Properties of Colloidal Solutions Chemistry Notes

Properties of Colloidal Solutions Chemistry Notes

Properties of Colloidal Solutions :

The important properties of colloidal solutions are given below:

Physical Properties : These are of following types:

→ Heterogeneity : A colloidal solution is heterogeneous in nature because it has two phases i.e., dispersed phase and dispersion medium. The size of colloidal particles is quite larger than the size of particles of true solutions. So, the colloidal solution has heterogeneity.

→ Filtrability : Colloidal solution cannot be filtered by normal filter paper as the pore size is quite larger than the size of colloidal particles. However, they cannot pass through parchment paper or animal and vegetable membrane or cellophane membrane. So, colloidal solution can be separated by crystalloid solution with the help of such filter paper.

→ Stability : Colloidal solutions are quite stable because their particles do not settle down under the influence of gravitational force as they are very minute and show constant motion. However, they can be settled down by centrifiguration. Lyophilic colloids are more stable than lyophobic colloids.

→ Visibility : Colloidal particles are invisible to naked eyes but with the help of ultramicrocope, their particles can be seen. The light scattered by the colloidal particles can be seen easily by naked eye.

→ Surface area : Surface area of colloidal particles present in colloidal solution is quite larger hence collodial solution can be worked as good adsorbent.

→ Surface tension and Viscosity : The surface tension and viscosity of lyophobic sols are not very different from those of the dispersion medium. On the other hand, lyophilic sols show higher viscosity and lower surface tension in comparison to the dispersion medium.

→ Shape and Colour: The colour of colloidal solution depends on the size, shape of colloidal particle as well as on the wavelength of light falling on the particles of colloids.

For example :

• Gold sol with very minute particles hase red colour, as soon as the size of particles increases, the colour becomes purple then blue and at last it becomes golden.
• The colour of silver sol is yellow-orange if the size of particle is 6 × 10^-5 mm while it becomes red-orange if the size becomes 9 × 10^-5 mm.
• A mixture of milk and water appears blue when viewed by the reflected light and red when viewed by the transmitted light.

→ Sedimentation: The particles of sol are centrifuged by rotating them rapidly in centrifuge machine. This process is called sedimentation. The molecular masses of macromolecules are determined by this method.

→ 2 Colligative Properties : The properties, which depend on the number of particles present in the solution but not on the nature of particles are known as colligative properties. The size of colloidal particles is larger than the size of particles present in true solutions hence the number of particles in a colloidal solution is comparatively smaller than the number of particles in true solution.

→ Hence, the values of colligative properties (i.e., lowering in vapour pressure, elevation in boiling point, depression in freezing point, osmotic pressure) are of small order as compared to values shown by true solutions at same concentrations.

→ Tyndall Effect: This effect was first observed by Faraday and later detail study was done by Tyndall When a beam of light enters a dark room it lights up the dust particles floating in the air. Similarly when a beam of light is passed through a colloidal solution, the path of the beam is illuminated and becomes visible as a blue bright streak when seen in a direction perpendicular to the incident beam.

→ This phenomenon is called as Tyndall effect and the blue streak is known as Tyndall cone. “This scattering of light by the colloidal particles is known as Tyndall effect.” True solution does not show this effect because in true solutions, there are no particles of sufficiently large size to scatter incident light. Hence the beam of light remains invisible.

→ Reason: Tyndall effect is due to the fact that colloidal particles scatter light in all directions in space. This scattering of light illuminates the path of beam in the colloidal dispersion. Tyndall effect is also observed during the projection of picture in the cinema hall due to scattering of light by dust and smoke particles present there.

The important conditions for the tyndall effect are as follow :

→ There should be large difference in the refractive indices of dispersed phase and dispersion medium. The greater thes difference, the brightness of the blue colour of Tyndall cone will be high. 2. The diameter of the sol particles should not be smaller than the wavelength of the light used to see the colloidal solution. The strength of Tyndall effect depends upon the extent of scattering of light. The scattering of light depends upon the following factors:

• Size of Particles of dispersed phase : Larger the size of the particles of dispersed phase, larger will be scattering. That’s why, the scattering of light is larger by lyophobic sol.
• Wavelength of light : Relation between wavelength and extent of scattering is as follows:
• Extent of scattering α \(\frac{1}{\lambda^{4}}\)
• As the wavelength of purple coloured light is lowest hence its scattering is maximum.

