Surface Chemistry : Colloidal Solution

Colloidal state

(1)     The foundation of colloidal chemistry was laid down by an English scientist, Thomas Graham, in 1861. The credit for the various advances in this field goes to eminent scientists like Tyndall, Hardy, Zsigmondy, N.R. Dhar, S.S. Bhatnagar and others.

 

(2)     Thomas Graham classified the soluble substances into two categories depending upon the rate of diffusion through animal and vegetable membranes or parchment paper.

(i)     Crystalloids : They have higher rate of diffusion and diffused from parchment paper.

Examples : All organic acids, bases and salts and organic compounds such as sugar, urea etc.

(ii)    Colloids (Greek word, kolla, meaning glue-like) : They have slower rate of diffusion and can not diffused from parchment paper.

Examples : Starch, gelatin, gums, silicic acid and hdemoglobin etc.

 

(3)     The above classification was discarded i.e., the terms colloid does not apply to a particular class of substances but is a state of matter like solid, liquid and gas. Any substance can be brought into colloidal state.

 

(4)     The colloidal state depends on the particle size. If is regarded as intermediate state between true solution and suspension.

  • True solution : In true solutions the size of the particles of solute is very small and thus, these cannot be detected by any optical means and freely diffuse through membranes. It is a homogenous system.
  • Suspension : The size of particles is large and, thus it can be seen by naked eye and do not pass through filter paper. It is a heterogeneous system.

The size of different solutions are sometimes expressed in other units also as given below :

Size (diameter) of particles in particles in different units

True solutions

Colloids

Suspensions

Relation

<10–9m

<1nm

<10 Å

<1000 pm

10–9 m to 10–7m

1 nm – 100 nm

10 Å – 1000 Å

1000 pm –105 pm

> 10−7 m

> 100 nm

> 1000 Å

>105 pm

 

1 nm = 10–9 m

1 Å = 10–10 m

1 pm = 10–12 m

 

The important distinguishing features of the three types of solutions

Property

Suspension

Colloid solution

True solution

Nature Heterogeneous Heterogeneous Homogeneous
Particle size > 100 nm 1 nm – 100 nm < 1 nm
Separation by

(i) Ordinary filtration

(ii) Ultra- filtration  

 

Possible

Possible

 

Not possible

Possible

 

Not possible

Not possible

Settling of particles Settle under gravity Settle only on centrifugation Do not settle
Appearance Opaque Generally transparent Transparent
Tyndall effect Shows Shows Does not show
Diffusion of particles Does not diffuse Diffuses slowly Diffuses rapidly
Brownian movement May show Shows Negligible

 

(5)     Roughly speaking the colloidal state is a heterogeneous dispersion of solute particles of size between true solution and suspension.

Note :

  • Colloidal particles do not settle down under the force of gravity even an long keeping.
  • The surface area of colloidal particle is very large in comparison to suspension.

 

Phases of colloids and their classification.    

(1)     Phases of colloids : We know that a colloidal solution is of heterogeneous nature. It consists of two phases which are as follows

(i)     Internal phase or Dispersed phase (Discontinuous phase) : It is the component present in small proportion and is just like a solute in a solution. For example in the colloidal solution of silver in water (silver acts as a dispersed phase)

(ii)    External phase or Dispersion medium (continuous phase) : It is generally component present in excess and is just like a solvent in a solution. For example, in the colloidal solution of silver in water. Water act as a dispersion medium.      

Note :

  • When dispersion medium is a gas, the colloidal system is called aerosol. When it is a liquid system is called solution (hydrosol for water, alcosol for alcohol, benzosol for benzene)
  • Colloidal system is a two phase system.

Colloidal system = Dispersed phase + Dispersion medium

 

(2)     Classification of colloids : The colloids are classified on the basis of the following criteria

(i)     Classification based on the physical state of the dispersed phase and dispersion medium  Depending upon the physical state of dispersed phase and dispersion medium whether these are solids, liquids or gases, eight types of colloidal systems are possible.

Different types of colloidal systems

Dispersed phase Dispersion Medium Colloidal System Examples
Liquid Gas Aerosol of liquids Fogs, clouds, mists, fine insecticide sprays
Solid Gas Aerosol of solids Smoke, volcanic dust, haze
Gas Liquid Foam or froth Soap lather. Lemonade froth, foam, whipped cream, soda water
Liquid Liquid Emulsions Milk, emulsified oils, medicines
Solid Liquid Sols Most paints, starch in water, proteins, gold sol, arsenic sulphide sol, ink
Gas Solid Solid foam Pumice stone, styrene rubber, foam rubber
Liquid Solid Gels Cheese, butter, boot polish, jelly, curd
Solid Solid Solid sols (coloured glass) Ruby glass, some gem stones and alloys

 

Note

  • A colloidal system of gas in gas is not possible as gases are completely miscible and always form homogenous true solution.

