Asphaltene Subfractions Responsible for Stabilizing Water-in-Crude Oil Emulsions. Part 3. Effect of Solvent Aromaticity

Abstract

Whole asphaltenes (WA) were fractionated by the E-SARA method according to their adsorption characteristics at oil−water interfaces from either toluene or heptol solutions. Heptol, a mixture of n-heptane and toluene at a 1:1 volume ratio, is a less aromatic solvent than toluene. The effect of solvent aromaticity on the composition of resulting asphaltene subfractions at oil−water interfaces was studied to determine the key functional groups that are critical to the asphaltene-induced stabilization of water-in-oil (W/O) petroleum emulsions. The interfacially active asphaltenes (IAA) were extracted as materials irreversibly adsorbed onto emulsified water droplets, while the asphaltenes remaining in the oil phase were considered as remaining asphaltenes (RA). Although toluene-extracted interfacially active asphaltenes (T-IAA) accounted for only 1.1 ± 0.3 wt % of WA, this subfraction of asphaltenes exhibited a greater interfacial activity and formed more rigid films at the oil−water interface than IAA extracted using heptol, known as HT-IAA which accounted for 4.2 ± 0.3 wt % of WA. The increased potential of T-IAA to stabilize W/O emulsions was attributed to their higher content of oxygen, resulting in a higher content of sulfoxide groups, as verified by elemental analysis, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Although the toluene-extracted remaining asphaltenes (T-RA) and heptol-extracted remaining asphaltenes (HT-RA) were shown to contain similar H/C ratios and nitrogen contents to those of T-IAA and HT-IAA, the two RA subfractions contained a much less amount of sulfur and oxygen, leading to a much reduced interfacial activity as compared with that of IAA subfractions. In spite of the small proportions in asphaltenes, oxygen-containing functional groups, in particular sulfoxides, were believed to contribute significantly to the increased stability of asphaltene-stabilized W/O petroleum emulsions.

Interactions of Asphaltene Subfractions in Organic Media of Varying Aromaticity

“…Based on the different organic solvents used in the E-SARA fractionation, two IAA subfractions and two corresponding RA subfractions were collected as follows: toluene- fractionation of whole asphaltenes can be found in our previous study. 40…”

“…In comparison with the RA subfractions, the IAA subfractions also contained more of other oxygenated polar functional groups, such as carbonyl and hydroxyl groups, more so for T-IAA than HT-IAA. 40 The T-IAA subfraction was therefore expected to result in the smallest value, followed by HT-IAA, and then the two RA subfractions. This trend was in good agreement with the order of fitted values, indicating the predominant role of polar interactions in the adsorption of asphaltenes onto hydrophilic silica.…”

“…In comparison, deposited asphaltenes following rinsing with pure toluene exhibited a layer of asphaltene aggregates of irregular shapes, suggesting a strong irreversible adsorption of asphaltenes onto hydrophilic silica surfaces (Figure 2b-2e Asphaltenes adsorb onto silica surfaces primarily through their polar functional groups. 52,53 The sulfoxide group was considered the single identifier for IAA subfractions 40 with the T-IAA containing more sulfoxides than the HT-IAA ( Table 1). The presence of polar oxygenated functional groups allows the asphaltene molecules to arrange themselves more favorably on hydrophilic solid surfaces 53,54 and oil-water interfaces, 12,39,40 mainly through hydrogen bonding interactions.…”

“…In the current work, the four different asphaltene subfractions were obtained according to the procedure outlined in our earlier study. 40 Dynamic light scattering (DLS) was used to measure the aggregation behaviors of these fractionated asphaltenes in organic solvents of varying aromaticity, while AFM colloidal force measurements were conducted to understand the mechanisms of their molecular interactions. Combined with the detailed characterizations of fractionated asphaltenes through elemental analysis and X-ray photoelectron spectroscopy (XPS), the results from this study provide molecular insights into the role of key functional groups in controlling asphaltene interactions.…”

The composition of acid/oil interface in acid oil emulsions

“…As to asphaltenes, there is evidence that only the most self-associated (least soluble) asphaltenes with increased aromaticity (Rocha et al 2018), decreased alkyl side chain lengths, and reduced chain branching (Khadim and Sarbar 1999), act as emulsion stabilizer. Yang et al (2014Yang et al ( , 2015 suggested that the most interfacial active asphaltene (IAA) subfraction having higher molecular weight and higher contents of heteroatoms, oxygen-containing functional groups, in particular the high-polarity sulfoxide groups (Qjao et al 2017), concentrates at the oil/water interface. It is assumed that IAA molecules self-associate in solvent, and as supramolecular structures with a porous network are concentrated at the oil/water interface.…”