are responsible for stabilizing oil emulsions against coalescence, consist essentially of asphaltenes. Films containing primarily resins are weak and tend to form unstable emulsions. Furthermore, the characteristics of the asphaltene fraction affect the interfacial film properties. Asphaltene fractions that have low aromaticity and a high content of heavy metals (mainly Ni and V) have been noted as the major component responsible for constructing films with high elasticity and high yield stress, whereas more aromatic asphaltenes possessing low heavy metal content produce weak films.

In this work, the effect of asphaltenes on the interfacial oil properties is investigated regarding the elasticity of interfacial films. N-pentane insoluble asphaltenes (C5I) were extracted from a heavy crude oil by means of the IP143 method. Compression isotherms obtained for the oil and its asphaltene fraction were compared. The samples were deposited at the air-water interface from a dichloromethane spreading solution to form two-dimensional monolayers. The film properties were evaluated using a Langmuir trough under compression at a constant rate of 10 mm.min^-1. Surface pressure-area isotherms were interpreted from a mathematic model of the surface pressure, assuming constant compressibility for each phase, such as previous work¹. The C5I monolayers displayed an extensive region containing liquid-expanded (LE) and liquid-condensed (LC) phases and a well-identified transition region in between these phases. Tests performed for oil shows a long gas region undergoing a phase transition at 0.20 m².mg^-1 and 1.83 mN.m^-1. The compression of the film produced a liquid expanded region to about 0.067 m².mg^-1 and 14.18 mN.m^-1. From that point on, a second phase transition occurs at 0.038 m².mg^-1 and 36.36 mN.m^-1 which corresponds to the maximum surface pressure. The interfacial behavior of the film containing C5I asphaltenes shows a phase transition to expanded liquid with a low surface pressure of 9.14 mN.m^-1, following a rapid phase transition to condensed liquid at 0.178 m².mg^-1 and 50.56 mN.m^-1 which is maintained up to the maximum pressure of 72.23 mN.m^-1, where the area corresponds to the minimum area of 0.039 m².mg^-1. Asphaltene films display a very large hysteresis area, which is related to irreversible strong interactions between the molecules adsorbed on the air-liquid interface.