(ii)    Classification based on Nature of interaction between dispersed phase and dispersion medium: Depending upon the nature of interactions between dispersed phase and the dispersion medium, the colloidal solutions can be classified into two types as (a) Lyophilic and (b) Lyophobic sols.

(a)    Lyophilic colloids (water loving) : Substances such as proteins, starch and rubber whose molecules are large enough to be close to the lower limit of colloidal range pass readily into colloidal state whenever mixed with suitable solvent. Thus these colloids have strong interaction with the dispersion medium and are called lyophilic colloids (hydrophilic when continuous phase is water)

“or”

“The colloidal solutions in which the particles of the dispersed phase have a great affinity for the dispersion medium, are called lyophilic collodis.”

(b)    Lyophobic colloids (water heating) : Substance such as arsenic sulphide, ferric hydroxide, gold and other metals, which are sparingly soluble and whose molecules are much smaller than the lower colloidal limit, change into colloidal state by aggregation of many individual molecules. “These substances; therefore, do not pass into colloidal state readily and are called lyophobic colloids (hydrophobic when continuous phase is water).

“or”

“The colloidal solutions in which there is no affinity between particles of the dispersed phase and the dispersion medium are called lyophobic colloids.

Distinction between Lyophilic and Lyophobic Sols

Property Lyophilic (suspensoid) Lyophobic Sols (Emulsoid )
Surface tension Lower than that of the medium Same as that of the medium
Viscosity Much higher than that of the medium Same as that of the medium
Reversibility Reversible Irreversible
Stability More stable Less stable
Visibility Particles can’t be detected even under ultramicroscope  Particles can be detected under ultramicroscope.
Migration Particles may migrate in either direction or do not migrate in an electric field because do not carry any charge. Particles migrate either towards cathode or anode in an electric field because they carry charge.
Action of electrolyte Addition of smaller quantity of electrolyte has little effect Coagulation takes place
Hydration Extensive hydration takes place No hydration
Examples Gum, gelatin, starch, proteins, rubber etc. Metals like Ag and Au, hydroxides like Al(OH)3, Fe(OH)3 metal sulphides like AS2S3 etc.

 

(iii)   Classification based on types of particle of dispersed phase : Depending upon the type of the particles of the dispersed phase, the colloids are classified as follows.

(a)    Multimolecular colloids

  • When on dissolution, atoms or smaller molecules of substances (having diameter less than 1nm) aggregate together to form particles of colloidal dimensions, the particles thus formed are called multimolecular colloids.
  • In these sols the dispersed phase consists of aggregates of atoms or molecules with molecular size less than 1
  • For example, sols of gold atoms and sulphur (S8) molecules. In these colloids, the particles are held together by Vander Waal’s forces. They have usually lyophilic character.

 

(b)    Macromolecular colloids

  • These are the substances having big size molecules (called macromolecules) which on dissolution form size in the colloidal range. Such substances are called macromolecular colloids.
  • These macromolecules forming the dispersed phase are generally polymers having very high molecular masses.
  • Naturally occurring macromolecules are starch, cellulose, proteins, enzymes, gelatin etc.
  • Artificial macromolecules are synthetic polymers such as nylon, polythene, plastics, polystyrene etc.
  • Their solutions are quite stable and resemble with true solution in many respects.
  • They have usually lyophobic character.
  • The molecules are flexible and can take any shape.

 

(c)     Associated colloids

  • These are the substances which on dissolved in a medium behave as normal electrolytes at low concentration but behave, as colloidal particles at higher concentration due to the formation of aggregated particles. The aggregates particles thus formed are called
  • Their molecules contain both lyophilic and lyophobic
  • The colloidal behaviour of such substances is due to the formation of aggregates or clusters in solutions. Such aggregated particles are called

Micelles

  • Micelles are the cluster or aggregated particles formed by association of colloid in solution.
  • The common examples of micelles are soaps and detergents.
  • The formation of micelles takes place above a particular temperature called Kraft temperature (Tk) and above a particular concentration called critical micellization concentration (CMC).
  • They are capable of forming ions. 
  • Micelles may contain as many as 100 molecules or more.
  • For example sodium stearate (C17H35COONa) is a typical example of such type of molecules.
  • When sodium stearate is dissolved in water, it gives Na+ and C17H35COO− ions.

