The Mechanism of Chromatographic Retention - The Association of Methanol with Water > Page 38
The curves shown in figure 15 can explain some of the unique characteristics of certain methanol/water mixtures when used as a mobile phase in reversed phase LC. For example, when the original mixture contains 50%v/v of methanol, there is little free methanol available in the mobile phase to elute the solutes, as it is mostly associated with water.
Courtesy of the Journal of Analytical Chemistry (ref.15)
Figure 15. Diagram of the Ternary Solvent System for Methanol/Water Mixtures