No abstract

Asphaltene molecular size and weight have been of concern since asphaltenes were first isolated from crude oils. Despite previous divergent results on this topic, in recent years, there has been a growing consensus among all mass spectral ionization techniques and all diffusion measurements that asphaltenes are fairly small molecules. In this paper, fluorescence correlation spectroscopy (FCS) is used to determine translational diffusion coefficients of asphaltene and model compounds under a variety of conditions. These FCS studies provide several stringent tests on asphaltene molecular size and architecture. A broad range of concentrations including ultralow concentrations is investigated to ensure the lack of potential aggregation difficulties. Large temperature variations are used to test the application of the simple diffusion equation. FCS results here clearly show the dependence of the diffusion constant on the molecular weight. Finally, FCS results on asphaltenes are in quantitative agreement with those of time-resolved fluorescence depolarization on asphaltenes. A comparison of the results herein with previous FCS and time-resolved fluorescence depolarization (TRFD) results on the same asphaltenes confirms the correlation between molecular size and asphaltene chromophore size; this supports a molecular architecture with one or two polycyclic aromatic hydrocarbons (PAHs) per molecule and counters proposed structures with many PAHs per asphaltene molecule.

Following the great success of traditional microfluidic devices across many disciplines, a new class of microfluidic systems emerged in recent years, which features finely tuned, localized surface modifications within the microstructures in order to keep up with the demand for devices of ever increasing complexity (lab on chip, assay on chip, etc.). Graft photopolymerization has become a powerful tool for such localized surface modifications particularly in combination with poly(dimethylsiloxane) (PDMS) devices, as it is compatible with many functional monomers and allows for high spatial resolution. However, application within enclosed PDMS microstructures and in particular well-controlled surface-directed polymerization remains challenging. Detailed understanding of the interaction between photoinitiator, benzophenone (BP), and polymer matrix is needed. We have developed a visualization technique, which allows for observation of reacted BP in situ within the PDMS matrix. We present a detailed study on solvent-driven BP diffusion providing results essential to successful surface treatment. We also identified and investigated photoinitiator inhibition by oxygen and provide appropriate mitigation strategies.