Phase behavior of surface films of SARA fractions extracted from heavy oil

Crude oil amphiphilics have been extensively investigated concerning their colloidal properties related to oilfield operation problems. The interfacial structure and elasticity of oil amphiphilic films built on fluid-fluid interfaces have a powerful effect on the stability and rheology of petroleum emulsions and foams. This work presents a description of interfacial films of amphiphilics extracted from petroleum at water-air interfaces using a Langmuir trough. The investigation evaluates surface pressure - area compression isotherms to describe properly the phase behavior of monolayers formed by SARA fractions on isothermal compression at different subphase pH. The SARA films displayed two distinct phase-transitions in different specific areas, according to the pH of the subphase. At pH 2, the surface pressure follows the order: asphaltene > aromatic > resin > saturate, for specific areas higher than 0.04 m(2)/mg. At neutral and basic pH, the previous order was changed to asphaltene > resin > aromatic > saturate. The surface pressure for asphaltene films was always higher than the surface pressure for other SARA fractions, which is in agreement with the higher surface activity of crude oil asphaltenes. The occurrence of the first phase transition of asphaltene films varies in the range 0.14?0.19 m(2)/mg with a maxima surface pressure (pi(max)) of 37.14 mN/m, whereas the second phase transition was held on range 0.069?0.099 m(2)/mg with a pi(max) = 55.23 mN/m. The modulus of elasticity (E) of asphaltene compressed-liquid films was found 65-80 mN/m (expect at pH 4, where E similar to 40 mN/m), which is related to the most compressible films. On the other hand, maltene films of SARA fractions showed almost always modulus of elasticity lower than 20 mN/m for both expanded-liquid and compressed-liquid films. The description of the interfacial film nature must be a useful tool to forecast the film stability in the disperse systems as foams and emulsions according to the SARA composition. (AU)