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

This article is the continuation of a preceding paper (Part I) in which the adsorption and desorption of asphaltenes from the oil/water interface by pure solvent and model demulsifiers was studied. In this second part, the composition of mixed interfaces of asphaltenes and two demulsifiers (Brij-93 and Pluronic PE8100) was studied. Desorption of asphaltenes by demulsifiers, and vice versa, was determined. First, the composition of a mixed interface (asphaltenes and demulsifiers) through the use of the Langmuir equation of state (EoS) was determined. Second, an experimental setup that mimics, to some extent, the chemical demulsification of water-in-crude oil emulsions during the production stages was used. Desorption of already-adsorbed asphaltenes at the liquid/liquid interface by the action of two demulsifiers was assessed. It was found that desorption is always initiated by interactions between demulsifiers and asphaltenes. It is followed by the plausible formation of complex-like structures to finally end in the replacement, by displacement from the interface, of asphaltenes by demulsifiers. Third, the assessment of Brij-93 and PE8100 desorption from the oil/water interface by the action of asphaltenes was also carried out. It was found that asphaltenes can desorb PE8100 at low surface coverage

Nanoaggregation of Polyaromatic Compounds Probed by Electrospray Ionization Mass Spectrometry

This paper reports the results of the first detailed experimental study on probing nanoaggregation of a polyaromatic compound. Electrospray ionization mass spectrometry (ESI–MS) was used to monitor the self-association of a well-defined polyaromatic compound, <i>N</i>-(1-hexylhepyl)-<i>N</i>′-(5-carboxylicpentyl)-perylene-3,4,9,10-tetracarboxylicbisimide (C5Pe), under various solution conditions. Gaseous ions corresponding to nanoaggregates of C5Pe molecules were directly observed on ESI mass spectra. The dominant aggregation number (<i>n</i>) was found to be less than 10, although larger nanoaggregates with an aggregation number larger than 10 were also observed. The aggregation number of C5Pe decreased by replacing toluene with xylene, while it increased with the C5Pe concentration or upon the addition of heptane to toluene as the solvent. The consecutive aggregation number was found only for small C5Pe nanoaggregates (2 ≤ <i>n</i> ≤ 11), which suggests a stepwise self-association at <i>n</i> ≤ 11. The larger nanoaggregates (<i>n</i> > 11) were formed by interactions between small nanoaggregates. The presence of naphthenic acids (NAs) was observed to hinder C5Pe self-association. The dispersive effect of NAs was found to be in the order of 1-methyl-1-cyclohexanecarboxylic acid ∼ cyclohexanebutyric acid < stearic acid < 5β-cholanic acid < 1-naphthalene pentanoic acid. The nanoaggregation behavior of C5Pe was compared to that of two other polyaromatic compounds

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

This article is the continuation of a preceding paper (Part I) in which the adsorption and desorption of asphaltenes from the oil/water interface by pure solvent and model demulsifiers was studied. In this second part, the composition of mixed interfaces of asphaltenes and two demulsifiers (Brij-93 and Pluronic PE8100) was studied. Desorption of asphaltenes by demulsifiers, and vice versa, was determined. First, the composition of a mixed interface (asphaltenes and demulsifiers) through the use of the Langmuir equation of state (EoS) was determined. Second, an experimental setup that mimics, to some extent, the chemical demulsification of water-in-crude oil emulsions during the production stages was used. Desorption of already-adsorbed asphaltenes at the liquid/liquid interface by the action of two demulsifiers was assessed. It was found that desorption is always initiated by interactions between demulsifiers and asphaltenes. It is followed by the plausible formation of complex-like structures to finally end in the replacement, by displacement from the interface, of asphaltenes by demulsifiers. Third, the assessment of Brij-93 and PE8100 desorption from the oil/water interface by the action of asphaltenes was also carried out. It was found that asphaltenes can desorb PE8100 at low surface coverage

Equilibrium Partitioning of Naphthenic Acid Mixture, Part 1: Commercial Naphthenic Acid Mixture

Crude oil contains naphthenic acids that can partition into water. This phenomenon is a function of several parameters, such as the naphthenic acid composition, pH, and water phase salinity. This article is a continuation of previous work regarding the partitioning between oil and water of model acids and bases in model systems, with regard to pH and salinity. The present work will focus on a commercial naphthenic acid mixture from Fluka, while the next work will deal with extracted naphthenic acids from a crude oil. The composition of the acid mixture was determined by GC/MS and it was found that the commercial naphthenic acid mixture is mostly composed of saturated acids with 0–3 ring structures. The partitioning of the commercial naphthenic acid mixture was determined. The equilibrium partitioning of acids with different molecular weight was determined over a large pH interval, using toluene as the oil phase and 3.5 wt % NaCl aqueous buffers as the water phase. The partitioning of the naphthenic acid mixture with pH was successfully modeled by dividing the naphthenic acid mixture into narrow molecular weight range fractions characterized by a single partitioning ratio (p<i>P</i>_wo). The variation of the p<i>P</i>_wo of the fractions with molecular weight was found to be linear. In the presence of calcium and high pH, the partitioning of higher-molecular-weight acids was reduced, likely because of the formation of oil-soluble calcium naphthenates, since no precipitation was observed. The partitioning of low-molecular-weight acids was not affected by calcium

