Assessing the Interfacial Activity of Insoluble Asphaltene Layers: Interfacial Rheology versus Interfacial Tension

Abstract

Asphaltenes have been suggested to play an important role in the remarkable stability of some water-in-crude oil emulsions, although the precise mechanisms by which they act are not yet fully understood. Being one of the more polar fractions in crude oils, asphaltenes are surface active and strongly adsorb at the oil/water interface, and as the interface becomes densely packed, solid-like mechanical properties emerge, which influence many typical interfacial experiments. The present work focuses on purposefully measuring the rheology in the limit of an insoluble, spread Langmuir monolayer in the absence of adsorption/desorption phenomena. Moreover, the changes in surface tension are deconvoluted from the purely mechanical contribution to the surface stress by experiments with precise interfacial kinematics. Compression “isotherms” are combined with the measurement of both shear and dilatational rheological properties to evaluate the relative contributions of mechanical versus thermodynamic aspects, i.e., to evaluate the “interfacial rheological” versus the standard interfacial activity. The experimental results suggest that asphaltene nanoaggregates are not very efficient in lowering interfacial tension but rather impart significant mechanical stresses. Interestingly, physical aging effects are not observed in the spread layers, contrary to results for adsorbed layers. By further studying asphaltene fractions of different polarity, we investigate whether mere packing effects or strong interactions determine the mechanical response of the dense asphaltene systems as either soft glassy or gel-like responses have been reported. The compressional and rheological data reflect the dense packing, and the behavior is captured well by the soft glassy rheology model, but a more complicated multilayer structure may develop as coverage is increased. Potential implications of the experimental observations on these model and insoluble interfaces for water-in-crude oil emulsion stability are briefly discussed.

Physicochemical Characterization of Asphaltenes Using Microfluidic Analysis

“…(A) Overview of compression, dilatational, and shear rheology techniques and the corresponding velocity profiles used to characterize asphaltene-stabilized interfacial films. Reprinted from ref . Copyright 2020 American Chemical Society.…”

The Oscillatory Spinning Drop Technique. An Innovative Method to Measure Dilational Interfacial Rheological Properties of Brine-Crude Oil Systems in the Presence of Asphaltenes

“…This is generally due to the formation of a rigid, viscoelastic layer with high mechanical resistance to flow in the film [10,62]. The actual case is somehow complex because this film is composed of natural surfactants from petroleum, such as asphaltenes, resins, and naphthenic acids, as well as particles of inorganic solids or waxes present in the system [52,56,[62][63][64].…”

“…The method most used for the study of dilational interfacial rheological properties have been the oscillating pendant drop and longitudinal waves, and although new equipment has been developed (based on oscillatory Langmuir trough or microfluidics [64,115,116]), these techniques cannot be used in systems with low or ultralow interfacial tensions.…”

Critical Review of Underlying Mechanisms of Interactions of Asphaltenes with Oil and Aqueous Phases in the Presence of Ions

“…Alicke et al 101 studied the mechanical and thermodynamic properties of the interface using interfacial rheological and activity experiments. The results revealed the decisive role of asphaltene nanoaggregates in mechanical stress and the minor role in reducing interfacial tension.…”