\[\underset{Sodium\,\,stearate}{\mathop{{{C}_{17}}{{H}_{35}}COONa}}\,\to \underset{Stearate\,\,ion}{\mathop{{{C}_{17}}{{H}_{35}}CO{{O}^{-}}}}\,+N{{a}^{+}}

  • The stearate ions associate to form ionic micelles of colloidal size.
  • It has long hydrocarbon part of C17H35 Which is lyophobic and COO part which is lyophilic.
  • In the figure, the chain corresponds to stearate ion, (C17H35COO). When the concentration of the solution is below from its CMC (10−3 mol L−1), it behaves as normal electrolyte. But above this concentration it is aggregated to behave as micelles.
  • The main function of a soap is to reduce oily and greasy dirt to colloidal particles (an emulsion). Soap therefore, are known as emulsifying agents.
  • Some other examples of micelles are sodium palmitate (C15H31COONa), Sodium lauryl sulphate [CH3(CH2)11SO3ONa+], Cetyl trimethyl ammonium bromide CH3(CH2)15(CH2)3N+Br

Note :      

  • Polydisperse and Monodisperse colloids : In multimolecular colloids, the colloidal particles consist of aggregates of atoms or small molecules with diameters less than 10−9 m of 1 nm Colloidal solutions in which colloidal particles are of different sizes are called polydisperse colloids. For example, a gold sol may contain particles of various sizes having several atoms of gold. The colloidal solutions in which all the colloidal particles are more or less of identical size are monodisperse colloids.

 

General methods of preparation of colloids.

Lyophilic and lyophobic colloidal solutions (or sols) are generally prepared by different types of methods. Some of the common methods are as follows.

(1)      Preparation of Lyophilic colloids

(i)       The lyophilic colloids have strong affinity between particles of dispersed phase and dispersion medium.

(ii)     These colloidal solutions are readily formed by simply mixing the dispersed phase and dispersion medium under ordinary conditions.

(iii)    For example, the substance like gelatin, gum, starch, egg, albumin etc. pass readily into water to give colloidal solution.

(iv)    They are reversible in nature become these can be precipitated and directly converted into colloidal state.

 

(2)      Preparation of Lyophobic colloids: Lyophobic colloids can be prepared by mainly two types of methods.

(i)       Condensation method : In these method, smaller particles of dispersed phase are condensed suitably to be of colloidal size. This is done by the following methods.

(a)      By oxidation : A colloidal solution of sulphur can be obtained by bubbling oxygen (or any other oxidising agent like HNO3, Br2 etc.)  through a solution of hydrogen sulphide in water.

2H2S + O2 (or any other oxidizing agent → 2H2O + 2S   

(b)    By reduction : A number of metals such as silver, gold and platinum, have been obtained in colloidal state by treating the aqueous solution of their salts, with a suitable reducing agent such as formaldehyde, phenyl hydrazine, hydrogen peroxide, stannous chloride etc.

\[2AuC{{l}_{3}}+3SnC{{l}_{2}}\to 3SnC{{l}_{4}}+\underset{Gold\,sol}{\mathop{2Au}}\,

\[2AuC{{l}_{3}}+3HCHO+3{{H}_{2}}O\to \underset{Gold\,\,sol}{\mathop{2Au}}\,+3HCOOH+6HCl

The gold sol, thus prepared, has a purple colour and is called purple of cassius.

\[FeC{{l}_{3}}+3{{H}_{2}}O\to \underset{Colloidal\,\,sol}{\mathop{Fe{{(OH)}_{3}}}}\,+3HCl

(c)     By hydrolysis : Many salt solutions are rapidly hydrolysed by boiling dilute solutions of their salts. For example, ferric hydroxide and aluminium hydroxide sols are obtained by boiling solutions of the corresponding chlorides. 

\[FeC{{l}_{3}}+3{{H}_{2}}O\to \underset{Colloidal\,\,sol}{\mathop{Fe{{(OH)}_{3}}}}\,+3HCl

Similarly silicic acid sol is obtained by the hydrolysis of sodium silicate.

(d)    By double decomposition : A sol of arsenic sulphide is obtained by passing hydrogen sulphide through a cold solution of arsenious oxide in water. As2O3 + 3H2S → As2S3 + 3H2O

(e)     By excessive cooling : A colloidal solution of ice in an organic solvent like ether or chloroform can be prepared by freezing a solution of water in the solvent. The molecules of water which can no longer be held in solution, separately combine to form particles of colloidal size.

(f)     By exchange of solvent : Colloidal solution of certain substances such as sulphur, phosphorus, which are soluble in alcohol but insoluble in water can be prepared by pouring their alcoholic solution in excess of water. For example, alcoholic solution of sulphur on pouring into water gives milky colloidal solution of sulphur.

