Influence of pressure and solvent composition on the density gradient in the analytical ultracentrifuge I extended Hermans-Ende equation for the equilibrium density gradient

An extended Hermans-Ende equation for real solutions has been derived in order to calculate the density as a function of the radial distance in a density gradient mixture (light/heavy medium) inside an ultracentrifugal cell. The equation has been tested with equilibrium density gradient measurements of the system H2O/methanol/metrizamide/polystyrene latex where metrizamide is the heavy medium. Additionally, density measurements of the systems with a vibrating tube densitometer at normal pressures and at pressures up to 400 bar have been performed in order to complete the measurements. The solvent compositions were 8–25% metrizamide, 20–100% water and 0–80% methanol. The latex concentration was 0.03%. It is demonstrated that the density gradient is highly influenced by nonideality effects with respect to the solvent composition and by the pressure gradient in the ultracentrifuge.