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Dynamic Asphaltene-Stearic Acid Competition at the Oil–Water Interface
journal contribution
posted on 2018-04-17, 00:00 authored by Bastian Sauerer, Mikhail Stukan, Jan Buiting, Wael Abdallah, Simon AndersenInterfacial tension
(IFT) is one of the major parameters which
govern the fluid flow in oil production and recovery. This paper investigates
the interfacial activity of different natural surfactants found in
crude oil. The main objective was to better understand the competition
between carboxylic acids and asphaltenes on toluene/water interfaces.
Dynamic IFT was measured for water-in-oil pendant drops contrary to
most studies using oil-in-water drops. Stearic acid (SA) was used
as model compound for surface-active carboxylic acids in crude. The
influence of concentration of these species on dynamic IFT between
model oil and deionized water was examined. The acid concentrations
were of realistic values (total acid number 0.1 to 2 mg KOH/g oil)
while asphaltene concentrations were low and set between 10 and 100
ppm. In mixtures, the initial surface pressure was entirely determined
by the SA content while asphaltenes showed a slow initial diffusion
to the interface followed by increased adsorption at longer times.
The final surface pressure was higher for asphaltenes compared to
SA, but for binaries, the final surface pressure was always lower
than the sum of the individuals. At high SA concentration, surface
pressures of mixtures were dominated entirely by the SA, although,
Langmuir isotherm analysis shows that asphaltenes bind to the interface
200–250 times stronger than SA. The surface area/molecule for
both SA and asphaltenes were found to be larger than the values reported
in recent literature. Various approaches to dynamic surface adsorption
were tested, showing that apparent diffusivity of asphaltenes is very
low, in agreement with other works. Hence, the adsorption is apparently
under barrier control. A possible hypothesis is that at the initial
phase of the experiment and at lower concentration of asphaltenes,
the interface is occupied by stearic acid molecules forming a dense
layer of hydrocarbon chains that may repel the asphaltenes.