The adsorption process of asphaltene onto molecularly smooth mica surfaces from toluene solutions of various concentrations (0.01 -1 wt%) was studied using a Surface Forces Apparatus (SFA). Adsorption of asphaltenes onto mica was found to be highly dependent on adsorption time and concentration of the solution. The adsorption of asphaltenes led to an attractive bridging force between the mica surfaces. The adsorption process was identified to be controlled by diffusion of asphaltenes from the bulk solution to the mica surface with a diffusion coefficient on the order of 10 -10 m 2 /s at room temperature, depending on asphaltene bulk concentration. This diffusion coefficient corresponds to a hydrodynamic molecular radius of approximately 0.5 nm, indicating that asphaltene diffuses to mica surfaces as individual molecules at very low concentration (e.g. 0.01 wt%). Atomic force microscopy (AFM) images of the adsorbed asphaltenes on mica support the results of the SFA force measurements. The results from the SFA force measurements provide an insight on the molecular interactions (e.g. steric interaction, bridging attraction as a function of distance) of asphaltenes in organic media and hence their roles in crude oil and bitumen production.

Recent studies have successfully measured surface forces using 8 atomic force microscope (AFM) and modeled surface deformations using the Stokes−Reynolds−Young−Laplace (SRYL) equations for particle−droplet, particle−bubble, droplet−droplet, and bubble−bubble systems in various solutions. The current work focuses on interactions between spherical silica particles and a viscoelastic interface of water droplets in crude oil. The self-assembly of surface active natural polyaromatic molecules (NPAMs) at the oil−water interface has previously been shown to change a viscous dominant oil−water interface to an elastic dominant interface upon aging, due to gradual formation of rigid interfacial networks. AFM was used to measure the interactions between a small silica sphere (D ≈ 8 m) and a deformable water droplet (D ≈ 70 m), which exhibits time-dependent interfacial viscoelasticity in NPAM solutions. Unlike the systems studied previously, the measured deformation shown as a repulsive force over the region of constant compliance could not be modeled adequately by the conventional SRYL equations which are applicable only to purely Laplacian interfaces. As the water droplet ages in NPAM solutions, a rigid "skin" forms at the oil−water interface, with the interface exhibiting increased elasticity. Over a short aging period (up to 15 min in NPAM-in-toluene solution), interfacial deformation is well predicted by the SRYL model. However, upon further exposure to the NPAM solution, droplet deformation is over predicted by the model. Physical properties of this mechanical barrier as a function of interfacial aging were further investigated by measuring interfacial tension, dilatational rheology, and interfacial "crumpling" (non-smooth, non-Laplacian interface) upon droplet volume reduction. By introducing a viscoelasticity parameter to account for interfacial stiffening and using experimentally determined elasticity, we are able to correct this discrepancy and predict droplet deformation under AFM cantilever compression. This parameter appears to be important for modeling non-Laplacian systems of significant viscoelastic contributions, such as biological cell membranes or polymer blends.

Atomic force microscopy (AFM) is frequently used to elucidate complex interactions in emulsion systems. However, comparing results obtained with "model" planar surfaces to curved emulsion interfaces often proves unreliable, since droplet curvature can affect adsorption and arrangement of surface-active species, while droplet deformation affects the net interaction force.In the current study, AFM was utilized to study the interactions between a colloidal probe and water droplet. Force magnitude and water droplet deformation were measured in asphaltene and bitumen solutions of different concentrations at various droplet aging times. Interfacial stiffening and an increase in particle-droplet adhesion force were observed upon droplet aging in bitumen solution. As reported in our previous study (Kuznicki, N. P.; Harbottle, D.; Masliyah, J.; Xu, Z. Dynamic Interactions between a Silica Sphere and Deformable Interfaces in Organic SolventsStudied by Atomic Force Microscopy. Langmuir 2016, 32 (38), 9797−9806), a viscoelasticity parameter should be included in the high force Stokes-Reynolds-Young-Laplace (SRYL) equations to account for the interfacial stiffening and non-Laplacian response of the water droplet at longer aging times. However, following the addition of a biodegradable demulsifier, ethyl cellulose (EC), an immediate reduction in both the particle-droplet adhesion force and the rigidity of the water droplet occurred. Following EC addition, the interface reverted back to a 2 Laplacian response and droplet deformation was once again accurately predicted by the classical SRYL model. These changes in both droplet deformation and particle-droplet adhesion, tracked by AFM, imply a rapid asphaltene/bitumen film displacement by EC molecules. The colloidal probe technique provides a convenient way to quantify forces at deformable oil/water interfaces and characterize the in-situ effectiveness of competing surface active species.

A surface forces apparatus (SFA) was used to measure the intermolecular forces between a biodegradable demulsifier (ethyl cellulose, EC) and asphaltenes immobilized on two molecularly smooth mica surfaces in an organic solvent. A steric repulsion on approach between the immobilized EC layers and asphaltenes was measured, despite strong adhesion during retraction. The measured adhesion was attributed to the interpenetration and tangling of aliphatic branches of swollen asphaltenes and solvated chains of EC macromolecules. Competitive adsorption of EC on/in immobilized asphaltene layers was confirmed by combining SFA force measurements and atomic force microscopy (AFM) imaging. Following the injection of EC-in-toluene solution, an immediate (< 5 min) increase in the confined layer thickness of the immobilized asphaltenes layers was measured. Irreversibly adsorbed asphaltenes were displaced by EC macromolecules through binding with unoccupied surface sites on mica, followed by the spreading of EC across the mica substrate due to increased surface activity governed by the higher number of hydroxyl groups per EC molecule. AFM imaging confirmed that the increase in confined layer thickness resulted from the formation of larger asphaltene aggregates/clusters protruding from the mica substrate.Molecular level topographical images showed that the asphaltenes were not re-solvated in the -2 -organic phase but more self-associated as the EC macromolecules spread across the hydrophilic mica substrate. The results from this study provide not only fundamental insights into the basic interaction mechanisms of asphaltenes and EC macromolecules as a demulsifier in organic media, but also directions towards enhancing demulsification of water-in-oil emulsions.

Small oil droplets (diameter <500 mm) in slow, steady, buoyancy-driven rise through pure or salty water at neutral pH exhibit a cascade partial-coalescence phenomenon upon soft collision with quasi-planar aqueous electrolyte solution-oil interfaces. For droplets of pure toluene, n-heptane, and their mixtures (heptol), and for moderate-strength electrolytes, the cascade partial-coalescence process continues until a critical droplet size is reached. We infer that this outcome is due to the oil-water interface being electrically charged and use the last partial-coalescence event in the sequence to estimate lower-and upper-bounds for the absolute value of the electrical potential of the interface.

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