→ Difference of Refractive index of dispersed phase and dispersion medium : Larger the difference in refractive index of dispersed phase and dispersion medium, larger will be the intensity of scattered light.

→ Diameter of particles and wavelength : The diameter of the particles of dispersed phase should not very less than the wave length of light used. Tyndall effect is used to distinguish collodial solution and true solution. Zsigmondy used tyndall effect to prepare ultramicroscope. Other examples of Tyndall effect are blue colour of sky. blue colour of the ocean, the redness of the sun at sunset, the visibility for the head light of a vehicle in the presence of smoke of ahead going vehicles etc.

→ Brownian Movement: When colloidal solution is observed with the help of ultramicroscope then colloidal particles are seen as bright spots of light in dark back ground in a state of rapid, random, zig-zag motion. This motion was first observed by Robert Brown. Hence, it is known as Brownian movement. This Brownian movement is responsible for the stability of colloidal solution because the movement has a string effect which does not permit the particles to settle.

• There is no effect of wavelength of light rays and the intensity of light rays on the Brownian movement.
• Brownian movement is shown by the particles of solution, colloidal solution and suspension.
• Brownian movement does not change with time. It happens at last. The speed of particles does not change with time.
  On increasing temperature, Brownian movement also increases.
• Brownian movement is responsible for the stability of colloidal solution because it do not allow to settle down the colloidal particles.
• As soon as the size of colloidal particles increases, the Brownian movement shown by particles decreases.
• As viscosity decreases, Brownian movement also decreases.

→ Electrical properties : The main reason of stability of colloidal sol is charge present on particles of dispersed phase. Infact only one type of charge is present on all particles of dispersed phase i.e., positive or negative particles. The particles having one type of charge repel each other. So they can not come close to each other and hence cannot form large particles by combination. There are following few positively and negatively colloidal sol by using water in the form of dispersion medium.

Colloidal sol having positively and negatively charged colloidal particles

Negatively charged colloidal sol	Positively charged colloidal sol
Metals like Au, Ag, Cu, Pt etc.	Hydrated metal compounds like Al2O3 . xH2O, Fe2O3 -xH2O or Al(OH)3 and Fe(OH)3
Metal suiphides like CdS, As2S3, SbS3.	Oxides like TiO2
Starch, gelatin, salicylic acid, soil, haemoglobin (blood) acidic dyes like eosin, congo red.	Basic dye like methylene blue.

Causes of Origin of Charges : The cause of charge on colloidal particle is due to the following reasons:

• Frictional Electrification : Electric charge on colloidal particle is produced due to the friction of particles of dispersed phase of colloid.
• Due to dissociation of surface molecules : Soaps are ionised as follow in its aqueous solution :
  C₁₇H₃₅COONa — C₁₇H₃₅COO^– + Na⁺
• Cations (Na⁺) go to solution but anions form negatively charged colloid by aggregation from inter molecular attractive forces of hydrocarbon chain.

→ Preferential Adsorption Theory: According to this theory, sol particles show preferential adsorption of ions present in colloidal solution. In this process, colloidal particles adsorb common ions i.e., similar ions from the dispersion medium on the surface of the colloidal particles. If colloidal particles adsorb positive ions then they acquire positive charge or if they adsorb negative ions then they acquire negative charge. For example:

→ Fe³⁺ ions are present in colloidal solution of Fe(OH)₃ formed by hydrolysis of FeCl₃. So, colloidal particles of ferric hydroxide are positively charged by adsorbing Fe³⁺ ions.

→ The colloid of Agl is formed by adding aqueous KI solution in excess of silver nitrate, which forms positive colloid by adsorbing Ag⁺ ions from dispersion medium. But if the excess of KI adds in AgNO₃ solution then AgI colloid is negatively charged, because AgI adsorbs I^– ion of KI electrolyte.

Arsenic sulphide colloid is obtained by passing H₂S gas in solution of arsenic oxide which is negatively charged by adsorbing S^2- ions.