(g)    By change of physical state : Sols of substances like mercury and sulphur are prepared by passing their vapour’s through a cold water containing a suitable stabilizer such as ammonium salt or citrate.

(ii)    Dispersion methods : In these methods, larger particles of a substance (suspensions) are broken into smaller particles. The following methods are employed.

(a)    Mechanical dispersion

  • In this method, the substance is first ground to coarse particles. 
  • It is then mixed with the dispersion medium to get a suspension.
  • The suspension is then grinded in colloidal mill.
  • It consists of two metallic discs nearly touching each other and rotating in opposite directions at a very high speed about 7000 revolution per minute.
  • The space between the discs of the mill is so adjusted that coarse suspension is subjected to great shearing force giving rise to particles of colloidal size.
  • Colloidal solutions of black ink, paints, varnishes, dyes etc. are obtained by this method.

 

(b)     By electrical dispersion or Bredig’s arc method : This method is used to prepare sols of platinum, silver, copper or gold. 

  • The metal whose sol is to be prepared is made as two electrodes which immerged in dispersion medium such as water etc.
  • The dispersion medium is kept cooled by ice.
  • An electric arc is struck between the electrodes.
  • The tremendous heat generate by this method and give colloidal solution.
  • The colloidal solution prepared is stabilised by adding a small amount of KOH to it.

 

(c)     By peptisation

  • The process of converting a freshly prepared precipitate into colloidal form by the addition of suitable electrolyte is called peptisation.
  • The electrolyte is used for this purpose is called peptizing agent or stabilizing agent.
  • Cause of peptisation is the adsorption of the ions of the electrolyte by the particles of the precipitate. 
  • Important peptizing agents are sugar, gum, gelatin and electrolytes.
  • Freshly prepared ferric hydroxide can be converted into colloidal state by shaking it with water containing Fe3+ or OH ions, viz. FeCl3 or NH4OH respectively. 

\[\underset{\text{Precipitate electrolyte}}{\mathop{Fe{{(OH)}_{3}}+FeC{{l}_{3}}}}\,\xrightarrow{{}}\underset{\text{Colloidal sol}}{\mathop{{{[Fe{{(OH)}_{3}}Fe]}^{3+}}}}\,+3C{{l}^{-}}

  • A stable sol of stannic oxide is obtained by adding a small amount of dilute HCl to stannic oxide precipitates.
  • Similarly, a colloidal solution of Al(OH)3 and AgCl are obtained by treating the corresponding freshly prepared precipitate with very dilute solution of HCl and AgNO3 or KCl respectively.
  • Gelatin stabilises the colloidal state of ice-cream.
  • Lamp black is peptised by gums to form Indian ink.
  • If precipitate of CuS, BaSO4 or Prussian blue are washed continuously with water, after sometime the precipitate are converted into colloidal state which thus pass through the fitter paper and thus can be detected in wash water.

 

Purification of colloidal solution.

The colloidal solutions prepared by the above methods usually contain  impurities especially electrolytes which can destabilize the sols. These impurities must be eliminated to make the colloidal solutions stable. The following methods are commonly used for the purification of colloidal solutions.

(1)     Dialysis

(i)     The process of separating the particles of colloid from those of crystalloid, by means of diffusion through a suitable membrane is called dialysis.

(ii)    It’s principle is based upon the fact that colloidal particles cannot pass through a parchment or cellophane membrane while the ions of the electrolyte can pass through it.

(iii)   The colloidal solution is taken in a bag (parchment paper).

(iv)   The bag is suspended in fresh water.

(v)    The impurities slowly diffused out of the bag leaving behind pure colloidal solution

(vi)   The distilled water is changed frequently to avoid accumulation of the crystalloids otherwise they may start diffusing back into the bag.

(vii) Dialysis can be used for removing HCl from the ferric hydroxide sol.

 

(2)     Electrodialysis

(i)     The ordinary process of dialysis is slow. 

(ii)    To increase the process of purification, the dialysis is carried out by applying electric field. This process is called electrodialysis.

(iii)   Kidneys in the human body act as dialysers to purify blood which is colloidal in nature.

(iv)   The important application of dialysis process in the artificial kidney machine used for the purification of blood of the patients whose kidneys have failed to work. The artificial kidney machine works on the principle of dialysis.

 

(3)     Ultra – filtration  

(i)     Sol particles directly pass through ordinary filter paper because their pores are larger (more than 1μ or 1000mμ) than the size of sol particles (less than 200mμ).

(ii)    If the pores of the ordinary filter paper are made smaller by soaking the filter paper in a solution of gelatin of colloidion and subsequently hardened by soaking in formaldehyde, the treated filter paper may retain colloidal particles and allow the true solution particles to escape. Such filter paper is known as ultra – filter and the process of separating colloids by using ultra – filters is known as ultra – filtration.

