Properties of colloidal solutions
The basic characteristic properties of the colloidal solutions are stated as follows.
(1) Physical properties
(i) Heterogeneous nature : Colloidal sols are heterogeneous in nature. They consists of two phases; the dispersed phase and the dispersion medium.
(ii) Stable nature: The colloidal solutions are quite stable. The particles of the colloidal solution are in a state of motion and do not settle down at the bottom of the container.
(iii) Filterability : Colloidal particles are readily passed through the ordinary filter papers. Although they can be retained by special filters known as ultrafilters (such as parchment paper).
(2) Colligative properties
(i) Due to formation of coupled molecules, observed values of colligative properties such as relative depression in freezing point, elevation in boiling point, decrease in vapour pressure, osmotic pressure are much smaller than expected.
(ii) For the given colloidal solution the number of particles will be very small as compared to the true solution.
(3) Mechanical properties
(i) Brownian movement
(a) Robert Brown, a botanist discovered in 1827 that the pollen grains suspended in water do not remain at rest but move about continuously and randomly in all directions.
(b) Later on, it was observed that the colloidal particles are moving at random in a zig - zag motion. This kind of motion is termed as Brownian movement.
(c) The molecules of the dispersion medium are constantly colloiding with the particles of the dispersed phase. It was stated by Wiener in 1863 that the impacts of the dispersion medium particles are unequal, thus creating a zig-zag motion of dispersed phase particles.
(d) The Brownian movement explains the force of gravity acting on colloidal particles. This helps in giving stability to colloidal solutions by not allowing them to settle down.
(ii) Diffusion : The sol particles diffuse from higher concentration to lower concentration region. Though, due to bigger size, they diffuse at a lesser speed.
(iii) Sedimentation : The colloidal particles settle down under the influence of gravity at a very slow rate. This process is used for determining the molecular mass of the macromolecules.
(4) Optical properties : Tyandall effect
(i) When light passes through a solution, its path becomes visible due to scattering of light by particles. It is termed as Tyndall effect. This phenomenon was studied by Tyndall for the first time. The illuminated path of the beam is termed as Tyndall cone.
(ii) The intensity of scattered light depends on the difference between refractive indices of the dispersed phase and the dispersion medium.
(iii) In lyophobic colloids, the difference is appreciable and, thus, the Tyndall effect is well - defined. But in the lyophilic solutions, the difference is very small and the Tyndall effect is very weak.
(iv) The Tyndall effect tells us the heterogeneous nature of the colloidal solution.
(v) The Tyndall effect has also been observed by an instrument called ultra - microscope.
Some example of the Tyndall effect are stated below
(a) Tail of the comets is seen as a Tyndall cone because of the scattering of light by the tiny solid particles left by the comet in its path.
(b) Because of the scattering the sky looks blue.
(c) Blue colour of the water in sea is because of scattering of the blue light by water molecules.
(d) The visibility of projector path and circus light.
(e) The visibility of sharp ray of the sunlight passing through a slit in dark room.
(5) Electrical properties
(i) Electrophoresis
(a) The process of movement of the colloidal particles under an applied electric field is called as electrophoresis.
(b) If the particles accumulate near the negative electrode, charge on the particles is positive in nature.
(c) On the other hand, if particles of the solution accumulate near the positive electrode, the charge on the particles is negative in nature.
(d) The apparatus comprises of a U-tube with two Pt-electrodes in each limb.
(e) When electrophoresis of a sol is carried out without stirring, the bottom layer slowly becomes more concentrated while the top layer which contains pure and concentrated colloidal solution may be decanted. This is termed as electro decanation and is used for the purification as well as for concentrating the sol.
(f) The reverse of electrophoresis is called Sedimentation potential or Dorn effect. The sedimentation potential is created when a particle is forced to move in the resting liquid. This process was discovered by Dorn and is also known as Dorn effect.
(ii) Electrical double layer theory
(a) The electrical properties of colloids can also be explained by electrical double layer theory. In accordance to this theory a double layer of ions appear at the surface of the solid.
(b) The ion hear is adsorbed preferentially and is held in fixed part and imparts charge to colloidal particles.
(c) The second part consists of a diffuse mobile layer of ions. This second layer comprises of both the type of charges. The average charge on second layer is exactly equal to that on the fixed part.
(d) The existence of opposite sign on fixed and diffuse parts of double layer leads to appearance of a difference of potential, called as zeta potential or electrokinetic potential. Currently when electric field is employed the particles move (electrophoresis)
(iii) Electro-osmosis
(a) In this the movement of the dispersed particles is prevented from moving by semipermeable membrane.
(b) Electro-osmosis is the process in which dispersion medium is allowed to move under influence of an electrical field, whereas colloidal particles are not allowed to move.
(c) The existence of electro-osmosis has suggested that when liquid forced through a porous material or a capillary tube, the potential difference is created between the two sides called as streaming potential. Thus the reverse of electro-osmosis is called streaming potential.
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