Properties of Colloidal Solution





Properties of Colloidal Solutions

All the properties of colloidal solutions can be grouped under three heads:

1. General Properties: General properties of colloidal solutions are given in difference between colloid and true solution.

 

2.  Optical Properties:

(i)  Brownian movement: Robert Brown (1927) an English Botanist, observed that the pollen grains in aqueous suspensions were in constant motion. Similar phenomenon was, later on, found in case of colloidal solution, when observed ultra-microscopically.

Brownian movement

Brownian movement

This continuous and rapid zig-zag motion of the colloidal particles is called Brownian movement. This motion is independent of the nature of the colloidal particles. It is more rapid when the size of the particles is small and the solution is less viscous.

Cause of Phenomenon:

The Brownian movement is due to the bombardment of colloidal particles by molecules of dispersion medium. The intensity of motion depends upon the size of the particles and the viscosity of the dispersion medium. The smaller the particles and the less viscous the dispersion medium, the more vigorous is the Brownian movement and vice versa.

 

Importance of Phenomenon

(a)    Confirmation of Kinetic Theory of Gases: It offers a visible proof of a rapid random kinetic motion of molecules in liquid as postulated by kinetic theory of gases.

(b)   Determination of Avogadro’s number: With the help of ultra-microscope the number of particles in a given mass of the colloidal solution can be counted and the Avogadro’s number being the number of molecules in one mole of any substance; can be calculated.

(c)    Stability of Colloidal Solution: The constant rapid zig-zag motion of the particles prevents them from setting due to gravity and thus helps in stabilising colloidal solution to some extent.

(d)   Size of Colloidal Particles: The colloidal particles remain under the influence of Brownian movement and gravitation force. Thus both these effects act on colloidal particles and these particles distribute themselves in a vertical column in accordance with the following equation:

\dfrac{RT}{N_0} ln \dfrac{n_1}{n_2} = \dfrac{4}{3} \pi r^3 ( h_2 n - h_1 ) (\rho - \rho')

where N_0 = Avogadro number, R = Gas constant, T = Temperature (K), n_1 \text{and} n_2 number of particles at depths h_1 \text{and} h_2 respectively, \rho \text{and} \rho' densities of particles and of the liquid medium respectively. The radius r, of the particle can be determined easily.

 

(ii)  Tyndall Effect: Tyndall (1869) observed that when a strong beam of light is focused on a colloidal solution the path of the beam becomes visible and when viewed through microscope placed at right angle to the path of light (ultramicroscopically), the colloidal particles appear as pin points of light moving against a dark background in a colloidal solution. This phenomenon is known as Tyndall effect and the illuminated path is called Tyndall cone. This phenomenon is not observed in case of true solution.

 

Cause of Phenomenon:

This phenomenon is due to scattering of light by colloidal particles. This scattering of light can be due to simple reflection because the size of the particles is smaller than wavelength of visible light which are, therefore, unable to reflect light waves. The colloidal particles become self luminous due to absorption of light energy which is then scattered from their surface. The maximum scattered intensity in the plane is at right angle to the path of the light and thus the path becomes visible when observed from the sides.

The intensity of scattered light the difference between the refractive indices of the dispersed phase and that of the medium

In case of lyophobic the difference is more so Tyndall effect is well observed; while in Lyophilic it is less so Tyndall effect is very poor.

 

Tyndall Effect

Tyndall Effect


 

Importance of Phenomenon:

1. This phenomenon has been employed as the basic principle for the construction of ultra-microscope. It has been used to detect solid suspended impurities in solution.

2. On the basis of Tyndall effect we can explain that sky is blue in day light. The reason is that dust particles along with water are in the atmosphere. These dust particles scatter blue light and other colours are absorbed therefore the colour of the sky is blue. Since in night scattering of light is not taking place hence sky is black in night.

3. Tail of comets.

4. Blue colour of sea water.

5. Blue tinge of smoke.

 

3. Electrical Properties:

(i)   Electrophoresis (or Cataphoresis): Since the colloidal particles are electrically charged (+ or -)with respect to the dispersion medium, hence on passing electric current through colloidal solution the charged particles move towards oppositely charged electrodes and get discharged to give precipitate. So, this migration of colloidal particles under the influence of electric field is called electrophoresis.

This phenomenon can be studied by a simple apparatus. It consists a U-tube fitted with a funnel shaped reservoir and a stop cock. An As_2S_3 sol is taken in the tube and two electrodes dipped in the solution. On passing electric current the As 25 3 particles move towards anode (positive electrodes) indicating that As_2S_3 particles are negatively charged particles and lose their charge and coagulate into coarse particles.

 

Applications:

(a)    Determination of charge: The nature of the charge of a colloidal particle can be ascertained by its migration in an electric field.

(b)   Electrodeposition of rubber: The negatively charged particles of rubber suspended in the latex of rubber plant can be deposited on another articles making them anode only as a result of electrophoresis.

(c)    Removal of carbon particles from smoke: The removal of negatively charged carbon particles from smoke can be done by passing through a chamber provided with highly positive charged metallic knob.

purification of water

Purification of water


(d)   Purification of water: The sewage contains negatively charged particles suspended in water. They may be removed by coagulating them on anode as a result of electrophoresis.