 

(4)     Ultra – centrifugation

(i)     The sol particles are prevented from setting out under the action of gravity by kinetic impacts of the molecules of the medium.       

(ii)    The setting force can be enhanced by using high speed centrifugal machines having 15,000 or more revolutions per minute. Such machines are known as ultra–centrifuges.

 

Properties of colloidal solutions.

The main characteristic properties of colloidal solutions are 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. Their particles 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. However they can be retained by special filters known as ultrafilters (parchment paper).                       

 

(2)     Colligative properties

(i)     Due to formation of associated molecules, observed values of colligative properties like relative decrease in vapour pressure, elevation in boiling point, depression in freezing point, osmotic pressure are smaller than expected.    

(ii)    For a given colloidal sol 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 type of motion is called 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 causing a zig-zag motion of the dispersed phase particles.

(d)    When a molecule of dispersion medium colloids with a colloidal particle, it is then displaced in one direction. Then another molecules strikes it, displacing it to another direction and so on. This process give rise to zig-zag motion.

(e)     This can be confirmed by the fact that the suspensions do not show any such movement due to large molecular size.

(f)     Brownian movement provides a direct demonstration of the ceaseless motion of molecules as postulated by kinetic energy.

(g)    The Brownian movement explains the force of gravity acting on colloidal particles. This helps in providing stability to colloidal sols by not allowing them to settle down.

(ii)    Diffusion : The sol particles diffuse from higher concentration to lower concentration region. However, 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 phenomenon is used for determining the molecular mass of the macromolecules.

 

(4)     Optical properties : Tyandall effect

(i)     When light passes through a sol, its path becomes visible because of scattering of light by particles. It is called Tyndall effect. This phenomenon was studied for the first time by Tyndall. The illuminated path of the beam is called Tyndall cone.

(ii)    In a true solution, there are no particles of sufficiently large diameter to scatter light and hence no Tyndall effect is observed.

(iii)   The intensity of the scattered light depends on the difference between the refractive indices of the dispersed phase and the dispersion medium.

(iv)   In lyophobic colloids, the difference is appreciable and, therefore, the Tyndall effect is well – defined. But in lyophilic sols, the difference is very small and the Tyndall effect is very weak.

(v)    The Tyndall effect confirms the heterogeneous nature of the colloidal solution.

(vi)   The Tyndall effect has also been observed by an instrument called ultra – microscope.      

Note :

  • The smoke is colloidal, so when it is viewed at an angle to the source of light, it appears blue due to Tydnall effect.
  • Dust in the atmosphere is often colloidal. When the sun is low down on the horizon, light from it has to pass through a great deal of dust to reach your eyes. The blue part of the light is scattered away from your eyes and you observe red part of the spectrum. Thus red sunsets are Tyndall effect on a large scale.
  • Tail of comets is seen as a Tyndall cone due to the scattering of light by the tiny solid particles left by the comet in its path.
  • Due to scattering the sky looks blue.
  • The blue colour of water in the sea is due to scattering of blue light by water molecules.
  • Visibility of projector path and circus light.
  • Visibility of sharp ray of sunlight passing through a slit in dark room.

 

(5)     Electrical properties : Colloidal particles carry an electric charge and the dispersion medium has an opposite and equal charge, the system as a whole being electrically neutral. The presence of equal and similar charges on colloidal particles is largely responsible in giving stability to the system because the mutual forces of repulsion between similarly charged particles prevent them from coalescing and coagulating when they come closer to one another.

(i)     Electrophoresis 

(a)    The phenomenon of movement of colloidal particles under an applied electric field is called electrophoresis.

(b)    If the particles accumulate near the negative electrode, the charge on the particles is positive.

(c)     On the other hand, if the sol particles accumulate near the positive electrode, the charge on the particles is negative.

(d)    The apparatus consists of a U-tube with two Pt-electrodes in each limb.

(e)     Take a sol of As2S3 in the U-tube.

(f)     The intensity of the colour of the sol in both the arms is same. Now pass the current through the sol.

(g)    After some time it is observed that the colour of sol near the positive electrode become dark. This indicates that the As2S3 particles are negatively charged and they move towards oppositely charged electrodes.

(h)    Similarly, when an electric current is passed through positively charged Fe(OH)3 sol, it is observed that they move towards negatively charged electrode and get accumulated there.

(i)     Thus, by observing the direction of movement of the colloidal particles, the sign of the charge carried by the particles can be determined.