Al(OH)₃ colloid is positively charged because this electrolyte adsorbs Al³⁺ ions of AlCl₃:

Hence the nature of charge on colloidal particles depend upon the nature of adsorbed common ions. In above given examples, we can understand it easily.

Distribution of Charge in Colloidal State : Electrical Double Layer Theory: Helmholtz initiated this theory which was later modified by Guoy and stern.

According to this theory :

There is a electrical double layered structure in every colloidal state.

Here one layer is due to the adsorption of positive or negative ions on the colloidal particles. This layer is known as fixed layer.

• Second layer is due to the dispersion medium and diffused positive or negative ions in it. This layer is known as diffused or movable layer or mobile layer.
• Both the layers are closer to each other and are separated by a hypothetical layer. This hypothetical layer is known as hydrodynamic plane or shear.
• The total charge on fixed layer is equal but opposite to the total charge on diffused layer.

→ Since separation of charge creates potential, the charges of opposite signs on the fixed and diffused parts of the double layer results in potential difference between the fixed layer and the diffused layer of opposite charges is called the electrokinetic potential or zeta potential.

→ The presence of equal and similar charges on colloidal particles is largely responsible for the stability of collodial solution, because similar charges do not allow them to coagulate due to repulsion created between these similar charges.

→ Electrophoresis : As we know that collodial particles are electrically charged and carry same type of charge either positive or negative. The dispersion medium has an equal but opposite charge to make the system neutral.

→ Due to similar nature of charge on the collodial particles then they repel each other and do not allow to combine to form bigger particles so the collodial solution is stable. The existence of the electric charge can be proved by the phenomenon of electrophoresis.

→ “When electric field is applied on colloidal solution then colloidal particles show movement either towards cathode or anode. This movement of particles towards oppositvely charged electrode is known as electrophoresis.”

→ When colloidal particles are negatively charged then they move towards anode and this phenomenon is known as anaphoresis. When collodial particles are positively charged then they move towards cathode and this phenomenon

→ Coagulation or Flocculation : The stability of any colloidal solution is due the presence of same charge on the particles of dispersed phase. If any how this charge is removed or reduced then the stability of colloidal solution also decreases or destroyed. Now the small particles collapse with each other, converted into bigger aggregate (or precipitate) and settle down under the force. This gravity precipitation of colloidal solution is called coagulation.

→ “The transformation of colloidal solution into a suspension with the help of an electrolyte is called coagulation or flocculation or precipitation.”

The coagulation can be done by following methods:

• By electrophoresis : If electrophoresis is done for a long time, then colloidal particles come closer to each other and got discharged on oppositely charged electrode and precipitated there.
• By Boiling : When colloidal solution is boiled then adsorbed layer is disturbed due to increase in collisions with the molecules of dispersion meidum. This reduces the charge on the particles and ultimately lead to settling down in the form of a precipitate.
• By mixing electrolytes : Presence of small amount of electrolyte causes peptisation or formation of colloidal soluiton. On the other hand if electrolytes are added in excess amount then colloidal particles are precipitated or coagulated.
• The reason is that colloids interact with ions carrying charge opposite to that present on themselves.
• This causes neutralisation leading to their coagulation or flocculation of precipitation The ions which causes neutralisation of charge on colloidal solution is called coagulating ions.
• A negative ion causes the precipitation of positively charged sol and a positive ion causes the precipitation of negatively charged sol.

For example : In the colloidal solution of As₂S₃, if BaCl₂ is added then negatively charged sol particles are neutralized by Ba²⁺ ions

• By mixing two oppositely charged collodial sols : When two oppositely charged colloidal solutions are mixed in almost equal proportions then they neutralise the charges of each other and get partially or completely coagulated.
• For Example : Mixing of hydrated ferric oxide which is a positively charged sol and aresenious sulphide which is a negatively charged sol, bring them in the precipitd forms. This type of coagulation is called mutual coagulation.
• By persistent Dialysis : On prolonged dialysis, traces of the eleotrolytes present in the sol are removed completely and the colloids become unstable and ulitmately coagulated.
• Hardy-Schulze’s Law : Hardy-Schulze gave this rule which is concerned with coagulation of a colloidal solution. According to this rule
• The ions carrying opposite charge to that of colloid or sol particles are effective in causing the coagulation of the sol. Such ions are called of flocculating ions or active ions or coagulating ions or effective ions.
• Coagulating power of an electrolyte is directly proportional to the fourth power of the valency of the active ions. Thus greater the valency of the coagulating ion, greater is its power of precipitation
• For example : For the coagulation of sols carrying negative charge like As, S, sol, the order of coagulation power of effective ions will be Th⁴⁺ > Al³⁺ > Ba²⁺ > Na⁺
• For the coagulation of sols carrying positive charge like Fe(OH)₃ sol, the order of coagulation power of effective ion will be