(ii)   Electro-osmosis: When electrophoresis of dispersed particles in a colloidal system is prevented by some suitable means, the dispersion medium itself begins to move in an electric field. This phenomenon  is known as electro-osmosis.

(iii) Coagulation: We know that the stability of the colloidal solution is due to mutual repulsion between similarly charged colloidal particles. When the charge on the colloidal particles is neutralized by the addition of an electrolyte or oppositely charged sol, the precipitation takes place. Thus “the process of precipitating a colloidal solution is known as coagulation or flocculation”.

For example, the negatively charged As_2S_3 sol is readily coagulated on addition of a solution of BaCl_2 (due to Ba^{2+} ions). The positively charged Fe(OH)_3 sol is readily coagulated on addition of a solution of NaOH (due to OH^- ions).

 

 Factors governing coagulation

(a)    A little amount of suitable electrolyte may bring coagulation.

(b)   Coagulation is brought about by oppositely charged ions of the electrolyte.

(c)    Coagulation also takes place by mixing oppositely charged sols. It is called mutual coagulation.

(d)   Coagulation of a sol is more pronounced at high temperature.

(e)   The coagulating power of different electrolytes is not equal, but depends on:

  • The valence of the effective ion: The coagulating power increases with the increase of the valency of the active cation i.e.

Sn^{+4} > Al^{3+} > Ba^{2+} > Na^+ and active anion I.e [Fe(CN)_ 6]^{- 4} > PO^{3-}_4 > SO^{2-}_4 > Cl^-(Hardy Schulze’s Law).

For example the coagulating values of Na^+ , Ba^{++} \text{and} La^{++} ion for the silver sol are 30, 0.5 and 0.003 milli-mole per litre respectively, hence their coagulating power may be expressed as the reciprocal of \dfrac{1}{30}, \dfrac{1}{0.5}, \dfrac{1}{0.003} or 0.003: 2.0: 333.3 or 1:60:1000 respectively for Na^+, Ba^{+ +} \text{and} La^{+++}  ions.

In the same way the coagulating power of Cl^-, SO^{2-}_4, \text{and} Po^{3-}_4  ions for Fe(OH)_3  Sol is 1:40:90.

  • The type of the colloidal solution: The lyophobic colloids are easily coagulated while lyophilic colloids require more amount of electrolyte.

When air saturated with water vapours, reaches a cool region, due to condensation bigger drops of water are formed which fall due to gravity in the form of rain. The other reason is that clouds carry positive and negative charges. When these opposite charge clouds mix, rain falls, due to coagulation of oppositely charged colloids. Thus by throwing electrified sand particles from an aero plane coagulation of mist hanging in the air takes place and comes down due to gravitational force. This is called artificial rain.

  • Peptization: If a freshly precipitated ferric hydroxide is treated with a small amount of ferric chloride solution, a reddish brown coloured sol of ferric hydroxide is obtained. “Thus the process of transferring  precipitate back into colloidal from is called peptization”. The FeCl_3, which has caused this dispersion, is called peptizing agent. It is evident that peptization is just reverse of coagulation.

 

Cause of Phenomenon:

Peptization is due to adsorption of common ion in colloidal solution and electrolyte. The common ion gives the colloidal particle a positive or negative charge according to the charge on the absorbed ion. It results again on mutual repulsion between similar charged particles and they are separated apart.

Peptization

Peptization

In the above example, the Fe^{3+} ions are absorbed on the precipitate of Fe(OH)_3 whereby the positive charge comes on their surface and repulsion takes place due to similar charge. Thus these particles are represented as [Fe(OH)_3]: Fe^{3+} . Similarly, a yellow precipitate of As_2S_3 obtained by passing H_2S gas through a solution of As_2 S_3 peptizes easily with excess of H_2S gas and is represented as [As_2S_3]: s^{2-}

 

  • Gold Number: Lyophobic colloids are readily coagulated by electrolyte, but it is difficult to coagulate lyophilic colloid. It has been seen that if a lyophilic colloid is added to the lyophobic one, the later is not coagulated easily by electrolytes and attains stability. The lyophobic colloid is supposed to be enveloped by the lyophilic colloid. Therefore the former remains protected against the action of electrolytes. The extent to which this protective action is exerted by lyophilic colloids differs from substance to substance and is measured quantitatively in terms of Gold number, an expression originated by Zigmondy (1901).

“Gold number is the number of milligrams of protective colloid which must be added to 10 mL of gold sol to prevent coagulation solution of sodium chloride is added to the gold sol”.

It is detected by a colour change from red to blue. it is obvious that the smaller the gold number the greater will be the protective action of the given hydrophilic colloid.

\text{Gold No.} \propto \dfrac{1}{\text{Protective power}}

 

The gold numbers of some of the colloids are given in the bracket with their names : gelatin (0.006 – 0.01), haemoglobin (0.03 — 0.97), gum arabic (0.15 – 0.25), albumin (0.19 – 0.20), dextrin (6.0 — 20.0) and potato starch (10.0 – 25.0).



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