(j)     When electrophoresis of a sol is carried out with out stirring, the bottom layer gradually becomes more concentrated while the top layer which contain pure and concentrated colloidal solution may be decanted. This is called electro decanation and is used for the purification as well as for concentrating the sol.   

(k)    The reverse of electrophoresis is called Sedimentation potential or Dorn effect. The sedimentation potential is setup when a particle is forced to move in a resting liquid. This phenomenon was discovered by Dorn and is also called Dorn effect.

 

(ii)    Electrical double layer theory

(a)    The electrical properties of colloids can also be explained by electrical double layer theory. According to this theory a double layer of ions appear at the surface of solid.          

(b)    The ion preferentially adsorbed 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 consists of both the type of charges. The net charge on the 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, known as zeta potential or electrokinetic potential. Now when electric field is employed the particles move (electrophoresis)

 

(iii)   Electro-osmosis

(a)    In it the movement of the dispersed particles are prevented from moving by semipermeable membrane.

(b)    Electro-osmosis is a phenomenon in which dispersion medium is allowed to move under the 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, a potential difference is setup between the two sides called as streaming potential. So the reverse of electro-osmosis is called streaming potential.       

Note :

  • Distance traveled by colloidal particles in one second under a potential gradient of one volt per cm is called electrophoretic mobility of the colloidal particles.
  • The principle of electrophoresis is employed for the separation of proteins from nucleic acids, removing sludge from sewage waste etc.

 

Origin of the charge on colloidal particles.

The origin of the charge on the sol particles in most cases is due to the preferential adsorption of either positive or negative ions on their surface. The sol particles acquire electrical charge in any one or more of the following ways.

(1)     Due to the dissociation of the surface molecules : Some colloidal particles develope electrical charge due to the dissociation / ionisation of the surface molecules. The charge on the colloidal particles is balanced by the oppositely charged ions in the sol. For example, an aqueous solution of soap (sodium palmitate) which dissociates into ions as, \[\underset{\text{Sodium palmitate}}{\mathop{{{C}_{15}}{{H}_{31}}COONa}}\,\to {{C}_{15}}{{H}_{31}}CO{{O}^{-}}+N{{a}^{+}}  The cations (Na+) pass into the solution while the anions (C15H31COO) have a tendency to form aggregates due to weak attractive forces present in the hydrocarbon chains.

 

(2)     Due to frictional electrification

(i)     It is believed that the frictional electrification due to the rubbing of the dispersed phase particles with that of dispersion medium results in some charge on the colloidal particles.

(ii)    The dispersion medium must also get some charge, because of the friction. Since it does not carry any charge, the theory does not seem to be correct.

 

(3)     Due to selective adsorption of ions

(i)     The particles constituting the dispersed phase adsorb only those ions preferentially which are common with their own lattice ions.    

(ii)    For example, when a small quantity of silver nitrate (AgNO3) solution is added to a large quantity of potassium iodide (KI) solution, the colloidal particles of silver iodide adsorb I from the solution to become negatively charged, (at this stage is in excess, and Ibeing common to Agl)

\[\underset{\text{(Colloidal}\,\,\text{particle)}}{\mathop{AgI}}\,+\underset{\text{(in}\,\,\text{excess}\,\,\text{in}\,\,\text{the}\,\,\text{medium)}}{\mathop{{{I}^{-}}}}\,\xrightarrow{{}}\underset{\text{(Colloidal}\,\,\text{particle}\,\,\text{becomes}\,\,\text{positively}\,\,\text{charged)}}{\mathop{(AgI){{I}^{-}}}}\,

But, when a small quantity of potassium iodide (KI) solution is added to a large quantity of silver nitrate solution (AgNO3); the colloidal silver iodide particles adsorb Ag+ from the solution to become positively charged, (at this stage AgNO3 is in excess and Ag+ is common to Agl),    

\[\underset{\text{(Colloidal}\,\,\text{particle)}}{\mathop{AgI}}\,+\underset{\text{(in}\,\,\text{excess}\,\,\text{in}\,\,\text{the}\,\,\text{medium)}}{\mathop{A{{g}^{+}}}}\,\xrightarrow{{}}\underset{\text{(Colloidal}\,\,\text{particle}\,\,\text{becomes}\,\,\text{positively}\,\,\text{charged)}}{\mathop{(AgI)A{{g}^{+}}}}\,

(iii)   Similarly, the ferric hydroxide colloidal particles develop positive charge due to the adsorption of Fe3+ ions from the solution.

