Suppression of Phase Separation as a Hypothesis to Account for Nuclei or Nanoaggregate Formation by Asphaltenes in Toluene

Abstract

Here, the concept of suppression of phase separation is proposed to account for the solubility behavior of asphaltenes at high dilution in toluene under ambient conditions. Nuclei formation at concentrations near 90 mg L–1 is the consequence of reaching A1 fraction solubility, and phase separation is suppressed by the intercalation of sufficient A2 in these nuclei or nanoaggregates. Presumably, such intercalation leads to media penetration of the nuclei periphery, hindering the growth and allowing for nuclei dispersion as a kinetic unit. Trapped compounds (TCs) or compounds trapped by asphaltene clusters were isolated, and their elemental analysis showed that they were neither resins nor asphaltenes. The information available regarding the A1 and A2 asphaltene subfractions is revised and complemented with new thermogravimetric analysis, simulation distillation (SimDis) curves, microcarbon Conradson, softening points, and nanoparticle results involving size-exclusion microchromatography. In general, physical results, such as solubility, SimDis, aggregation, and the softening point, differ substantially, whereas structural results, such as elemental analysis, DBE, and 13C nuclear magnetic resonance spectra, are similar. These results suggest that minor structural differences strongly affect the solubility, softening point, and other physical characteristics.

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“…In this way and from a qualitative point of view, it can be expected that a material represented by the M2 model should have greater solubility than the M1 model. Consistent with these ideas, it should be noted that the A1-based aggregates form at very low concentrations and the addition of A2 on these aggregates prevents their growth beyond the colloidal size, i.e., keeps the A1 molecules dispersed [70]. In this sense, Figure 2 shows a model of molecular mechanisms that simulate the nanoaggregates between four asphaltene A1 molecules represented under the M1 model and three A2 represented under the M2 model.…”

“…In this way and from a qualitative point of view, it can be expected that a material represented by the M 2 model should have greater solubility than the M 1 model. Consistent with these ideas, it should be noted that the A 1 -based aggregates form at very low concentrations and the addition of A 2 on these aggregates prevents their growth beyond the colloidal size, i.e., keeps the A 1 molecules dispersed [70]. In this sense, Figure 2 shows a model of molecular mechanisms that simulate the nanoaggregates between four asphaltene A 1 molecules represented under the M 1 model and three A 2 represented under the M 2 However, the stability of asphaltenes depends to a large extent on the content of resins present in the crude oil matrix, because they can avoid asphaltene-asphaltene interactions by positioning on the surface of the asphaltene aggregate, penetrating its microporous structure and finally breaking the aggregation system [72][73][74] Previous studies have reported that higher adsorption of the resins on the colloidal fractions showing a higher affinity for A1 than for A2, indicating that the resin-A1 interactions are more favorable than the A1-A1 interactions [70].…”

“…From Figure 1 is observed that the A1 fraction is formed by the bond of aromatic and aliphatic rings, which suggests a nucleus composed of polycyclic and naphthenic aromatic units (PANU), fused in a large flat nucleus. In the case of the A2 fraction, a large number of aromatic carbons is observed, and the presence of small amounts of PANU bound by aliphatic chains is suggested [70]. In this way and from a qualitative point of view, it can be expected that a material represented by the M2 model should have greater solubility than the M1 model.…”

“…The A 2 fraction has a high solubility (57 g•L −1 ), which is very similar to the solubility of the total mixture. In response to this definition, it should be noted that the strong trend of aggregation of asphaltenes is due to the presence of the insoluble fraction (A 1 ), which is present in lower quantity [70]. In this sense, studying the colloidal aggregation processes of A 1 and A 2 fractions allows an understanding of the properties of the total asphaltene fraction [71].…”

“…In this sense, studying the colloidal aggregation processes of A 1 and A 2 fractions allows an understanding of the properties of the total asphaltene fraction [71]. Nuclear magnetic resonance spectroscopy (NMR) studies combined with elemental analyses suggests that there are structural differences for the A 1 and A 2 fractions, such as those shown in the M 1 and M 2 models, where the molecular structure of the M2 was constructed from the opening of two aliphatic rings in M1 [70]. The proposed molecular models M1 and M2 for the graphical description of asphaltene fractions A 1 and A 2 , respectively, are shown in Figure 1.…”