Asphaltic micelles

Storm, D.A. Barresi, R.J. Sheu, E.Y. Bhattacharya, A.K. DeRosa, T.F. Microphase behavior of asphaltic micelles during catalytic and thermal upgrading. Energy Fuels 1998, 12, 120-128. 

Colloidal State. The principal outcome of many of the composition studies has been the delineation of the asphalt system as a colloidal system at ambient or normal service conditions. This particular concept was proposed in 1924 and described the system as an oil medium in which the asphaltene fraction was dispersed. The transition from a coUoid to a Newtonian Hquid is dependent on temperature, hardness, shear rate, chemical nature, etc. At normal service temperatures asphalt is viscoelastic, and viscous at higher temperatures. The disperse phase is a micelle composed of the molecular species that make up the asphaltenes and the higher molecular weight aromatic components of the petrolenes or the maltenes (ie, the nonasphaltene components). Complete peptization of the micelle seems probable if the system contains sufficient aromatic constituents, in relation to the concentration of asphaltenes, to allow the asphaltenes to remain in the dispersed phase. 

Asphalt emulsions are dispersioas of asphalt ia water that are stabilized iato micelles with either an anionic or cationic surfactant. To manufacture an emulsion, hot asphalt is mixed with water and surfactant ia a coUoid mill that produces very small particles of asphalt oa the order of 3 p.m. These small particles of asphalt are preveated from agglomerating iato larger particles by a coatiag of water that is held ia place by the surfactant. If the asphalt particles agglomerate, they could settle out of the emulsion. The decision on whether a cationic or anionic surfactant is used depends on the appHcation. Cationic stabilized emulsions are broken, ie, have the asphalt settle out, by contact with metal or siHcate materials as weU as by evaporation of the water. Siace most rocks are siHcate-based materials, cationic emulsions are commonly used for subbase stabilization and other similar appHcations. In contrast, anionic emulsions only set or break by water evaporation thus an anionic emulsion would be used to make a cold patch compound. 

Mack (58, 59) points out that asphaltenes from different sources in the same petro-lenes give mixtures of approximately the same rheological type, but sols of the same asphaltenes in different petrolenes differ in flow behavior. Those in aromatic petrolenes show viscous behavior and presumably approach true solution. Those in paraffinic media show complex flow and are considered to be true colloidal systems. Pfeiffer and associates (91) consider that degree of peptization of asphaltene micelles determines the flow behavior. Thus, a low concentration of asphaltenes well peptized by aromatic petrolenes leads to purely viscous flow. High concentrations of asphaltenes and petrolenes of low aromatic content result in gel-type asphalts. All shades of flow behavior between these extremes are observed. 

Brule (27) deduced from his experiments on asphalts that isolated asphaltenes form agglomerates which may be dissociated into micelles by simple dilution and that the intensity of the phenomena should characterize the force of interaction. He also noticed that in some cases there is a drift towards small sizes which implies a continual departure by molecules of all dimensions. On different types of material there was a general trend towards molecules in the approximate range of 50 A moreover, our experiments have clearly demonstrated that some of the processes involved may be very slow. Also, the redistribution of material across the chromatogram might be attributable to various types of reassociation once some part of the aggregate dissociated to small MW species. 

In 1991, Andersen and Birdi (25) first reported a critical micelle concentration (erne) of asphaltenes in a mixture of n-aUcane and toluene, using a ealorimetric titration method. From their study, the aggregation process of asphaltic molecules in solutions was suggested as the following ... 

Many of the properties of asphalt are determined by the variety of chemical types and their divergent properties. The asphaltenes and saturates are immiscible. Mixtures of asphaltenes and naphthene aromatics are highly non-Newtonian at 100°F, but polar aromatics and asphaltene mixtures are Newtonian (63). It has long been proposed (74,75) that asphalt exists as asphaltene micelles or clusters solubilized by polar aromatics. 

MORPHOLOGICAL SIZE OF ASPHALTENE MICELLES IN ASPHALT AND HEAVY RESIDUE... 

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. 

Asphaltene dispersed in the asphalt can form micelles, supeimicelles, and even giant supermicelles or liquid crystals, depending on the content of asphaltene and resins. All units exist in the asphiit systems but their distributions are different. The micelles and supeimicelles are predominantly sol type asphalts liquid crystal are predominantly gel type asphalts and giant supermicelles are predominantly sol-gel types. For different types of asphalt, the physical and chemical properties are different and, therefore, their uses and applications will differ. 

From the Table n, it can be found that no matter what type of asphalt, all basic units of asphaltene cells are the same size. No matter what solvent is used, the asphaltene micelle is ca. 10 nm, and is consistent with the results of other methods. 