Study of the Aqueous Chemical Interactions between a Synthetic Tetra-acid and Divalent Cations as a Model for the Formation of Metal Naphthenate Deposits

The previously presented synthetic tetra-acid model compound BP10 was used to investigate the chemistry behind the formation of metal naphthenate deposits. The interactions between BP10 and the cations Ba²⁺, Ca²⁺, H⁺, Mg²⁺, and Sr²⁺ were investigated using potentiometric titrations, metal ion depletion by inductively coupled plasma−atomic emission spectrometry (ICP−AES), pH measurements, and elemental analysis of precipitates, in 20−600 mM NaCl ionic medium. The interactions of BP10 with the monovalent Na⁺ are discussed on the basis of a previous study. The data given indicate that Ca²⁺ shows the strongest affinity toward BP10 and Ba²⁺, and Sr²⁺ form approximately equally stable solid phases with BP10, while Mg²⁺ is less tightly bound to the tetra-acid. H⁺ interacts more strongly than the Me²⁺ ions, and Na⁺ shows a rather small affinity for BP10. No soluble complexes could be detected, and all products in the chemical reactions are therefore believed to be solid materials. We suggest that BP10 show the following preference of cations: H⁺ ≫ Ca²⁺ > Ba²⁺ ≈ Sr²⁺ > Mg²⁺ ≫ Na⁺. This order could be due to the hydration state and size of the cations. In comparison to typical concentrations found of each in saline water, it is proposed that the dominance of Ca²⁺ in naphthenate deposits is dependent upon both availability and selectivity

Wax-Inhibitor Interactions Studied by Isothermal Titration Calorimetry and Effect of Wax Inhibitor on Wax Crystallization

Isothermal titration calorimetry is applied to investigate the interactions of polymeric pour-point depressants (PPDs) or asphaltenes with macrocrystalline wax in a model oil system. This represents a novel approach to measure and compare the heat of interaction for solid wax crystals with different wax inhibitors (WIs). In addition, the PPDs were characterized via size-exclusion chromatography and differential scanning calorimetry (DSC), and the effect of PPDs or asphaltenes onto the wax appearance temperature (WAT) and the formed wax-oil gel was investigated using DSC, cross-polarized microscopy (CPM), and rheometry. The results show that there is detectable interaction heat with wax crystals for all PPDs and asphaltenes. DSC and viscometry show a decrease in observed WAT for all WIs as compared to the additive-free blank case. CPM imaging shows differences in structure, shape, and size of the wax crystals formed in the presence of particular PPDs or asphaltenes, which is also seen as a decrease in gel-breakage strength. Overall, there is no direct correlation between interaction heat and WI performance characteristics, such as a high decrease in WAT or gel strength. Ethylene-vinyl acetate copolymer (EVA) with 25% vinyl actetate accounts for the highest interaction heat measured but shows effective decrease in wax crystals size for only part of the crystals while acting as a flocculate to others, which resulted in a low effect on gel strength. Commercial PPDs based on polycarboxylate have the best performance for the model oil system used but show only overage interaction heat values. The interaction heat of asphaltenes with wax is measurable but lower than for the PPDs tested. The presence of asphaltenes significantly lowered the gel strength and changed the wax crystal morphology to rounder and dendrite-like shapes. These findings suggest that asphaltene compounds are incorporated into the wax crystals, changing the structure and shape of the crystals

Role of Naphthenic Acids in Controlling Self-Aggregation of a Polyaromatic Compound in Toluene

In this work, a series of molecular dynamics simulations were performed to investigate the effect of naphthenic acids (NAs) in early stage self-assembly of polyaromatic (PA) molecules in toluene. By exploiting NA molecules of the same polar functional group but different aliphatic/cycloaliphatic nonpolar tails, it was found that irrespective of the presence of the NA molecules in the system, the dominant mode of π–π stacking is a twisted, offset parallel stacking of a slightly larger overlapping area. Unlike large NA molecules, the presence of small NA molecules enhanced the number of π–π stacked PA molecules by suppressing the hydrogen bonding interactions among the PA molecules. Smaller NA molecules were found to have a higher tendency to associate with PA molecules than larger NA molecules. Moreover, the size and distribution of π–π stacking structures were affected to different degrees by changing the size and structural features of the NA molecules in the system. It was further revealed that the association between NA and PA molecules, mainly through hydrogen bonding, creates a favorable local environment for the overlap of PA cores (i.e., π–π stacking growth) by depressing the hydrogen bonding between PA molecules, which results in the removal of some toluene molecules from the vicinity of the PA molecules

