Recent articles proposed that asphaltenes behave as surfactants insofar they follow an equation of state, which can be approximated by a Langmir model. The resulting estimate of molecular area circa 0.3 nm<sup>2</sup> could correspond to an average asphaltenes aromatic core parallel to the water surface, in line with Sum Frequency Generation spectroscopy observations. The caveat of this approach is that the Langmuir model does not predict any transition to a solid interface at high coverage as observed experimentally by shear rheology. On the other hand the use of a surfactant molecular area larger than the area of an adsorption site (here a water molecule) contradicts the localized adsorption assumption of the Langmuir model. This discrepancy can be resolved using ideal adsorption models accounting for the relative size of adsorbate and adsorption site, i.e. Lattice Gas models. It is here reported the comparison between asphaltenes experimental data and a Lattice Gas model of hexagonal molecules covering 4 adsorption sites on a triangular lattice (a choice dictated by the original estimate of asphaltenes molecular area and the geometry of their aromatic core). With respect to dilatational rheology, the relationship between high frequency modulus and surface pressure is very well captured by the Lattice gas model. The same applies to the relationship between surface pressure and area of droplet suddenly contracted or expanded. The Lattice Gas model also predicts a transition from a fully fluid to a fully solid interface around 70-80% relative coverage. The corresponding surface pressure values almost exactly match the experimental values at which a contracted droplet loses its Laplacian shape. Finally the Lattice Gas model exhibits dynamic frustration: if coverage increases fast compared to interfacial diffusion the interfacial layer enters into a metastable glass state that slowly relaxes towards a crystalline state. Such a scenario is perfectly consistent with the observation of Soft Glass rheology features during interfacial shear experiments on asphaltenes laden surfaces. It is also consistent with the observation of birefringence within asphaltenes laden interfaces upon aging of a contracted droplet.

Lattice gas model for asphaltenes adsorption at water/oil interface

254th National Meeting (2017)

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