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%.

Preparation of synthetic asphaltenes: a) 1,2-bis(4-bromophenyl)-3,4,5,6-tetraphenylbenzene A mixture of tetraphenylcyclopentadienone (5.0 mmol), and bis(p-bromophenyl) acetylene (5.0 mmol) in diphenyl ether (20 ml) was heated to 260 °C overnight. Then the temperature was raised to 270 °C. After 69 h, the reaction mixture was cooled, and methanol (100 ml) was added. After stirring for 1 h, the product was filtered off, washed with methanol, and finally dried in vacuum overnight. Yield: Quantitative. b) 1,2-bis(4-dodecylphenyl)-3,4,5,6-tetraphenylbenzene 1-dodecene (30.0 mmol) was slowly added to a 0.5 M solution of 9-borabicyclo[4.4.1] nonane in THF (65 ml), and the mixture was stirred at room temperature overnight. Then a solution of NaOH (45.0 mmol) in water (15 ml) was slowly added, and the mixture was stirred for 20 min. The dibromide (2.5 mmol) was added, followed by Pd(dppf)Cl2 (80 mg). The reaction mixture was stirred at room temperature for 5 h. Then TLC analysis indicated incomplete reaction, and the temperature was increased. After boiling at reflux overnight, the reaction mixture was cooled to room temperature and concentrated in vacuo. The residue was dissolved in CH2Cl2, washed with water and with brine, and then dried (Na2SO4). The dry solution was diluted with hexane, filtered thru celite, and concentrated in vacuo. Finally the product was purified by flash chromatography (SiO2, pentane w/ 5-10% CH2Cl2). Yield: 80 %. c) 2,5-didodecylhexaperihexabenzocoroneneThe didodecylhexaphenylbenzene (1.5 mmol) was dissolved in dry CH2Cl2 (750) ml, and argon was bubbled thru the solution for 15 min. Then anhydrous FeCl3 (45 mmol) dissolved in nitromethane (15 ml) was added, and the mixture was stirred at room temperature while being bubbled with argon. After 75 min, the reaction mixture was poured into methanol (1 l). The reaction mixture was then concentrated in vacuo to remove most of the CH2Cl2, and the precipitated product was filtered off, washed thoroughly with dilute hydrochloric acid and with methanol, and then dried in vacuo. The crude product was dissolved in hot THF, precipitated once again with methanol, and finally dried in vacuum overnight. Yield: 55 %.Confirmation of expected structure was first sought for by solid state NMR. 1H-MAS was performed to qualitatively evaluate the relative contribution of aromatic and aliphatic protons to the total resonance signal. 1H MAS solid state NMR signal shows that aromatic and aliphatic contributions are well separated. Deconvolution and integration of peaks leads to: Aliphatic protons: 72 %, Intermediate protons: 3 % Aromatic protons: 25 % The match with the expected structure (50-16 ratio between aliphatic and aromatic protons) appears to

Asphaltenes are "n-alkane insoluble" species in crude oil that stabilize water-in-oil emulsions. To understand asphaltene adsorption mechanisms at oil-water interfaces and coalescence blockage, we first studied the behavior in aliphatic oil-water systems in which asphaltenes are almost insoluble. They adsorbed as monomers, giving a unique master curve relating interfacial tension (IFT) to interfacial coverage through a Langmuir equation of state (EoS). The long-time surface coverage was independent of asphaltene bulk concentration and asymptotically approached the 2-D packing limit for polydisperse disks. On coalescence, the surface coverage exceeded the 2-D limit and the asphaltene film appeared to become solidlike, apparently undergoing a transition to a soft glassy material and blocking further coalescence. However, real systems consist of mixtures of aliphatic and aromatic components in which asphaltenes may be quite soluble. To understand solubility effects, we focus here on how the increased bulk solubility of asphaltenes affects their interfacial properties in comparison to aliphatic oil-water systems. Unlike the "almost irreversible" adsorption of asphaltenes where the asymptotic interfacial coverage was independent of the bulk concentration, an equilibrium surface pressure, dependent on bulk concentration, was obtained for toluene-water systems because of adsorption being balanced by desorption. The equilibrium surface coverage could be obtained from the short- and long-term Ward-Tordai approximations. The behavior of the equilibrium surface pressure with the equilibrium surface coverage was then derived. These data for various asphaltene concentrations were used to determine the EoS, which for toluene-water could also be fitted by the Langmuir EoS with Γ∞ = 3.3 molecule/nm(2), the same value as that found for these asphaltenes in aliphatic media. Asphaltene solubility in the bulk phase only appears to affect the adsorption isotherm but not the EoS. Further support for these observations is provided by dilatational rheology experiments for the EoS and contraction experiments in which desorption to the equilibrium surface pressure was observed.