Competitive Adsorption of Naphthenic Acids and Polyaromatic Molecules at a Toluene–Water Interface

The early-stage competitive co-adsorption of interfacially active naphthenic acids (NAs) and polyaromatic (PA) molecules to a toluene–water interface from the bulk toluene phase was studied using molecular dynamics (MD) simulation. The NA molecules studied had the same polar functional group but different cycloaliphatic nonpolar tails, and a perylene bisimide (PBI)-based molecule was used as a representative PA compound. The results from our simulations suggest that the size and structural features of NA molecules greatly influence the interfacial activity of PA molecules and partitioning of NA molecules at the toluene–water interface. At low concentrations of PA (∼2.3 wt %) and NA (∼0.4 wt %) molecules, NA molecules containing large cycloaliphatic rings (e.g., four rings) or with a very long aliphatic tail (e.g., carbon chain length of 14) were observed to impede the migration of PA molecules to the interface, whereas small NA molecules containing two cycloaliphatic rings had little effect on the adsorption of PA molecules at the toluene–water interface. At high NA concentrations, the adsorption of PA molecules (∼5.75–17.25 wt %) was greatly hindered by the presence of small NA molecules (∼1.6–4.8 wt %) due to the solvation of PA nanoaggregates in the bulk. Adsorption mechanisms of PA and NA molecules at toluene–water interfaces were clarified through a detailed analysis on the interactions among different species in the system. The results obtained from this work provide insights into designing appropriate chemical demulsifiers or co-demulsifiers for breaking water-in-oil emulsions of great industrial applications

Sorption and Interfacial Rheology Study of Model Asphaltene Compounds

The sorption and rheological properties of an acidic polyaromatic compound (C5PeC11), which can be used to further our understanding of the behavior of asphaltenes, are determined experimentally. The results show that C5PeC11 exhibits the type of pH-dependent surface activity and interfacial shear rheology observed in C₆-asphaltenes with a decrease in the interfacial tension concomitant with the elastic modulus when the pH increases. Surface pressure–area (Π–<i>A</i>) isotherms show evidence of aggregation behavior and π–π stacking at both the air/water and oil/water interfaces. Similarly, interactions between adsorbed C5PeC11 compounds are evidenced through desorption experiments at the oil/water interface. Contrary to indigenous asphaltenes, adsorption is reversible, but desorption is slower than for noninteracting species. The reversibility enables us to create layers reproducibly, whereas the presence of interactions between the compounds enables us to mimic the key aspects of interfacial activity in asphaltenes. Shear and dilatational rheology show that C5PeC11 forms a predominantly elastic film both at the liquid/air and the liquid/liquid interfaces. Furthermore, a soft glassy rheology model (SGR) fits the data obtained at the liquid/liquid interface. However, it is shown that the effective noise temperature determined from the SGR model for C5PeC11 is higher than for indigenous asphaltenes measured under similar conditions. Finally, from a colloidal and rheological standpoint, the results highlight the importance of adequately addressing the distinction between the material functions and true elasticity extracted from a shear measurement and the apparent elasticity measured in dilatational–pendant drop setups

Initial Partition and Aggregation of Uncharged Polyaromatic Molecules at the Oil–Water Interface: A Molecular Dynamics Simulation Study

Initial partitioning and aggregation of several uncharged polyaromatic (PA) molecules with the same polyaromatic core but different terminal moieties at oil–water interfaces from the bulk oil phase were studied by molecular dynamics simulation. The partition of the PA molecules between the bulk organic phase and oil–water interface was highly dependent on the terminal moiety structure of the PA molecules and aromaticity of the organic phase. The polarity ratio between the oil and water phases showed a significant influence on adsorption of the PA molecules at the oil–water interface. The presence of hydrophobic aromatic moieties in PA molecules hindered the adsorption process. Larger aromatic rings in PA molecules lowered the interfacial activity due to strong intermolecular π–π interactions and molecular aggregation in the bulk oil phase. The presence of a terminal carboxylic functional group on the side chain enhanced the adsorption of the PA molecules at the oil–water interface. The fused ring plane of the uncharged PA molecules was found to preferentially adsorb at the oil–water interface in a <i>head-on</i> or <i>side-on</i> orientation with the polyaromatic core staying in the nonaqueous phase (i.e., the principal plane of the molecule perpendicular to the oil–water interface). The results obtained from this study could provide a scientific direction for the design of proper chemical demulsifiers for PA molecule-mediated emulsions formed under specific process conditions of temperature, pressure, and pH