- Organic Nanopores Structure Materials - Pyrobitumen
The carbon-rich residue of catagenesis, pyrobitumen constitutes the material of organic part of the rock. The Pyrobitumen is a type of solid, amorphous organic matter, which is insoluble to organic solvent (different from bitumen). However, one should not confuse pyrobitumen with residual kerogen in a mature source rock and the distinction is based on microscopic evidence of fluid flow within the rock fabric and is usually not determined. The thermal processes (bitumen to pyrobitumen) driving the molecular cross-linking also decrease the atomic ratio of hydrogen to carbon from greater than one to less than one and ultimately to approximately one half.
- Gas Adsorption
Similar to surface tension, adsorption is a consequence of surface energy. In a bulk material, all the bonding requirements (be they ionic, covalent, or metallic) of the constituent atoms of the material are filled by other atoms in the material. However, atoms on the surface of the adsorbent are not wholly surrounded by other adsorbent atoms and therefore can attract adsorbates. The exact nature of the bonding depends on the details of the species involved, but the adsorption process is generally classified as physisorption (characteristic of weak van der Waals forces) or chemisorption (characteristic of covalent bonding). It may also occur due to electrostatic attraction.
Affecting Factors: (i) In general, easily liquefiable gases e.g., CO2, NH3, Cl2 and SO2 etc. are adsorbed to a greater extent than the elemental gases e.g. H2, O2, N2, He etc. (while chemisorption is specific in nature.)
(ii) Porous and finely powdered solid e.g. charcoal, fullers earth, adsorb more as compared to the hard non-porous materials. Due to this property powdered charcoal is used in gas masks.
However, most importantly, the extent of adsorbate adsorption depends directly upon the surface area of the adsorbent.
Adsorption forces and potentials: Methane is generally considered as non-polar molecules. The inter-molecular forces between gas molecules and nanopore surface can be obtained using atom–atom pair potentials, namely a Lennard–Jones (LJ) potential.
[\V_{LJ} = 4\varepsilon \left[ \left(\frac{\sigma}{r}\right)^{12} - \left(\frac{\sigma}{r}\right)^{6} \right] = \varepsilon \left[ \left(\frac{r_{m}}{r}\right)^{12} - 2\left(\frac{r_{m}}{r}\right)^{6} \right]
\]
where ε is the depth of the potential well, σ is the finite distance at which the inter-particle potential is zero, r is the distance between the particles, and rm is the distance at which the potential reaches its minimum.
The r−12 term, which is the repulsive term, describes Pauli repulsion at short ranges due to overlapping electron orbitals and the r−6 term, which is the attractive long-range term, describes attraction at long ranges (van der Waals force, or dispersion force).
Isotherm models: Irving Langmuir was the first to derive a scientifically based adsorption isotherm in 1918. The model applies to gases adsorbed on solid surfaces. It is a semi-empirical isotherm with a kinetic basis and was derived based on statistical thermodynamics. It is the most common isotherm equation to use due to its simplicity and its ability to fit a variety of adsorption data. The mono-layer assumption is addressed by the BET isotherm for relatively flat (non-microporous) surfaces.
