Applicability of the Langmuir Equation of State for Asphaltene Adsorption at the Oil–Water Interface: Coal-Derived, Petroleum, and Synthetic Asphaltenes

In recent studies of asphaltenes from the Norwegian continental shelf at water–oil interfaces, adsorption from largely aliphatic oils appeared to be governed at early times by molecular diffusion and at later times by molecular random sequential adsorption. In addition, the interfacial tension of asphaltenes at the oil–water interface was a a unique function of interfacial coverage, and no apparent rearrangement of asphaltenes at the interface was observed within an experimental time frame of up to 12 h. Confirmation of this result was obtained by showing that high frequency elasticity is a unique function of asphaltene interfacial tension. Both interfacial tension versus coverage and elasticity versus interfacial tension data have been successfully captured by the Langmuir Equation of State (EoS), which also allows estimates of the sizes of the asphaltene polyaromatic cores to be made. The value obtained in experiments with asphaltenes from a Norwegian crude oil is 0.32 nm<sup>2</sup>/molecule, which corresponds to an average size of ∼6.2 ring polyaromatic core resting flat on the oil–water interface, which is supported by estimates made by other means. In the current study, we provide further investigation of these findings. UG8 asphaltenes, which have been well-studied previously, show interfacial behavior similar to the asphaltenes from the Norwegian oil. Experiments with coal-derived asphaltenes, which are known to have smaller cores, and model asphaltenes, with a larger cores, both confirm the applicability of the Langmuir EoS to fit the interfacial tension data and enable estimates of the polyaromatic core sizes from tensiometric measurements.