Bonded stationary phases hydrophobic properties

Reversed-phase stationary phases are more or less hydrophobic, and the degree of this property is characterized by their hydrophobicity H. As a general rule, retention times are longer the more C atoms the bonded stationary phase contains. (The reason is that the volume taken up by the bonded nonpolar groups, i.e. that required by the actual stationary phase, is greater with long chains than it is with shorter chains retention is directly proportional to the volume ratio between the stationary and mobile phases see Section 2.3.) Figure 10.7 demonstrates this effect. 

Alhedai et al also examined the exclusion properties of a reversed phase material The stationary phase chosen was a Cg hydrocarbon bonded to the silica, and the mobile phase chosen was 2-octane. As the solutes, solvent and stationary phase were all dispersive (hydrophobic in character) and both the stationary phase and the mobile phase contained Cg interacting moieties, the solute would experience the same interactions in both phases. Thus, any differential retention would be solely due to exclusion and not due to molecular interactions. This could be confirmed by carrying out the experiments at two different temperatures. If any interactive mechanism was present that caused retention, then different retention volumes would be obtained for the same solute at different temperatures. Solutes ranging from n-hexane to n hexatriacontane were chromatographed at 30°C and 50°C respectively. The results obtained are shown in Figure 8. 

Application of pollutant chemodynamic models, which neglect the DHS phase, may result in inaccurate estimations of apparent solubility and transport parameters. The impact of a DHS solubility enhancement is most pronounced for the least water-soluble solutes. The affinity of a solute for a DHS is a function of the same properties, which drive a complex organic mixture(s) to sorb onto the stationary solid phase, namely bonding interactions and hydrophobicity. 

Apart from the hydrophobic interactions provided by the alkyl part of the molecule, octanol has also hydrogen-bond acceptor and donor functions like lipid membranes have. This property of n-octanol made the octanol-water distribution coefficient that widely used. However, n-octanol or reversed phase materials cannot mimic the interfacial character of the bilayer structure. The ionic interactions between membrane phospholipids and solute are also not represented in the properties of octanol or reversed phase materials. To overcome this issue, alternative stationary phases... 

Reversed-phase chromatography is now widely used for the fractionation of biological molecules. The technique is based on the use of a non-polar stationary phase and a polar mobile phase. The stationary phase is usually made of a hydrocarbonaceous layer, either n-octyl. Cfl, or n-octadecyl, Cib, ligands, chemically bonded to the surface of a silica matrix via siloxane bonds. Separation Is achieved by exploiting the difference in the hydrophobic properties of the molecules. 

This finding has been verified by several experiments phases with polar properties show a good steric selectivity (see Fig. 6). Thus, isomeric steroids (double-bond isomerism) cannot be separated selectively using hydrophobic phases (co-elution of peaks 2 and 3 in the upper and middle chromatograms), but they can if polar groups are present on the surface of the stationary phase, as in the lower chromatogram in Fig. 6. The stationary phase is Resolve, a non-endcapped material. 

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