A Molecular Dynamics Study of the Effect of Asphaltenes
on Toluene/Water
Interfacial Tension: Surfactant or Solute?
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Abstract
A series of molecular dynamics simulations
were performed to investigate
the effects of model asphaltenes on the toluene/water interfacial
tension (IFT) under high temperature and pressure conditions. In the
absence of model asphaltenes, the toluene/water IFT monotonically
decreases with increasing temperature, whereas, with the presence
of model asphaltenes, especially at high concentrations, such monotonic
dependence no longer holds. Furthermore, in contrast with the decreasing
trend of IFT with increasing model asphaltene concentration at low
temperature (300 K), increasing concentration at high temperature
(473 K) leads to increasing IFT. This relation can even be nonmonotonic
at moderate temperatures (373 and 423 K). Through detailed analysis
on the distribution of model asphaltenes with respect to the interface,
such complex behaviors are found to result from the delicate balance
between miscibility of toluene/water phases, solubility of model asphaltenes,
and hydrogen bonds formed between water and model asphaltenes. By
increasing the temperature, the solubility of model asphaltenes in
toluene is enhanced, leading to their transition from being a surfactant
to being a solute. The effect of pressure was found to be very limited
under all model asphaltene concentrations. Our results here present,
for the first time, a complete picture of the coupled effect of (high)
temperature and asphaltene concentration on IFT, and the methodology
employed can be extended to many other two-phase or multiphase systems
in the presence of interface-active chemicals