Diffusivity of Spheres at Infinite Dilution

Theoretieal predietion of the diffusion coeffieient of spheres moving through a low-molecular-weight liquid is a problem that was examined by Einstein at the turn of this century [39]. This situation is of interest to the polymer scientist because isolated polymer molecules in solution act as random cods. 

The analysis begins by showing that the osmotic pressure is the driving force for diffusion. This is done by carrying out the experiment dlustrated in 

The right-hand side of Eq. (13.4.1) must be the drag force exerted by the fluid on the sphere. This is given by Stokes law and equals 6nt vR, because the 

Example 13.6 Northrop and Anson measured the diffusion coefficient of hemoglobin in water to be 4.86 x 10 cm /sec at 5°C [14]. Estimate the radius of the hemoglobin molecule. The viscosity of water at 5°C is 0.0152 P. 

If the diffusing species is not a sphere, we merely need to use an appropriately defined equivalent radius R instead of if in Eq. (13.4.3) [41]. Thus, we have the following  

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