\[Fe{{(OH)}_{3}}+F{{e}^{3+}}\xrightarrow{{}}Fe{{(OH)}_{3}}.F{{e}^{3+}}                                     

Ferric hydroxide colloidal particles develop negative charge due to adsorption of either OH or Cl

\[\underset{(Colloidal\,\,particle)}{\mathop{Fe{{(OH)}_{3}}}}\,+\underset{(in\,\,excess)}{\mathop{O{{H}^{-}}}}\,\xrightarrow{{}}\underset{(Negatively\,\,ch\arg ed\,\,ferric\,\,hydroxide\,\,colloidal\,particle)}{\mathop{Fe{{(OH)}_{3}}.O{{H}^{-}}}}\,

\[\underset{(Colloidal\,\,particle)}{\mathop{Fe{{(OH)}_{3}}}}\,+\underset{(in\,\,excess)}{\mathop{C{{l}^{-}}}}\,\xrightarrow{{}}\underset{(Negatively\,\,ch\arg ed\,\,ferric\,\,hydroxide\,\,colloidal)}{\mathop{Fe{{(OH)}_{3}}.C{{l}^{-}}}}\,                

(iv)   Depending upon the nature of charge on the particles of the dispersed phase, the colloidal solutions are classified into positively charged and negatively charged colloids. Some typical examples are as follows

(a) Negatively charged colloids (b) Positively charged colloids
Metal sulphides : As2S3, CdS

Metal dispersions : Ag, Au, Pt

Acid dyes : Eosin, congo red

Sols of starch, gums, gold, gelatin etc.

Metal hydroxides :

Al(OH)3, Fe(OH)3

Metal oxide : TiO2

Basic dyes : Methylene blue

Haemoglobin

Sulphur sol

 

Note :

  • SnO2 forms positively charged colloidal sol in acidic medium and negative charged colloidal in basic medium this is due to SnO2is amphoteric reacting with acid and base both. In acidic medium (say HCl) Sn4+ ion is formed which is preferentially adsorbed on SnO2 giving positively charged colloidal sol.

\[Sn{{O}_{2}}+4HCl\to SnC{{l}_{4}}+2{{H}_{2}}O\,\,;\,\,\,Sn{{O}_{2}}+SnC{{l}_{4}}\to \underset{Positivity\,\,ch\arg ed}{\mathop{[Sn{{O}_{2}}]}}\,S{{n}^{4+}}\,\,:\,\,4C{{l}^{-}}

In basic medium, \[SnO_{3}^{2-} is formed which is preferentially adsorbed on SnO2 giving negatively charged colloidal sol.

\[2NaOH+Sn{{O}_{2}}\to N{{a}_{2}}Sn{{O}_{3}}+{{H}_{2}}O\,\,;\,\,\,\,\,\,(Negatively\,\,ch\arg ed)

\[Sn{{O}_{2}}+N{{a}_{2}}Sn{{O}_{3}}\to [Sn{{O}_{2}}]\,SnO_{3}^{2-}\,\,;\,\,2N{{a}^{+}}

 

Stability of sols.

Sols are thermodynamically unstable and the dispersed phase (colloidal particles) tend to separate out on long standing due to the Vander Waal’s attractive forces. However sols tend to exhibit some stability due to

(1)     Stronger repulsive forces between the similarly charged particles : All colloidal particles in any sol possess similar charge. Therefore, due to the electrostatic repulsion these are not able to come closer and form aggregates. Thus stronger repulsive forces between the similarly charged particles in a sol promote its stability.

 

(2)     Particle-solvent interactions

(i)     Due to strong particle-solvent (dispersion medium) interactions, the colloidal particles get strongly solvated.

(ii)    Due to solvation, the effective distance between the colloidal particles increases, and therefore, the Vander Waal’s force of attraction decreases. As a result, the particles are not able to form aggregates.

(iii)   Lyophilic sols are mainly stabilized by solvation effects due to strong interactions between the sol particles and the dispersion medium.      

 

Coagulation or Flocculation or Precipitation.

The phenomenon of the precipitation of a colloidal solution by the addition of the excess of an electrolyte is called coagulation or flocculation.

or

The stability of the lyophobic sol is due to the presence of charge on colloidal particles. If the charge is removed, the particles will come nearer to each other and thus, aggregate or flocculate and settle down under the force of gravity. This phenomena is known as coagulation or flocculation.”

The coagulation of the lyophobic sols can be carried out by following methods.

(1)     By electrophoresis : In electrophoresis the colloidal particles move towards oppositely charged electrode. When these come in contact with the electrode for long these are discharged and precipitated.

(2)     By mixing two oppositely charged sols : When oppositely charged sols are mixed in almost equal proportions, their charges are neutralised. Both sols may be partially or completely precipitated as the mixing of ferric hydroxide (+ve sol) and arsenious sulphide (–ve sol) bring them in precipitated form. This type of coagulation is called mutual coagulation or meteral coagulation.

