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.
KeywordsAnalytical ultracentrifuge density gradient equilibrium Hermans-Ende/it equation
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