1

VISCOSITY, ADSORPTION, SURFACE TENSION AND

OSMOSISCourse: BSc Biotech Sem-I

Subject: Biophysics and Bioinstrumentation

Unit - 2

2

Viscosity

The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. For liquids, it corresponds to the informal concept of "thickness". For example, honey has a much higher viscosity than water.

The resistance experienced by one layer of a liquid in moving over another layer is called ‘viscosity’.

3

Viscosity is due to the friction between neighboring particles in a fluid that are moving at different velocities. When the fluid is forced through a tube, the fluid generally moves faster near the axis and very slowly near the walls.

A liquid's viscosity depends on the size and shape of its particles and the attractions between the particles.

A fluid that has no resistance to shear stress is known as an ideal fluid or inviscid fluid. Zero viscosity is observed only at very low temperatures, in superfluids. Otherwise all fluids have positive viscosity.

4

Ether and gasoline have little viscosity and are quite mobile.

But honey and coal-tar have high viscosities.

The unit of viscosity is the ‘Poise’ (1 P = 0.1 Pa·s), named after Poiseuille.

It is expressed as,

1 2

5

Factors affecting viscosity

Temperature: The viscosity of liquids decreases by about 2% for each degree rise in temperature.

Chemical composition: The viscosity of liquids generally depends upon the size, shape and chemical nature of their molecules. It is greater with larger than with smaller molecules; with elongated than with spherical molecules. Large amounts of dissolved solids generally increase viscosity. Small amounts of electrolytes lower the viscosity of water slightly.

6

Colloid systems: The viscosity of lyophilic colloid solution is generally relatively high.

Suspended material: Suspended particles cause an increase in the viscosity. The viscosity of blood is important in relation to the resistance offered to the heart in circulating the blood. The heart muscle functions best while working against a certain resistance. The viscosity of blood is due largely to the emulsoid colloid system present in plasma and to the great proportion of suspended corpuscles.

7

Physiological Importance

Blood cells increase the viscosity of blood. Higher the number of blood cells, the greater is the viscosity.

Blood viscosity helps in streamlining blood flow by reducing turbulence.

Blood viscosity helps in hemodynamics especially in capillaries, blood flow through narrow vessels.

8

Adsorption

The process of taking up substances from solution on surface is called adsorption.

Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface.

This process creates a film of the adsorbate on the surface of the adsorbent.

The word "adsorption" was coined in 1881 by German physicist Heinrich Kayser (1853-1940).

9

Characteristics of Adsorption

It is a surface phenomenon. The attractive forces on the surface are limited to

distances one molecule deep. The extent to which adsorption takes place is

dependent upon the nature of both adsorbing agent and the substances absorbed.

The greater the surface of the adsorbing agent, the greater is the adsorption.

Charcoal becomes activated when it is heated at 700-800 C in a closed container and adsorption takes place on the activated charcoal due to the attraction of oppositely charged ions. Salts, acids and alkalis restrict it.

It has got much importance in industry.

10

Principles governing adsorption

Adsorption is a reversible process. It decreases with the rise in temperature. This process takes place relatively

quickly. Equilibrium is reached within one hour.

Adsorption is proportional to the surface area and it varies with the nature of the surface of the adsorbent and of the substances to be adsorbed.

11

Cont…

It proceeds best from dilute solutions. Narrow pores on the surface of the

adsorbing agent are more effective than globular openings.

Heat is given off in all adsorption. The molecules adsorbed on the surface

are oriented and arranged in a definite manner.

12

Factors affecting Adsorption

Nature of the adsorbate Surface area of the adsorbent Effect of pressure Effect of temperature

13

Importance of adsorption

Many chemical reactions are speeded up by the presence of adsorptive surface.

Surface adsorption helps to combine enzymes with substrates to give reaction products.

Adsorption processes taking place on the cell membranes promote many vital chemical reactions.

Drugs and poisons which are adsorbed on cell surfaces exert their effects from that location.

The process of adsorption is applied in the purification of enzyme.

14

Surface Tension

The force with which the surface molecules are held together is called surface tension.

Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force.

Surface tension is an important property that markedly influences the ecosystem.

