Graphitized carbon molecular interaction

Porous graphitic carbon (section 4.2.5) is an inert but highly retentive sorbent under supercritical fluid chromatography conditions. Supercritical carbon dioxide is a weak eluent for porous graphitic carbon and even compounds such as naphthalene are difficult to elute in a reasonable time [72]. Low molecular mass polar compounds generally have poor peak shapes, but in this case most likely due to limited solubility in tbe mobile phase rather than undesirable interactions with active sites on the stationary phase. The flat surface of porous graphitic carbon leads to preferential adsorption of... 

The atomic partial charge of the model phase and hydrocarbon does not change significantly after optimization of the eomplex form, while the electron potential is slightly shifted toward the molecular interaction side. These results clearly indicate the existence of different retention mechanisms on graphitized carbon phases, a hydrophobic interaction and hydrogen bonding. 

Table 5.1 Molecular interaction energy values for saccharides on graphitized carbon calculated using the MM2 program. MIPS, MIVW, MIHB and MIES represent the molecular interaction energy value of the final (optimized) structure, the van der Waals energy, the hydrogen-bonding energy, and the electrostatic energy (kcal mol ), respectively. tR represents the retention time. Reproduced by permission of Elsevier, ref. 24.

The molecular interaction energy values of alkenes were smaller than for their related alkanes. This result supports the idea that the hydrophobic interaction due to the van der Waals energy is the predominant molecular interaction in reversed-phase liquid chromatography. No dipole-dipole or 71-71 interactions influenced the direct interaction. The lack of dipole-dipole or 71-71 interactions can be studied from chromatographic behavior on a graphitized carbon phase. 

Figure 6.10 Relationship between molecular interaction energy and log on a model graphitized carbon phase at (a) pH 10 and (b) pH 2.

The precision of the correlation between the log k and molecular interaction energy values vras high, as long as the analyte structure was simple and flat, as demonstrated in Section 6.9.2. Specifically, such analyses are most successful when studying retention mechanisms on graphitized carbon phases (Section 6.3). This is because the most effective system for such analyses is a homogeneous and flexible model phase where the docking process may not cause errors. 

In SPE, choice of an appropriate solid phase is based on their possible molecular interactions with the targeted compounds. For historical reasons, reversed-phase type adsorbents have been largely utilized to extract compounds from aqueous samples, given the fact that many of them have been developed already for use in reversed-phase chromatography applications and are commercially available. Ion exchange adsorbents have been used to retain counter ions based on electrostatic interactions. Solid phase adsorbents combining hydrophobic and ionic interactions are also present, for example graphitized carbon black (GCB) obtained by... 

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