It is demonstrated that asphaltene controls the physical properties and performance of the asphalt system. The micelles, giant supermicelles, and liquid crystals are shown dominating... 

Asphaltene particles are dispersed in saturates and aromatic hydrocarbons (gas-oil) with resins as peptizing agents in asphalt or heavy oil. The interaction between resin and asphaltene micelles is not well understood. In the present study, asphaltene has been dispersed into aromatic hydrocarbons (such as toluene), and the precipitations due to additions of paraffinic hydrocarbons (such as pentane) in the presence of a number of amphiphiles have been studied. These amphiphiles certainly affect the asphaltene precipitation, either by retardation or by enhancement, depending on the structural types and quantities of the amphiphiles. We have found that the nature of resin is that it behaves as an amphiphile, since the polar fractions of resin do contain amphiphiles. The solubility parameter spectra of these asphaltenes are discussed. 

Because the asphalt system is not a true solution, it can be fractionated into saturates, aromatics, resins, and asphaltenes by the solvent fraction method, SARA method, or TLC method. The polarity of these four fractions is increased in the order of saturates, aromatics, resins, asphaltenes. In crude oil, asphaltene micelles are present as discrete or dispersed particles in the oily phase. Although the asphaltenes themselves are insoluble in gas-oil (saturates and aromatics), they can exist as fine or coarse dispersions, depending on the resin content. The resins are part of the oily medium but have a polarity higher than gas-oil. This property enables the molecules to be easily adsorbed onto the asphaltene micelles and to act as a peptizing agent of the colloid stabilizer by charge neutralization. 

From the above demonstration, it can be noted that micelle structures are predominant in asphalt with a higher asphaltene content. Three different types of asphalt such as sol (micelle, supermicelle, giant supermicelle), sol-gel (supermicelle, giant supermicelle), gel (liquid crystal) asphalt, can be defined. Most of the paving asphalts belong to the sol-gel type of asphalt, and roofing asphalt belong to the gel (air blown) type of asphalt. 

Anionic Surfactants onto Kaolinite and lUite. In the investigation of the adsorption of sodium dodecylbenzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS) onto asphalt covered kaolinite and illite surfaces, Siffert et al. [5S] observed Langmuir type I isotherms for SDS adsorption onto Na kaolinite and Na illite while the SDBS exhibited a maximum in adsorption with a decrease beginning near the CMC. Adsorption maxima were observed near the CMC for both surfactants in the Ca kaolinite and Ca illite systems. The adsorption behavior was explained as precipitation of the calcium salt of the surfactants (an idea supported by other studies), and the interaction of the aromatic ring in SDBS with the asphalt. This interaction favors desorption of the asphalt rather than adsorption of the SDBS. The amount of asphalt desorbed by SDBS was twice that desorbed by SDS. Other explanations for adsorption maxima include mixed micelle formation [55] and electrostatic repulsion of micelles from the bdayer covered surface [59]. 

Monomeric. isphaltcne Moiiomcric resin Asphalt-free oil specie. Micelle... 

The micelle sketched in Fig. 5.10 contains an asphaltene core with asphaltene molecules 2 resin molecules are adsorbed onto the surface of the core. In addition to resins that are part of the solvation shell surrounding the core, asphalt-free oil species are also present in the shell. The formation of the solvation shell around the asphaltene core lowers the Gibbs free energy. The standard Gibbs free energy of micelli-zation, AG , represents the standaird Gibbs free energy difference between (1) asphaltene molecules in the core, resin molecules in the shell, and (2) those and ri2 molecules in an infinite-dilution petroleum mixture ... 

As the sketch in Fig. 5.10 shows, the bulk liquid phase consists of micelles, monomeric asphalts and resins, and asphalt-free oil monomers in the bulk phase and in the shell. Let us denote asphaltenes and resins as the solute and the rest of the species (that is, asphalt-free oil species) as the solvent. Then the Gibbs free energy of the liquid phase, can be written as... 

Solution Figure 5.11 provides the sketch for a micelle when an aromatic solvent is added to the crude oil. The effect of aromatic solvent in asphaltene precipitation is mainly because of the decrease of the interfacial tension. In other words, when aromatics are added to a crude oil, the term (AG ), , = ri2first step is, therefore, the calculation of a between the pure asphaltene liquid and the liquid mixture of asphalt-free crude oil and the aromatic. Pan and Firoozabadi (1998b) provide the following expression for the interfacial tension between the asphaltene liquid phase and the surrounding liquid ... 

The interaction Gibbs free energy of solute species, Ginters which represents the interactions between the micelles, the monomeric asphaltenes and resins, and the asphalt-free oil species in the liquid solution Ly, can be estimated using the mean-field approximation. 

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