Long-Term Adsorption Kinetics of Asphaltenes at the Oil–Water Interface: A Random Sequential Adsorption Perspective

Abstract

Previous studies indicated that asphaltenes adsorbed as monomers on oil-water interfaces and the early stage kinetics of the process was controlled by diffusion and hence dependent on oil viscosity. By measuring interfacial tension (IFT) as a function of surface coverage during droplet expansions in pendant drop experiments, it was also concluded that the IFT data could be interpreted with a Langmuir equation of state (EoS), which was independent of oil viscosity, time of adsorption, and bulk asphaltenes concentration. The surface excess coverage was calculated to be ∼0.3 nm(2)/molecule, which suggested adsorption in face-on configuration of asphaltenes monomers at the interface and average PAH core per molecule of about 6 for the asphaltenes investigated, consistent with the Yen-Mullins model. The current study focuses on the kinetics of asphaltenes adsorption at longer times and higher interfacial coverage. Long-term IFT data have been measured by the pendant drop method for different asphaltenes concentrations and for different bulk viscosities of the oil phase (0.5-28 cP). The data indicate that when coverage reaches 35-40%, the adsorption rates slow down considerably compared to the diffusion-controlled rates at the very early stages. The surface pressure increase rate (or IFT decrease rate) at these higher coverages is now independent of oil viscosity but dependent upon both surface pressure itself and asphaltene monomer concentration. The long-term asymptotic behavior of surface coverage is found to be consistent with the predictions from surface diffusion-mediated random sequential adsorption (RSA) theory which indicates a linear dependency of surface coverage on 1/√t and an asymptotic limit very close to 2D random close packing of polydispersed disks (85%). From these observations RSA theory parameters were extracted that enabled description of adsorption kinetics for the range of conditions above surface coverage of 35%.

Probing the Adsorption of Polycyclic Aromatic Compounds onto Water Droplets Using Molecular Dynamics Simulations

“…In addition, the perpendicular orientation of PAC molecules observed here can be partly attributed to the oxygen functional groups, such as ketone and ester, present in the chemical structures. For PAC molecules without oxygen moieties, their polyaromatic core may prefer parallel to the interface, as reported by Banerjee and co-workers [15][16][17][18]. Indeed, it has been suggested by Mullins and co-workers 19,20 that the presence of oxygen moiety can alter the preferred orientations of PAC molecules from being parallel to being perpendicular.…”

“…That is, in ݊-heptane, PAC molecules migrate to the interface and their adsorption tends to be irreversible, 17 while in toluene, they are more prone to being solvated in the bulk toluene phase, and completely detachment, i.e. reversible adsorption, is observed.…”

Mixed Interfaces of Asphaltenes and Model Demulsifiers, Part II: Study of Desorption Mechanisms at Liquid/Liquid Interfaces

“…On the other hand, other studies 30,31 showed that there is no reorganization or cross-linking at the interface and that asphaltenes are adsorbed as monomers instead of nanoaggregates. This was concluded after fitting adsorption data to an equation of state (Langmuir EoS) which yielded an adsorbed amount that would only be possible if a flat-on adsorption of asphaltene monomers occurred.…”

Understanding the lateral movement of particles adsorbed at a solid–liquid interface

“…The RSA particle adsorption simulations (governing logic can be found in [10,11]) indicate that the additional of 65 nm diameter particles in the second step is due to their adsorption in the interstices of the bigger particles adsorbed in the first step and the smaller particles introduced in step 2 do not cause the bigger particles adsorbed in the first step to move laterally on the surface. This was further confirmed from the coinciding pair co-relation function plots for the 90 nm diameter particles (Fig.…”