Abstract
Asphaltenes are a group of exotic hydrocarbons found in bitumen and other forms of heavy crude oil derivatives. These hydrocarbons, with elusive chemistry, give crude oil derivatives (CODs), such as bitumen, its characteristic properties. In bitumen, they form stable aggregates by interacting with other molecules, called asphaltene aggregates. Attempts have been made to enhance bitumen with nanoparticles, like graphene derivatives. Such studies have been successful in displaying the enhanceability of bitumen, but no studies have been directly focused on how the structural stability of asphaltene aggregates present in bitumen is affected by the incorporation of nanoparticles. The phase stability of the asphaltene aggregates is a pertinent question, which is often ignored. In this study, we investigate the physical impact of incorporating graphitic nanoparticles on the structure of bitumen. For this, we utilise graphene oxide (GO). GO is a form of polyaromatic nanoparticle with a similar structure to asphaltenes, such that both have molecular defects induced by heteroatoms. We have experimentally investigated the structural stability of the asphaltenes, using XPS, XRD and SEM-EDX to elucidate the interaction between asphaltenes and GO, and its implications for the stability of bitumen used for e.g., the surface layer of roads. In roads, asphaltene aggregates exist as stable structures, until GO has been introduced. The experimental results show that the introduction of GO initiates destabilisation of the asphaltene aggregates, and we discuss the destabilisation mechanism in this paper. Thereby, we conclude that counter intuitively, the introduction of graphene or GO has a negative impact on the structure of bitumen, thus hindering any functional enhancements to bituminous roads.
Highlights
Bitumen is a well-known and comprehensively studied colloidal system, with a viscosity of nearly a million pascal-seconds (Pa.s) [1]
It can be concluded that both asphaltenes and Dodecylbenzene sulfonic acid (DBSA) are adsorbed on the surface of graphene oxide (GO) and that the particles observed by the scanning electron microscope (SEM) images (Fig. 4) on the precipitate are destabilised asphaltene aggregates
This study is the first with the focus on determining how the struc tural stability of asphaltene aggregates is affected by the introduction of nanoparticles, especially GO, with the goal to understand the impact of using GO as a demulsification agent of bitumen-water emulsion, and its subsequent impact on the function of bitumen, post demulsification
Summary
Bitumen is a well-known and comprehensively studied colloidal system, with a viscosity of nearly a million pascal-seconds (Pa.s) [1]. The asphaltene nanoaggregates interact with less polar molecules called resins, that act as surfactants, to form larger aggregates structures, i.e., asphaltene aggregates. In emulsifi cation, the heavy CODs are dispersed in water as droplets stabilized by surfactants [33], which results in a reduction of the viscosity In this case, the asphaltene aggregates contribute to forming stable water-in-oil emulsions [32,33,34,35,36,37]. The general conclusions drawn from the results obtained by these studies, is that asphaltenes at the oil-water interface adsorb onto GO surfaces through an acid-base interaction, which causes an increase in the interfacial tension and eventual a phase separation of the emulsion. From the results it is obvious that asphaltenes adsorb on GO irreversibly and thereby GO will cause detrimental effects of bitumen properties, the high viscosity of bitumen makes the physical interaction between GO and asphaltenes difficult to observe on a macroscopic scale
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