Hydrodynamic Properties of Coal Extracts in Pyridine

Abstract

Hydrodynamic properties, such as solution viscosity and diffusivity, were measured at 298 K to investigate the solution state characteristics of coal extracts in pyridine obtained from Upper Freeport (UF) and Illinois No. 6 (IL) coals. The solvation constants obtained by applying the Pal-Rhodes equation to the viscosity of the extracts in pyridine are 2.9 and 4.6 for the UF and IL extracts, respectively. A larger content of acidic hydroxyl groups in IL extract than UF extract make pyridine solvate IL extract more extensively than UF extract. However, the solvation constant of the O-methylated IL extracts is 9.3, unexpectedly larger than that of the original IL extract. To have further insight into this observation, the change in the molecular shape of the extracts by the O-methylation were examined, using the combination of intrinsic viscosity and diffusion coefficient. The shapes of the coal extracts have been found to be prolate ellipsoids with axial ratio of 6.5 and 4.0 for UF and IL extracts, respectively. The axial ratio of the O-methylated IL extract is 13.1, suggesting that the elimination of the hydrogen bonds or the steric effect of added methyl makes the shape of IL extract stretched out. This deformable nature of coal extracts makes the interpretation of the solvation constants from Pal-Rhodes equation inaccurate, as it assumes that the particles are monodispersed hard spheres.

NMR Spectroscopy Analysis of Asphaltenes

“…Stokes–Einstein equation shows that the hydrodynamic radius (equivalent to a sphere) of the asphaltene raises from 1.8 to 2.8 nm as the concentration of the asphaltene solution is increased from 0.1 to 2.0 wt %. The Taylor dispersion method , has yielded the asphaltene diffusion coefficient ( D ∞ ) of 2.4 × 10 –10 m 2 /s in pyridine solution at 303 K, which is equivalent to the hydrodynamic radius of 1.1 nm. Although it is not possible to compare the D obtained from PFG NMR with D ∞ by Taylor dispersion directly, the nucleation and growth of molecular asphaltene accumulates is one of the most rational clarifications for the great gap between D and D ∞ at the concentration of 0.1 wt %.…”

“…Attractive interactions between the solute particles themselves initiate an accumulation procedure, and interactions between solvent and solute molecules hinder the general solvent diffusion. Then solvent hindrance effects are observed as explained by Ostlund et al The second important issue is the determination of molecular weights from diffusion measurements. Combining Flory’s scaling law ( R g ∝ M –α , R g : the radius of gyration) and Stokes–Einstein equation (for a spherical particle, R g ∝ R H ) establishes the following correlation between D and the molecular weight ( M ): where K is a constant depending on the characteristic of the molecule and α is a coefficient depending on the form of the particle-tagged structure factor. , For sample A, α is determined as 0.6, thus the molecular weight for sample A is calculated as 280 g/mol approximately.…”

“…Stokes−Einstein equation shows that the hydrodynamic radius (equivalent to a sphere) of the asphaltene raises from 1.8 to 2.8 nm as the concentration of the asphaltene solution is increased from 0.1 to 2.0 wt %. The Taylor dispersion method 79,80 77 have mentioned that the beginning of asphaltene accumulation in pyridine solution can happen below 0.1 wt %. In another report, Ostlund et al have measured diffusion coefficients of asphaltenes derived using n-heptane in deuterated toluene with different concentrations ranging between 0.044 and 5 wt % by utilizing pulsed field gradient spin echo (PFG-SE) NMR 81 to gain insight into the geometry of asphaltenes.…”

Review of the Molecular Structure and Aggregation of Asphaltenes and Petroleomics

“…Because the blue-emitting chromophores undergo rotation diffusion 10 times faster than red-emitting chromophores, they are not attached to each other, again showing that asphaltene molecules are small. Taylor dispersions measurements obtained the translational diffusion constant of coal asphaltenes (Wargadalam et al 2002). The detection method is optical absorption.…”

“…The detection method is optical absorption. These studies measured the exact same molecular sizes for coal asphaltenes as did the TRFD studies (Wargadalam et al 2002), yet different diffusion constants (translation vs. rotation) and different detection methods are em-ployed. Further confirmation comes from a series of papers measuring translational diffusion constants of asphaltenes using fluorescence correlation spectroscopy (FCS) in ultradilute solutions of asphaltenes (Andrews et al 2006;Guerra et al 2007;Schneider et al 2007).…”