Coagulation values of few electrolytes are given below in table 5.7:

→ Coagulating or precipitating power : The capacity of oppositvely charged ion to coagulate the sols known as its coagulating power or precipitating power.

→ Coagulation value or Flocculation Value : The minimum amount of an electrolyte in milli moles that must be added to flocculate one litre of a sol is called flocculation value of the electrolyte for the sol. Greater the coagulating power of effective ion, lower will be the flocculation value of the electrolyte containing the effective ion. Thus,

→ Coagulation of Lyophilic sols : The lyophilic sols are more stable than lyophobic sols. The stability of lyophilic sols is due to presence of following two factors:

• Charge on colloidal particles
• Solvation of colloidal particles because of the presence of a layer of dispersion medium on the surface of colloidal particles due to greater attractive forces between dispersed phase and dispersion medium.

→ So, the layer of the dispersion medium is first removed by adding any suitable dehydrating agent in the hydrophilic sols and then electrolyte is added which can neutralize the charge on colloidal particles. In this way lyophilic sols are coagulated.

→ For example : In a hydrophilic sol, first alcohol or acetone is added which causes dehydration of dispersion medium. Now small quantity of electrolyte is added for complete coagulation.

→ Protection of Colloids : Lyophilic sols are more stable than lyophobic sols. It is due to the fact that lyophilic colloids are highly solvated i.e., colloidal particles are covered by a layer of the liquid in which they are dispersed Lyophilie colloids have a unique property to protect lyophobic colloida.

→ When a lyophilic collid is added to a lyophobic colloid then the particles of lyophilic colloid form a layer around the particles of lyophobic sol. That is why lyophilic colloids are known as protective colloids and the phenomenon of protection of lyophobic colloids by lyophilic colloid is known as protective action.

For example:

• To prevent the coagulation of milk, ice and sugar present in ice cream, small amount of gelatin is added to it as protective colloid.
• To prevent the coagulation of colloidal particles of carbon present in black ink, small amount of gum arabic is added to it as protective colloid.
• Gelatin is used to stabilise the colloidal dispersion of silver halides used in the preparation of photographic films.

→ Gold Number : The protective power of different lyophilic sols is different. The protective action of different colloids is expressed in terms of gold number introduced by Zsigmondy in 1901. Gold number can be defined as:

→ “The number of milligrams of a hydrophilic colloid which just prevent the precipitation of 10 mL of gold sol on the addition of 1 mL of ten percent sodium chloride solution is known as Gold number.”

→ The protective power is inversely related to gold number i.e., smaller the gold number of a protective colloid, greater is its protective action. i.e.,

→ The gold number values of few protective colloids are given in table 5.8.

Some protective colloids and their gold number

Protective Colloids	Gold number
Gelatin	0.005-0.01
Casein	0.01-0.02
Haemoglobin	0.03-0.07
Egg Albumin	0.1-0.2
Gum Arabic	0.15-0.25
Sodium Oleate	0.4
Gum tragacanth	2
Dextrin	6-20
Potato starch	20-25

→ Thus, out of the list given above, gelatin has maximum protective power while potato starch has the least protective power.

→ Electro-Osmosis : When colloidal solution is placed under the influence of electric field and anyhow the movement of colloidal particles is stopped by using a membrane. Then particles of dispersion medium move towards oppsitely charged electrode.

→ This movement of dispersion medium under the influence of electric current is known as electro-osmosis. The process of electro-osmosis proves that the particles of dispersion medium also carry a charge but opposite to that on sol particles.

Chemistry Notes