Influence of pressure and solvent composition on the density gradient in the analytical ultracentrifuge. II direct refractometric determination of the equilibrium and non-equilibrium density gradient

A method for the direct refractometric determination of the density gradient in an analytical ultracentrifuge has been described. It is based on two assumptions: 1) the Schlieren optics measure the refractive index gradient, the interference optics the refractive index, and the absorption optics the absorption of the density gradient solution (consisting of a light and a heavy solvent component) and 2) there is a relationship between the refractive index or the absorption, on the one hand and the density of the density gradient solution, on the other. The method has been demonstrated on water/metrizamide/polystyrene latex solutions with different concentrations of metrizamide (a heavy sugar). Advantages of this method are: first, the pressure and the composition effect are included in this method and must not be corrected as usual and second, it is not necessary to measure at sedimentation equilibrium; it would be possible to measure at times before the equilibrium is achieved. That means the method represents a quasi-dynamic density gradient method.