Pressure changes with MeOH/water

[Ameroid]

Dear all

I am using a C-18 column (Nucleosil), 150x4.60mm, 5 micron for my reverse phase chromatography.

My mobile phases are a mix of water and methanol. I noticed that when my percentage of water increases, so does the pressure in the column.

While this does not interfere with my separations, I am merely curious. Can anyone enlighten? Just a guess: Has it something to do with the "wetability" of different solvents along the stationary phase of the column ?

[tom jupille]

Water+methanol is very much a "non-ideal" mixture (in a thermodynamic sense); the properties of the mixture are quite different from the average of the properties of the components. You see this with volume (there can be as much as a 4% decrease in total volume), refractive index, boiling point, and solubility of air. You also see it with viscosity: the viscosity of MeOH/water mixtures can be higher than the viscosity of either MeOH or water.

Higher viscosity -> greater flow resistance -> greater back pressure.