Eluent viscosity

The viscosity of the eluent changes with its composition nonlinearly, so column backpressure will depend on the eluent composition. It also may have an effect on the analyte diffusivity and thus on the equilibrium, but this effect is so small that it never been observed experimentally.

Picture below shows the dependencies of the viscosity for some RP HPLC binary mixtures most commonly used as an eluents.

This picture shows that if you have to use for example either 40% of MeCN in water, or 50% of MeOH in water, you may expect two times higher backpressure on the same column for MeOH/water eluent than for MeCN/water.