VISCOMETRY 



(he diffusion constant or the sedinirnla(if)n constant in terms of the 

 molecular weight and dimensions of nonsphcrical particles. Thus, 

 these quantities can he evaluated through viscosity and diffusion meas- 

 urements or through viscosity and sedimentation velocity measure- 

 ments. Viscometry, therefore, assumes dignity as a tool useful in 

 determining the dimensions of suspended particles. 



The interpretation of viscosity data is rendered difficult by three 

 complications. Of these, the disturbing question of hydration is the 

 most serious. Until a reliable independent method for estimating the 

 degree of hydration is developed, there will be uncertainty in the 

 evaluation of the shapes of proteins or other biocolloids from viscosity 

 data and also from other physical data. The other two complications 

 are the effect of the charge of the particles upon their intrinsic viscosity 

 and the effect of the velocity gradient in the viscometer upon the 

 magnitude of the apparent viscosity coefffcient. 



That there should be an effect upon viscosity due to the electric 

 charge of colloidal particles was first predicted from theoretical con- 

 siderations by von Smolochowski. The equation expressing this re- 

 lationship has been rederived by Krasny-Ergen (15) and can be pre-, 

 sented in the form: 



bVK = 1 + ^ 



87r2 X77or2 



The symbols f and r are the electrokinetic potential and the radius of 

 the colloidal particles, e and rjo are the dielectric constant and viscosity 

 of the solvent, respectively, and X is the specific conductance of the 

 solution. Kruyt and Bungenberg de Jong, Bull (4), and Briggs and 

 his associates (3) have all verified the existence of the electroviscous 

 effect. The intrinsic viscosity of colloidal particles with high electro- 

 kinetic potentials in solvents of low conductance can be many times 

 greater than the intrinsic viscosity of the same particles measured 

 under conditions under which the charge effects are suppressed. The 

 studies of Bull and of Briggs, however, show that the Krasny-Ergen 

 equation does not afford an adequate quantitative representation of 

 the effect of the electric charge. Thus, the electroviscous effect is 

 reduced to a pure hazard. The results of Bull and of Briggs show that 

 it can be suppressed by adjusting the pH to the isoelectric point of the 

 biocolloid or by adding neutral electrolytes such as sodium chloride. 



