Reverse-phase packing, description

Lamb s group has adsorbed the hydrophobic crown ethers and cryptands shown in Fig 1 onto reversed phase chromatography packings for application to ion chromatography in the analysis of cations and anions. A brief introduction to ion chromatography (IC) is in order to lay the foundation for a description of this work. 

This brief descriptive overview of the reversed-phase process emphasizes the complexity of the retention mechanism and the necessity to consider the influence of different and independent processes on the analyte retention. Since the governing process in the analyte retention is the adsorption equilibrium, the influence of the surface packing material (stationary phase) on the analyte retention in RPLC is described in Section 4.3. 

The sequence in which reversed phases occur is much more complicated than that for the normal phases and is not yet understood in terms of surfactant molecular structures. The main reason is (probably) that there is no limitation on the radius of the inverse micelles such as that imposed by the length of the paraffin chain on normal micelles. Water could swell the micelles indefinitely (this does not happen because the size and shape of inverse micelles is controlled by limits on surfactant packing on a curved surface). Figure 21.12 (or something similar) is often employed to describe the general pattern of mesophase behaviour as a function of surfactant (water) concentration. In the description... 

Retention in chromatography is controlled by thermodynamic equilibria. The partition ofthe analyte between the mobile and the stationary phase is in control of the retention factor. This partition can be described by the laws of reversible thermodynamics. Therefore, we also borrow the thermodynamic description of the temperature dependence of equilibria. This is the so-called van t Hoff equation, which is the quantitative expression of the Le Chatelier principle. According to this, the temperature dependence of the retention factor k can be described by 2.9, with R being the general gas constant, AH° the molar enthalpy (heat tone) related to the transition of the analyte from mobile to stationary phase, AS° the molar entropy change for this transition, andj( the so-called phase ratio of the packed stationary phase in the column. 

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