15

The interior molecules of a homogeneous liquid are equally attracted in all directions by surrounding molecules. They are free to move in all directions.

But the molecules in the surface of the liquid are attracted downward and sideways but not upward.

As a result, the molecules of the surface are not so free to move.

They are held together and form a membrane over the surface of the liquid.

3

16

A great part of energy required to convert a liquid into a gas is essential to overcome surface tension and drag the molecules free from the surface of liquid.

Surface tension x surface area = Surface energy. A falling drop of liquid assumes a spherical form because the ratio of surface energy is the least.

17

Surface tension = ½ hdgr Where, h=height of the liquid

d=density of the liquid g=acceleration due to gravity r=radius of the capillary tube

18

Gibbs – Thomson principle

1. Substances that lower the surface tension become concentrated at the interface.

2. Substances that increase surface tension tend to move away from interface.

3. Lipids and proteins which are both effective in lowering surface tension are found concentrated on the cell wall.

19

Methods of surface tension measurements

There are several methods of surface tension measurements:

1. Capillary rise method 2. Stallagmometer3. Wilhelmy plate or ring method 4. Maximum bulk pressure method5. Hanging liquid drop or gas bubble

method 6. Dynamic method

20

Factors affecting surface tension

Surface tension decrease with increase in temperature.

Most inorganic salts slightly raise surface tension of water.

Organic substances usually lower surface tension.

Alkalis increase surface tension. Acid and ammonia lowers it.

21

The presence of impurities either on the surface or dissolved in it, affect surface tension of the liquid. Highly soluble substances increase the surface tension of water, whereas sparingly soluble substances reduce the surface tension of water

the stronger the intermolecular attractive forces, the higher the surface tension will be

22

Physiological importance

Surface tension is involved in the process of digestion; because bile salts reduce the surface tension of lipids and thus assist emulsification. As a result, surface area is increased which favours lipase activity on lipids.

23

Osmosis

Osmosis is defined as spontaneous flow of water from a more dilute to a more concentrated solution when the two solutions are separated from each other by a semi-permeable membrane.

Osmosis occurs in the direction opposite to that in which diffusion occurs.

Osmotic pressure rise with the rise in temperature.

24

If the cell is kept in hypotonic solution, the cell membrane will allow water to pass into it and will set up an excess pressure in the interior of cell causing the cytoplasm to be forced tightly against cell wall. This condition is known as ‘turgor’ and the cell is said to be turgid.

If the cell is immersed in a concentrated solution (high osmotic pressure), water will pass out of the interior of the cell. The cytoplasm will then shrink and detach itself from the cell wall. This phenomenon is said to be ‘plasmolysis’.

25

Iso-osmotics: Solution with the same pressure are termed iso-osmotics.

Isotonic solutions: A pair of solutions which produce no flow through a semipermeable membrane are said to be isotonic solution.

26

Physiological importance

Absorption from gastro-intestinal tract, as also fluid interchange in various compartments of body follows the principle of osmosis.

Living red cells, if suspended in 0.92% NaCl solution, neither gain nor lose water. Briefly speaking, intracellular fluid of red cells is isotonic with the red cell membrane in 0.92% NaCl solution.

27

Vant Hoff’s laws of osmotic pressure

The osmotic pressure of a solution varies directly with the concentration of the solute in the solution and is equal to the pressure the solute would exert if it would be a gas in the volume occupied by the solution, if the volume of the solute molecules relative to volume of solvent be negligible.

The osmotic pressure of a solution varies directly with absolute temperature in the same way as the pressure of a gas varies when its volume is kept constant.

28

Measurements of osmotic pressure

Following methods are used for the measurement of osmotic pressure,

    (a) Pfeffer’s method,     (b) Morse and Frazer’s method,           (c) Berkeley and Hartley’s method,    (d) Townsend’s negative pressure

method,    (e) De Vries plasmolytic method.

29

References

READING IMAGES

Text book of biochemistry by West and Todd

Biophysics and Bioinstrumentation by N Arumugam and V Kumaresan

Biophysical chemistry: Principles and Techniques by Upadhyay and Nath

1-3: https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/liquids-and-solids-11/liquid-properties-85/surface-tension-376-6402/