**3.4. Stability of water-in-crude-oil emulsions**

metals like nickel, vanadium and iron, which confer polarity and amphiphilic characteristics to these macromolecules [16]. Asphaltenes can generate a wide distribution of molecular

Asphaltenes are composed of several polynuclear aromatic sheets surrounded by hydrocarbon tails, and form particles whose molar masses are between 500 and 20.000 g. They contain many

The specific structure of asphaltenes is unknown; however, molecular weight averages about 750 and there is a planar aromatic structure surrounded by alkane groups, some with heteroatoms, sulfur (S), nitrogen (N), and oxygen (O) [19]. Deposition of asphaltenes often causes some changes in the flow behavior of the oil fields through processing facilities, and difficulties in oil recovery. The problems caused by asphaltenes are closely related to its stability. Due to pressure variations, oil composition, and temperature, asphaltenes tend to

Experimental evidence from a combination of spectroscopy techniques and elemental analysis suggest that resins contains several fused five-and-six carbons rings in addition to alkyl chains.

Crude oil can be defined as a dispersed system where asphaltenes are colloids. Asphaltene molecules are in the center of the structure of the liquid oil, the other components are organized around the layer of asphaltenes decreasing polarity (resins, saturated and aromatic hydrocar‐ bons). This simple model allows reconciling the compatibility presence within the same fluid

Asphaltenes can be considered as a complex polydisperse system of molecules in which subcomponents with significantly different characteristics can be distinguished as the result

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,

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),

structures that can vary greatly from one crude to another [17].

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

emphasizing the strong aggregation tendency of this fraction [23].

functional groups, including some acids and bases [18].

form aggregates [14].

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and asphaltenes.

of solubility differences [17, 20].

and metal distribution [17].

In terms oftime,the stability ofthe emulsion has a range from few minutes to years and it varies by the characteristics of the crude oil and water [27]. The stability mechanism is, of course, related to the composition of crude oil, such as type and the concentration of natural surfac‐ tants, the physical properties of oil, and the interfacial rheology of the interface around water droplet,whichprovidesinformationaboutelasticityandviscosityand,forinstance,thepresence of a film (called skin) for the specific case of petroleum crude oil at this interface [10, 28].

Gao et al. [29] quantitatively studied the formation of interfacial film by the asphaltenes on the oil–water interface. They propose to measure the appearance of crumpling in the interfacial film, using imaging drop following compression by extraction volume, by the following relationship:

$$\text{CR} = \frac{A\_f}{A\_l} = \frac{\pi R\_{f^2}}{\pi R\_{f^2}} = \frac{R\_{f^2}}{R\_{f^2}}\tag{3}$$

where CR is crumpling ratio; A*<sup>i</sup>* is the projected drop initial area; A*<sup>f</sup>* is the projected area of the drop observed just before crumpling; R*<sup>i</sup>* is the projected drop initial radio; and R*<sup>f</sup>* is the radio of the drop observed just before crumpling. Figure 2 shows a schema of the contraction of a drop of oil in water. In Figure 2 (a), crumpling phenomenon does not occur; drop reduces its volume to be sucked into the syringe. In Figure 2 (b), volume drop is contracted when the drop crumpling phenomenon occurs in the interfacial film. The scheme shows its building. CR parameter is related to the irreversible adsorption of asphaltenes at the water–crude interface; an increasing of CR means an increasing in emulsion stability.

**Figure 2.** Volume contraction scheme of a drop of oil in water system: (a) absence of crumpling phenomenon and (b) presence of crumpling phenomenon occurs.

As part of the stabilization mechanism, it is accepted that the water droplets are stabilized by the formation of a mechanically strong and viscoelastic film at the interface, composed of asphaltenes [30].

Subsequently, several workers reviewed emulsions and concluded that asphaltene content is the most important factor in the formation of emulsions [7-10]. Asphaltene fraction is mainly responsible for the high emulsion stability of water-in-crude-oil; thus, they were found to be capable of forming rigid, cross-linked, and elastic films [19].

Asphaltenes stabilize water-in-oil emulsions if they are near or above the point of incipient flocculation, i.e., solid particles which can be. Other investigators have also suggested that asphaltene colloids are responsible for stable emulsions. Asphaltenes may be at the interface as fine solid particles or resin-asphaltene colloids. However, Yarranton et al. [31] reported that, at low asphaltene concentrations (<0.2 wt %), molecular state of asphaltenes predominant onto the colloidal form. It is also necessary to consider the effect of resins as they may influence asphaltene aggregation, adsorption on the interface, and emulsion stability [32].

One of the most used methods for studying emulsion stability is the bottle test. Figure 3 shows the bottle test sample for evaluating the stability of water-in-oil emulsion. It was observed that in the tubes on the left side there are stable emulsions, while in the tubes in the center and on the right side, there is a good separation of the two phases.

**Figure 3.** Stability bottle test of water-in-crude-oil emulsion (Silva, 2012).

where CR is crumpling ratio; A*<sup>i</sup>*

74 Advances in Petrochemicals

presence of crumpling phenomenon occurs.

capable of forming rigid, cross-linked, and elastic films [19].

asphaltenes [30].

is the projected drop initial area; A*<sup>f</sup>* is the projected area of the

is the radio

drop observed just before crumpling; R*<sup>i</sup>* is the projected drop initial radio; and R*<sup>f</sup>*

an increasing of CR means an increasing in emulsion stability.

of the drop observed just before crumpling. Figure 2 shows a schema of the contraction of a drop of oil in water. In Figure 2 (a), crumpling phenomenon does not occur; drop reduces its volume to be sucked into the syringe. In Figure 2 (b), volume drop is contracted when the drop crumpling phenomenon occurs in the interfacial film. The scheme shows its building. CR parameter is related to the irreversible adsorption of asphaltenes at the water–crude interface;

**Figure 2.** Volume contraction scheme of a drop of oil in water system: (a) absence of crumpling phenomenon and (b)

As part of the stabilization mechanism, it is accepted that the water droplets are stabilized by the formation of a mechanically strong and viscoelastic film at the interface, composed of

Subsequently, several workers reviewed emulsions and concluded that asphaltene content is the most important factor in the formation of emulsions [7-10]. Asphaltene fraction is mainly responsible for the high emulsion stability of water-in-crude-oil; thus, they were found to be

Langmuir technique has been widely used for the characterization of the interfacial film formed by natural surfactants from crude oil [33-37]. Isotherms provide surface-pressure-area information on the compressibility of the film formed by the asphaltenes and other natural surfactants. The interfacial film can be collected with the Langmuir-Blodgett technique. Extensive morphological characterization of the interfacial film has been performed, including properties such as contact angle, thickness, and aggregate size, among other. Images obtained using Atomic Force Microscopy have contributed to the understanding of the mechanisms of interfacial adsorption, swelling, aggregation of colloidal interactions, and asphaltene [38, 39].

Aromaticity, length of the alkyl side chains, and branch reduction in the aliphatic chains of resins and asphaltenes are associated with the formation of stable emulsions. Another important parameter is the pH of the aqueous phase. Asphaltenes and resins are composed of polar functional groups; these groups can be ionized at alkaline pH. Surface charge density increases due to ionization of polar groups, generating internal repulsive forces destroying the mechanical properties preventing droplet coalescence [14].
