Viscosity and Constitution

another laver, some relationship between viscosity of the substance and its constitution has Since viscosity depends on resistance offered when a molecular ever been observed as given below:

      (i) In a homologous Series, the increase in viscosity per C*H_{2} group is approximately The constant.

      (ii) Dunsten (1909) discovered a useful relationship between viscosity and inverse melar volume. The relationship is

d/M * eta * 10 ^ 6 =40 10 60

where d is the density and M is the molecular mass of the liquid. This relationship is true only for non-associated (normal) liquids, where as for associated liquids the value is much greater than 60. This relationship is very useful in deciding whether a given liquid is associated or not. (In CGS units, the multiplication factor is 10 ^ 6 but in SI units it is reduced to 10)

Molecular Structure Effects the Viscosity of Liquids. A spherical shaped molecule offers least resistance to flow. The shorter the chain length of a molecule, the lower will be its viscosity coefficient. s-isomers have usually greater viscosity than branched chain isomers. The viscosity of a trans-isomer is greater than cis-isomer. 

(iv) Intermolecular forces. The stronger the intermolecular forces in the liquid, the greater will be its viscosity. H_{2}*O and C_{2}*H_{5} OH molecules are about the same size, but the viscosity of H_{2}*O is greater to presence of strong hydrogen bonding which makes the How more difficult.

Rheochor. Newton Friend(1943) showed that if the molar volume (M/d) multiplied by the eighth root of the coefficient of viscosity, a constant value is obtained which is known as Rheochor.

R = M/d * gamma ^ (1/8)

 The Rheochor R may be defined as the molar volume of the liquid at the temperature at which its viscosity is unity. Like parachor, rheochor is both additive and constitutive. It should be noted that the rheochor is less useful for determining chemical structures than parachor.

Applications of Viscosity Measurement

The viscosity measurements of polymer solutions have been used to determine the molecular mass of polymers. The relationship between intrinsic viscosity In] ans molecular mass of a polymer is

(Mark Houwink equation) k M^ prime prime

12 where In I is the intrinsic viscosity which is the limiting viscosity number wh concentration of the polymer in a given solvent approaches zero. & and a are empiri parameters depending on the nature of the solvent as well as the solute. Factor a is cal the shape factor. For a randomly coiled molecule a = 0.5 for a rod like molecule a = 2 for spherically coiled molecule a = 0 The relation seems to hold well for a given poly and solvent when M exceeds 30 kg mol-1

The viscosity measurements are applied for controlling the manufacture of painta, rubber, glue and synthetic polymers.

This study is also of great significance for the solution of many technical problems for example, the amount of energy required for stirring of a liquid, mixing of liquids. making a slurry or sending of a liquid through a pipe depends directly on the viscosity