Surface Chemistry and Spectroscopy of UG8 Asphaltene Langmuir Film, Part 1

Abstract

This research focuses on a systematic investigation of UG8 asphaltene Langmuir films at the air-water interface using toluene as the spreading solvent. From the surface pressure-area isotherms, it was concluded that small-sized aggregates are spread on the water surface and the compression of the film leads to formation of large aggregates. Our methods provide a stringent test and confirmation for the formation of corresponding asphaltene nanoaggregates that have recently been proposed for bulk solutions. These results were confirmed by compression-decompression isotherms, Brewster angle microscopy, and p-polarized infrared reflection-absorption spectroscopy. The transfer of a single layer using both the Langmuir-Schaefer and Langmuir-Blodgett deposition techniques shows different aggregate shapes depending on the technique used as imaged using atomic force microscopy. The films reveal the existence of nanoaggregates spread on the water surface that coexist with large aggregates formed during compression. For the nanoaggregate, the thickness of the Langmuir-Schaefer and Langmuir-Blodgett films determined by AFM is consistent with small aggregation numbers of nanoaggregates determined by Langmuir film compression. In addition to these findings, the spreading solvent, toluene, was found to be trapped within the aggregates as confirmed by in situ UV-vis spectroscopy at the air-water interface. This result was possible only after waiting a time period of 1 h to allow the complete evaporation of the spreading solvent. This is the only study that reveals the presence of the in situ toluene within the UG8 aggregates directly at the air-water interface.

Probing Interfacial Structure and Dynamics of Model and Natural Asphaltenes at Fluid–Fluid Interfaces

“…Results show that based on the same mass of asphaltenes spread at the interface, the slope of the surface pressure curve (between 10 and 40 mN/m) for the first compression increases as the concentration of the spreading solution decreases. These results are in agreement with other works ,, that relate the active molecular area with the size of the asphaltene aggregate at the interface. Spreading solutions of higher concentrations provide larger asphaltene aggregates reducing their interfacial activity.…”

“…Images showed that these clusters are adsorbed at the interface as a whole domain. Orbulescu et al worked with asphaltene nanoaggregates and showed, using spectroscopy techniques, that polycyclic groups tend to position parallel to interface while peripheral alkanes are perpendicular to the interface at surface coverage values close to that of a theoretical monolayer (∼20 Å 2 /molecule). Zhang et al studied the effect of the spreading solvent in asphaltene interfacial behavior during compression–expansion experiments and found that the use of toluene favors the expansion process and weakens the compressibility of asphaltene monolayers .…”

A model of compression isotherms for analyzing particle layers

“…(10) and correlation coefficients calculated for the consecutive layers of multilayer films. accordance with the results published in the literature [38,39].…”

“…It was proposed in the literature that formation of stable films at the oil-water or air-water interfaces is due to the polar aromatic ring system and apolar alkyl side chains present in asphaltenes [30,31,36,37]. However, at concentrations above 1 mg ml −1 , asphaltenes form nanoaggregates [38,39] in solution prior to spreading, or upon film compression. Due to interplay of complex steric and energetic factors, the films formed with asphaltenes are challenging for a theoretical description.…”

Advances in Asphaltene Science and the Yen–Mullins Model

“…The films of asphaltene nanoaggregates are found to be ∼2 nm, whether grown from toluene or chloroform, which is in accordance with small aggregation numbers. 70,71 The size of asphaltenes and clusters has been investigated by a series of studies specifically analyzing the absolute crosssection of SAXS and SANS together. 45,46 Figure 18 shows an example of this analysis.…”