Asphaltenes hydrogen bonding

In addition to adsorbing at mineral-oil interfaces, asphaltene molecules also adsorb at oil-water interfaces. Strong intermolecular dipole-dipole, hydrogen bonding, electron donor-acceptor and acid-base interactions cause the surface-adsorbed asphaltene molecules to form rigid skins" at oil-water interfaces (41 43). When water droplets are dispersed in an oil which contains asphaltene molecules, molecularly thick, viscous asphaltene films form around the water droplets, inhibit the drainage of intervening oil and sterically stabilize the water-inoil emulsion. 

Sternberg et al. (7) showed that the presence of asphaltenes in coal-derived oils caused a marked increase in the viscosity. This group also showed that these asphaltenes were acid-base complexes and that hydrogen bonding occurs between the acidic and basic components of asphaltenes (9, 10). Recent work (J3, 11) on coal liquefaction bottoms has shown the importance of hydrogen bonding on the viscosity of coal liquids. 

Macrostructure. A very distinctive feature of coal-derived asphaltenes is that they appear to be associated both in concentrated and diluted solutions, and, particularly, in nonpolar solvents. This fact is supported by the viscosity measurement (40). The fitting of a model of monomer-dimer-trimer for the coal-derived asphaltenes is almost perfect (34). The hydrogen-bonding nature of the interaction within the coal-derived asphaltenes is important (35, 41, 42). The other interaction is still the tt-tt association. X-ray data give a low value of... 

The high polarity and low association of coal-derived asphaltenes can be used to explain the nature (hydrogen-bonding) and reactivity of coal conversion. 

The results of this study imply that the asphaltenes present in the straight-reduced asphalt primarily consist of compounds capable of hydrogen bonding, that is, acids and bases. 

The high viscosity at ambient temperature of coal liquids derived from hydrogenation processes has been related to the asphaltene (toluene-soluble, pentane-insoluble) and preasphaltene (toluene-insoluble, pyridine-soluble) fractions (1-5). Although the effect of preasphaltene concentration on the viscosity of coal liquids is dramatic, the increase caused by asphaltene materials has been attributed to hydrogen-bonding (6) and acid-base salt... 

Asphaltenes are generally defined as those components in petroleum and coal liquids that under certain conditions are soluble in benzene but insoluble in aliphatic solvents, such as n-pentane, n-heptane, or cyclohexane. This definition obviously includes a broad variety of components, as insolubility in the above solvents can be caused by high molecular weight, high polarity, hydrogen bonding, acid-base complexing, or combinations of these parameters. 

Separation of the Asphaltene. Table I shows the weight percent of the asphaltene fractions and subfractions produced by the separation scheme. The acid fraction, amounting to 81% of the total asphaltene, is the largest fraction isolated by the separation scheme. The primary prerequisite for a compound type to be defined as an acid by the anion, resin appears to be the ability of the compound type to hydrogen bond to the anion resin. Earlier work with distillates and residues identified compound types such as carboxylic acids, phenols, amides, and carbazoles as the major components of an acid fraction (6). Table I shows that Subfraction 3, the subfraction containing the strongest (most readily hydrogen bondable) acids, is more than half of the total acid fraction. 

The data in Table I are significant because they suggest that a one-to-one relationship of acids and bases does not exist for petroleum asphaltenes. The precipitation of asphaltenes may be attributed to a phenomenon other than precipitation of acid-base complexes or salts. The data strongly imply that the asphaltenes primarily consist of compounds capable of association through the hydrogen bonding mechanism. 

The precise mechanism of asphaltene association has not been conclusively established, but hydrogen bonding (66, 95, 96) and the formation of charge-transfer complexes (66) have been cited as the causative mechanisms. Evidence exists that asphaltenes participate in such complexes (97, 98), but the exact chemical or physical manner in which they would form in petroleum is still open to discussion. Intermolecular hydrogen-bonding could also be involved in asphaltene association and may have a significant effect on observed molecular weights (95). 

The concept of hydrogen-bonding interactions as one of the means of association between the asphaltenes and resins has, however, led to a reconsideration of the assumed cluster as part of the micelle. Indeed, it appears that when resins and asphaltenes are present together, hydrogen bonding may be one of the mechanisms by which resin-asphaltene interactions are achieved. In some instances, it appears that resin-asphaltene interactions may be preferred over asphaltene-asphaltene interactions. 

However, a more recent report indicates that pentane insolubility of the bulk of the coal asphaltene constituents cannot be ascribed to hydrogen-bonding effects between the acidic and basic components. 

Studies have shown that increases in asphalt viscosity with oxidation can be correlated with increases in carbonyl formation (20). Almost certainly this hardening results from hydrogen bonding between heteroatom groups in asphaltene molecules and also between polar aromatics, which then may become asphaltenes (66-69). This association strongly impacts attempts to measure molecular size by SEC or colligative properties. 

Asphalt is a lyophilic colloid in which asphaltene interacts with the dispersion medium through the peptizing agent. Because of the hydrogen bonding and dipole-dipole interaction, the asphaltene particles form micelles and even associations/clusters in the asphalt system. 

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