The influence of strong centrifugal force fields on solutions: I. ImprovedHermans-Ende equation

An improved Hermans-Ende equation for real solutions has been derived in or der to calculate the density as a function of the radial distance in a dens ity gradient mixture (components of low and high density) inside an ultrace ntrifugal cell. The equation is based on the model with vanishing mixing vo lume with the barometric equation and the extended Flory-Huggins-van Santen -Staverman equation for the free energy of mixing. The equation has been te sted with equilibrium density gradient measurements of the system water/met rizamide/polystyrene latex where metrizamide is the high density medium. Th e overall or initial concentration of metrizamide in the mixture was 8-12% by weight. The latex concentration was 0.03% by weight. It is demonstrated that the density gradient is highly influenced due to nonideality effects w ith respect to the solvent composition. (C) 1998 Published by Elsevier Scie nce Ltd. All rights reserved.