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6.1. SOLUTION VISCOSITY AND
INTRINSIC VISCOSITY
6.1.1. Viscosity
(symbol
η
) of dilute
solutions
Many polysaccharides, glycoproteins and some proteins produce very thick
(viscous) solutions when they are dissolved in water, even at low
concentrations. Practical examples include making a sauce or a gravy. Even
blood is more viscous than water because of both blood cells and dissolved
proteins
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. Other examples you may think of?
The viscosity of solutions is both a technologically and biologically important
property. In addition, measurement of the dilute solution viscosity is often a
simple way to obtain information about the physical properties of the dissolved
macromolecules. This includes both the molecular weight and the shape and
extension of the molecules. In this course we will focus on these aspects:
a)
Defining and understanding the intrinsic viscosity
b)
How to determine the intrinsic viscosity experimentally
c)
Using intrinsic viscosity measurements to determine the molecular
weight or monitor molecular weight changes of a biopolymer
d)
Using analysis of the intrinsic viscosity and the molecular weight to
quantify the shape and extension of a biopolymer (spherical? rodlike?
C
∞
or persistence length?)
First, we define the term dilute solution. It means the concentration range
below which polymer-polymer interactions do not contribute to the viscosity.
The concentration where such interactions emerge is called the critical
overlap concentration (c*). It depends very much on the shape of the
polymers, but can generally be approximated by c*
≈
2.5/
[η]
, where
[η]
is the
intrinsic viscosity (defined and explained below).
The viscosity of a liquid is a result of internal friction. For macromolecules in
solution the viscosity depends on several factors:
-
The polymer concentration
-
The molecular weight
-
Flexibility, shape and extension of the polymer
-
The solvent properties (including pH and ionic strength)
-
The temperature
-
The type of measurement and type of viscometer
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What is the concentration of protein in blood? Check literature or web