Molecular Insights into Glass Transition in Condensed Core Asphaltenes

Glass transition in a condensed core asphaltene model was investigated using molecular dynamics simulations performed in the isobaric–isothermal ensemble. Glass transition temperature obtained from the discontinuities in the slope of specific volume versus temperature plots was in close agreement with experimental results reported in the literature. These discontinuities also correspond to those in isothermal compressibility versus temperature plots. In this paper, we separate the contributions of aliphatic and aromatic regions of the asphaltene molecule to the glass transition behavior. We demonstrate that the aliphatic chains contribute significantly to volumetric changes and impose restrictions to the molecular orientations. Glass transition is accompanied by breaking of π–π stacking of the asphaltene molecule. Therefore, the size of the fused aromatic region in the condensed core determines the strength of intermolecular interactions and the glass transition temperature <i>T</i>_g

Water-in-Trichloroethylene Emulsions Stabilized by Uniform Carbon Microspheres

Uniform hard carbon spheres (HCS), synthesized by the hydrothermal decomposition of sucrose followed by pyrolysis, are effective at stabilizing water-in-trichloroethylene (TCE) emulsions. The irreversible adsorption of carbon particles at the TCE–water interface resulting in the formation of a monolayer around the water droplet in the emulsion phase is identified as the key reason for emulsion stability. Cryogenic scanning electron microscopy was used to image the assembly of carbon particles clearly at the TCE–water interface and the formation of bilayers in regions of droplet–droplet contact. The results of this study have potential implications to the subsurface injection of carbon submicrometer particles containing zero-valent iron nanoparticles to treat pools of chlorinated hydrocarbons that are sequestered in fractured bedrock

Attachment of a Hydrophobically Modified Biopolymer at the Oil–Water Interface in the Treatment of Oil Spills

The stability of crude oil droplets formed by adding chemical dispersants can be considerably enhanced by the use of the biopolymer, hydrophobically modified chitosan. Turbidimetric analyses show that emulsions of crude oil in saline water prepared using a combination of the biopolymer and the well-studied chemical dispersant (Corexit 9500A) remain stable for extended periods in comparison to emulsions stabilized by the dispersant alone. We hypothesize that the hydrophobic residues from the polymer preferentially anchor in the oil droplets, thereby forming a layer of the polymer around the droplets. The enhanced stability of the droplets is due to the polymer layer providing an increase in electrostatic and steric repulsions and thereby a large barrier to droplet coalescence. Our results show that the addition of hydrophobically modified chitosan following the application of chemical dispersant to an oil spill can potentially reduce the use of chemical dispersants. Increasing the molecular weight of the biopolymer changes the rheological properties of the oil-in-water emulsion to that of a weak gel. The ability of the biopolymer to tether the oil droplets in a gel-like matrix has potential applications in the immobilization of surface oil spills for enhanced removal