(168b) Effects of the Subphase pH on the Isothermal Compressibility of Interfacial Molecular Films Containing Crude Oil Amphiphilics

Petroleum amphiphilics present in the SARA fractions have the ability to aggregate in solution, adsorb on interfaces, and reduce the solvent interfacial tension. The aggregation phenomena have also been shown to be responsible for problems caused by phase separation and solid deposition of oil components, causing damage to the oil pipeline and the formation-rock. The adsorption of crude oil amphiphilics on oil-water and oil-air interfaces is responsible for the many oilfied problems. The asphaltenes and resins are able to adsorb onto interfaces, constructing resistant films with viscoelastic characteristics that contribute to the stability of crude oil emulsions. The properties of the molecular film adsorbed on the interface are crucial to define the main aspects of oil dispersion and wetting phenomena. The description of rheological and interfacial properties of interfacial films constituted of oil amphiphilics is of great relevance for oil recovery and production activities.

Interfacial phenomena are present in many operations of the petroleum production and transportation processes. Petroleum components have been extensively investigated with regard to their colloidal characteristics because of the effects on the emulsion and foam stabilization, wettability inversion, solid deposition, and adhesion on solid surfaces. The present work presents a study on the properties of amphiphilic components extracted from petroleum at water-air interfaces. The investigation describes the phase behavior of monolayers containing SARA fractions. The results obtained from Langmuir trough experiments identified the phase transition in saturates, aromatics, resins, and asphaltene films. The description of the phase behavior of asphaltenes films should contribute to the assessment of the rheological behavior and the stability of petroleum emulsions. Asphaltene films displayed the highest surface pressure, reaching 70 mN.m^-1 at the minima area, according to previous values reported¹. Asphaltene and resin films show a continuous increase in surface pressure as a function of pH from the first phase transition. On the other hand, a minimum point in surface pressure is found in aromatic and saturated films at pH 6. The reduction of the modulus of elasticity with the increase in pressure denotes the substantial intermolecular interaction of molecules present in the fractions. Factors such as ionization and temperature play an important role in determining the nature of the film. The pH effect on the isothermal compressibility is related to the electric double-layer forces acting in the air-water interface. Isothermal compressibility of asphaltene films was found from 0.012 (LC) to 0.158 (LE) under the full pH range. The highest isothermal compressibility was found to saturate films (3.47) at pH 12. The ionization, dissolution and/or ion adsorption phenomena have a great influence on the distribution of ions in the polar environment. The occurrence of ionizable groups on the amphiphilic molecular structure is the major responsible by the changes of the film elasticity with the pH changes.