Bonded Phases - The Relationship between (K) and the Carbon Number of a Solvent/Solute
The Relationship between (K) and the Carbon Number of a Solvent/Solute
The linear relationship between Log(K) and the carbon number of the aliphatic chain is substantiated for both the aliphatic alcohols and the two aliphatic acids in figure 20.
Figure 20. Graphs of Log(K) against Carbon Number of Solute
It shows that the graph for the homologous series of alcohols is an excellent straight line and this linear relationship between Log (K) and carbon number was observed by Martin (38) Guiochon (39) and Berendson (40). It should also be noted that the slope of the line for the alcohols and that the two acids is very similar (although the line for the aliphatic acids is drawn through only through two points!). The similarity in slopes would indicate that the contribution to log(K) by each methylene group when only dispersive interactions are effective, tends to be independent of the terminal functional group. In conclusion, employing solvent mixtures as mobile phases in reverse phase chromatography modifies the reverse phase surface in a predictable manner and is described by the Langmuir adsorption isotherm. The adsorption isotherm of any solvent or moderator, on a reverse phase, can be measured by a relatively simple and accurate chromatographic procedure without involving measurements where the methodology is uncertain or a subject of controversy. If the adsorption isotherm is known for the moderator or solvent that is being employed, then the character of the interacting surface can also be identified and this character can be changed in a predictable manner to achieve specific changes in solute retention.
The Adsorption of Ion Exchange Reagents on the Surface of Reverse Phases
Due to charged ions being excluded from the pores of a bonded phase due to ion exclusion it is sometimes difficult to synthesize a phase system that will efficiently retain and separate ionic materials. One effective solution to this problem is the use of soluble ion exchange materials, the self-styled "Paired Ion Chromatography" (PIC) reagents. Ion pair reagents can be anion or cation exchangers such as tertiary butyl ammonium iodide or hexane sulphonate and these substances can be added to the mobile phase in small quantities and can control the retention of ionic materials in two ways.
Firstly, at low solvent concentrations the aliphatic groups of PIC reagents will interact strongly with the reverse phase and form a layer of ion exchange material adsorbed on the surface. It must be understood, however, that the PIC reagents themselves cannot enter the pores of the reverse phase due to ion exclusion (i.e. in a similar manner that acetic acid and propionic acid are excluded). Thus, in practice the PIC reagents will only be adsorbed on the external surface of the stationary phase. Nevertheless, within limits, ions will interact with the surface charges and materials will be retained and separated as a result of significant ionic interactions.