**3.3. Mechanism of asphaltene aggregation: Colloidal behavior**

Asphaltenes can be considered as a complex polydisperse system of molecules in which subcomponents with significantly different characteristics can be distinguished as the result of solubility differences [17, 20].

From the colloidal point of view, it has been stated that asphaltenes in general are a blend of two main fractions. Acevedo et al. [21] proposed a model for asphaltene colloids in toluene which consists of two subfractions with different solubilities, called A1 and A2. The subfraction A1 (90 mg/L in toluene), which is insoluble in aromatic solvents such as toluene, cumene, and others, is found in greater proportion than a second subfraction A2 (57 g/L in toluene), which is soluble in these solvents and acts as a promotor in the dispersion of A1. The solubility of the latter is similar to the total mixture. Asphaltene aggregation phenomena is well understanding under the vision of colloidal nature of both fractions. [22]. Furthermore, this colloidal model has been found very useful in accounting for other phenomena such as trapping of compounds, flocculation, aggregation promoted by A1, asphaltene aggregation at very low concentration, and metal distribution [17].

In the vapor pressure osmometry (VPO) technique for molecular weight measures, the polarity of solvent is a critical parameter. Using o-dichlorobenzene, a 1000 g mol-1 value was reported for A2 solutions at 120 °C. On the other side, A1 afforded higher values (around 2600 g mol-1), emphasizing the strong aggregation tendency of this fraction [23].

Asphaltenes can form different types of aggregates depending on the environment: oil, solvent, interface, and others. Mullins et al. [24] proposed a scheme to explain the aggregation of asphaltenes in toluene. At low concentration, asphaltene is a group of individual molecules that evolve into nanoclusters and are associated in particles until reaching a system susceptible to flocculate. Floccules are unstable aggregates that eventually precipitate. In certain cases, stable aggregates that give the crude viscoelastic properties are formed.

Due to the presence of polar groups, asphaltenes can behave as surfactants, exhibiting the phenomena of adsorption and aggregation. However, it is wrong to attribute a critical micelle concentration (CMC) for asphaltenes and other macromolecules from crude oil. CMC is a parameter exclusive for surfactants, such as sodium dodecyl sulfate (8.3 \* 10-3 M in water). Parameters such as aggregation number, shape and size of the micelle, electrolyte effect on the micellization and other additives are well known for surfactants. For petroleum asphaltenes this knowledge does not exist, mainly because the structure of an asphaltenes molecule is unknown. Aggregates phenomena best describes asphaltenes behavior. Asphaltene in toluene begins to form aggregates at 50 ppm concentration [25].

In summary, the use of CMC term for asphaltenes and other natural surfactants contained in the crude oil should be avoided. The emulsions in crude oil are produced for a mechanism starting with asphaltenes migrate into the oil–water interface, a process which is regulated by the diffusion of the soluble asphaltenes [19]. Adsorption of asphaltenes at the interfaces is a very slow process. The initial diffusion step toward the interface is rapid and it is followed by a long reorganization and the progressive building of multilayers. Asphaltenes diffuse from the bulk of crude oil to oil–water interface and are then adsorbed, decreasing the interfacial tension [26]. Asphaltenes can also form multilayers on solid surfaces such as silica. It has been found that increasing the concentration of asphaltenes in solution tends to increase its state of aggregation, which decreases the rate of adsorption on the surface [22].
