M. A. LAUFFER 



sented symbolically as follows: A = [r]]= [(v/vo - l)/C]c_o- Obviously, 

 the numerical value of the intrinsic viscosity is dependent upon the 

 manner in which the concentration is expressed. The latter must be 

 specified for the former to have meaning. The intrinsic viscosity is 

 an important constant describing a solute. As will be shown in the 

 paragraphs to follow, it is related to the molecular structure of the 

 solute particles and, in some cases, to the thermodynamic properties 

 of the solution. The principal concern of viscometry is the evaluation 

 of intrinsic viscosities. 



All of the constants which have been defined thus far for solu- 

 tions apply only to those which exhibit Newtonian flow, that is, to 

 solutions for which a true viscosity coefficient can be assigned. Many, 

 if not most, of the solutions of interest to biologists and biochemists 

 do not exhibit Newtonian flow. For such systems, one obtains only 

 apparent viscosity coefficients, which depend upon the rate of shear. 

 Such systems must be described by a series of constants analogous to 

 the relative, specific, and intrinsic viscosities of Newtonian solutions. 

 These can be given the names: apparent relative viscosity, apparent 

 specific viscosity, and apparent intrinsic viscosity. The latter differ 

 from the former only in that each is dependent upon rate of shear. 

 Thus, in a study of a non-Newtonian solution, rate of shear must be 

 held essentially constant, or it must be specified, or the nature of the 

 variation with rate of shear must be described. Apparent intrinsic 

 viscosities are not necessarily any less useful than intrinsic viscosities. 



It is the purpose of this discussion to consider the usefulness of 

 \iscosity in the study of biological and biochemical systems. In the 

 remaining paragraphs, a brief consideration of the methods of measur- 

 ing viscosity and a discussion of the meaning of the viscosity of solutions 

 or dispersions of rigid colloidal particles will be presented. Particular 

 attention will be devoted to the examination of direct experimental 

 evidence concerning the correctness of theories used to interpret the 

 various aspects of viscous behavior. 



Intrinsic viscosities and apparent intrinsic viscosities are cal- 

 culated from measurements of the viscosity of the solvent, the viscosity 

 or the apparent viscosity of the solution, and the concentration of the 

 solute. It is entirely outside the scope of this article to discuss the 

 measurement of solute concentration. Suflftce it to say that any of 

 the methods of quantitative chemical and biochemical analysis might 



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