Transitions in Films of Asphaltenes Formed at the Water-Air and Water- Oil Interface(ABSTRACT)

摘要

Water insoluble amphophilic, compounds may form monomolecular films on the water surface. Properties of these films depend on molecular orientation of these molecules in respect to the water surface. Non-amphiphilic compounds don't form monomolecular films, but nanoparticles built with these molecules can do. Asphaltenes are present on the water surface as small non-amphiphilic aggregates. Therefore, they form monolayers built with nanoparticles (aggregates) the properties of which are mainly determined by the contact angle asphaltenes - water and by lateral interactions between the aggregates. Asphaltene films were extensively studied as model systems of water/oil emulsions. Moreover, experiments with films allow to following asphaltene aggregation in two-dimension matrix and to extrapolating conclusions towards aggregation occurring in the bulk. We think that the latter point is particularly important. The purpose of this study was to obtaining thermodynamic characterisation of asphaltene aggregation using experimental data concerning the Langmuir films formed by asphaltenes.Asphaltenes were deposed on the water surface in the volatile solvent solution. The film was formed after the solvent evaporation. Three solvent were used: chloroform, chloroform/methanol and chloroform/methylnaphthalene. While, two first solvents evaporate leaving at the surface asphaltenes only, the methylnaphthalene forms upper liquid layer. In the latter case, asphaltene film is formed at the water-oil interface, fj = f (A) isotherms, were established using a Langmuir through. J] is the surface pressure corresponding to the lateral pressure acting on the film and A is the unit area of molecules/aggregates forming the film. At each stage of the film compression the surface potential and viscoelastic modulus of the film and the surface images obtained by Brewster angle microscopy were recorded. Therefore, consecutive changes of asphaltene organization in the film were observed in real time. The simultaneous characterization of the film properties with several techniques made it possible to assessing physical basis of the mechanism of the film formation. We have demonstrated that the asphaltenic films may form gel-like and solid-like mono-aggregate and poly-aggregate films. Analysis of experimental results led to an analytical model capable to representing transitions occurring in the asphaltenic film. We proposed a new equation of state of the film (relationship between JT> A, and T) capable to representing phase transition in asphaltenic films induced by changes of the surface pressure.