The Mechanism of Chromatographic Retention - The Association of Methanol with Water > Page 30

 

The column was operated in the 'normal phase' mode using mixtures of n-hexane and ethanol as the mobile phase. Equation (5) is validated by the curves relating the corrected retention volume to the reciprocal of the volume fraction of ethanol shown in Figure 13. An excellent linear relationship is obtained between the corrected retention volume and the reciprocal of the volume fraction of ethanol. However, there is little or no strong association between ethanol and n-hexane and, thus, the mixtures behave as true binary mixtures.

The Association of Methanol with Water

The apparent anomalous behavior of methanol-water and other water-solvent mixtures with respect to retention provoked Katz et al. (11) to investigate the association of methanol and water. This work was carried out to determine whether, in fact, such mixtures did not constitute binary mixtures but were actually ternary mixtures, the third component being the methanol-water associate. Their, arguments and subsequent experiments were as follows.

An equilibrium between methanol and water can be expressed as follows,

(7)

where (M) is the molar concentration of water,
(W) is the molar concentration of methanol,
(MW) isthemolar concentration ofmethanol/water associate,
and (k) is the "association" constant