New evidence on the origin of 'bee structures' on bitumen and oils, by atomic force microscopy (AFM) and confocal laser scanning microscopy (CLSM). (1st November 2021)

• Record Type: Journal Article
• Authors: Blom, Johan; Soenen, Hilde; Van den Brande, Niko; Van den bergh, Wim
• Is Part Of: Fuel. Volume 303(2021)
• Publication Date: 2021-11-01
• Subjects: AFM -- CLSM -- Mineral oil -- Bitumen -- Maltene -- Bees -- Wax; Fuel -- Periodicals; Coal -- Periodicals; Coal; Fuel; Periodicals; 662.6
• DOI: 10.1016/j.fuel.2021.121265
• Languages: English
• ISSNs: 0016-2361

Abstract

Graphical abstract: Highlights: CLSM is complementary to AFM when investigating bee structures. A mixture of alkanes, differing in melting points, can create islands around the bee structures. Bee structures are not limited to bitumen and can be created on maltenes and mineral oil surfaces. No bee structures are observed on surfaces in contact with glass or water. In the bulk of a mineral oil sample waxes crystallize as small particles. Abstract: Bitumen is a complex mixture whose composition–structure-property relationships are not well-understood. Therefore, it is essential to understand which components may develop microstructures, how and when these features appear, and if they are initiated or influenced by other components. Many studies have reported so-called bee structures as well as other phases on bitumen surfaces. A large number of observations point towards the crystallinity of waxy compounds as an explanation for the bees. However, doubt remains about the origin of the other phases and the possibility of other bitumen components as promoters of the bee formation. In this study, bee structures were investigated using two microscopy techniques: atomic force microscopy (AFM) and confocal laser scanning microscopy (CLSM). By using model wax compounds, especially blends of waxes differing in melting points and chain lengths, bee structures and surrounding islands were created on featureless bitumen surfaces. Structures very similar to what is observed on a paraffinic bitumen were obtained. This indicates that bees and the surrounding areas both consist of crystallisable components. Moreover, bee structures were also generated on other surfaces, such as mineral oil and a maltene fraction. Bee formation is not limited to bituminous materials and bitumen components are not needed as promotors. Furthermore, CLSM offers possibilities to scan through transparent layers and to investigate surfaces under glass, for example. In contact with such a solid substrate, as well as after water submersion, no microstructural features were observed. Adding a combination of n-alkanes, to transparent oil, induced bee structures at the oil-air interface. The transparency of the oil allowed the visualization of wax crystallization in the bulk, where the added waxes crystallize as small particles. These small crystals are expected to influence the mechanical properties, such as low-temperature stiffening effects, which have been reported for paraffinic bitumen.