Also called mobile-phase viscosity. The viscosity of the mobile phase varies with the temperature of the column. Low-viscosity mobile phases generally provide better efficiency than less-viscous ones because diffusion coefficients are inversely related to solvent viscosity. For example, column efficiency is higher in reversed-phase chromatography with acetonitrile as an organic modifier than with isopropanol, which is more viscous. Column back pressure is directly proportional to solvent viscosity.