(3)     By boiling : When a sol is boiled, the adsorbed layer is disturbed due to increased collisions with the molecules of dispersion medium. This reduces the charge on the particles and ultimately they settle down to form a precipitate.

(4)     By persistent dialysis : On prolonged dialysis, the traces of the electrolyte present in the sol are removed almost completely and the colloids become unstable.           

(5)     By addition of electrolytes : The particles of the dispersed phase i.e., colloids bear some charge. When an electrolyte is added to sol, the colloidal particles take up ions carrying opposite charge from the electrolyte. As a result, their charge gets neutralised and this causes the uncharged, particles to come closer and to get coagulated or precipitated. For example, if BaCl2 solution is added to As2S3 sol the Ba2+ ions are attracted by the negatively charged sol particles and their charge gets neutralised. This lead to coagulation.        

(6)     Hardy schulze rule : The coagulation capacity of different electrolytes is different. It depends upon the valency of the active ion are called flocculating ion, which is the ion carrying charge opposite to the charge on the colloidal particles. “According to Hardy Schulze rule, greater the valency of the active ion or flocculating ion, greater will be its coagulating power” thus, Hardy Schulze law state:      

(i)     The ions carrying the charge opposite to that of sol particles are effective in causing coagulation of the sol.

(ii)    Coagulating power of an electrolyte is directly proportional to the valency of the active ions (ions causing coagulation).

For example to coagulate negative sol of As2S3, the coagulation power of different cations has been found to decrease in the order as, 

Al3+ > Mg2+ > Na+                

Similarly, to coagulate a positive sol such as Fe(OH)3, the coagulating power of different anions has been found to decrease in the order :

\[{{[Fe{{(CN)}_{6}}]}^{4-}}>PO_{4}^{3-}>SO_{4}^{2-}>C{{l}^{-}}                                      

(7)     Coagulation or flocculation value

“The minimum concentration of an electrolyte which is required to cause the coagulation or flocculation of a sol is known as flocculation value.”

or

“The number of millimoles of an electrolyte required to bring about the coagulation of one litre of a colloidal solution is called its flocculation value.”

Thus, a more efficient flocculating agent shall have lower flocculating value.

 

Flocculation values of some electrolytes

Sol Electrolyte Flocculation value (mM) Sol Electrolyte Flocculation value (mM)
As2S3

(-vely charged )

NaCl 51.0 Fe(OH)3

(+ vely charged)

KCl 9.5
KCl 49.5 BaCl2 9.3
CaCl2 0.65 K2SO4 0.20
MgCl2 0.72 MgSO4 0.22
MgSO4 0.81
AlCl3 0.093
Al2(SO4)3 0.096
Al(NO3)3 0.095  

 

Note :      Coagulating value or flocculating value . \[\propto \frac{1}{\text{coagulating}\,\,\,\text{power}}

 

(8)     Coagulation of lyophilic sols

(i)     There are two factors which are responsible for the stability of lyophilic sols.

(ii)    These factors are the charge and solvation of the colloidal particles.

(iii)   When these two factors are removed, a lyophilic sol can be coagulated.

(iv)   This is done (i) by adding electrolyte (ii) and by adding suitable solvent.

(v)    When solvent such as alcohol and acetone are added to hydrophilic sols the dehydration of dispersed phase occurs. Under this condition a small quantity of electrolyte can bring about coagulation.

Note :     Hydrophilic sols show greater stability than hydrophobic sols.

 

Protection of colloids and Gold number.

(1)     Lyophilic sols are more stable than lyophobic sols.

(2)     Lyophobic sols can be easily coagulated by the addition of small quantity of an electrolyte.

(3)     When a lyophilic sol is added to any lyophobic sol, it becomes less sensitive towards electrolytes. Thus, lyophilic colloids can prevent the coagulation of any lyophobic sol.

“The phenomenon of preventing the coagulation of a lyophobic sol due to the addition of some lyophilic colloid is called sol protection or protection of colloids.”    

(4)     The protecting power of different protective (lyophilic) colloids is different. The efficiency of any protective colloid is expressed in terms of gold number.

Gold number : Zsigmondy introduced a term called gold number to describe the protective power of different colloids. This is defined as, “weight of the dried protective agent in milligrams, which when added to 10 ml of a standard gold sol (0.0053 to 0.0058%) is just sufficient to prevent a colour change from red to blue on the addition of 1 ml of 10 % sodium chloride solution, is equal to the gold number of that protective colloid.”

Thus, smaller is the gold number, higher is the protective action of the protective agent.

\[\text{Protective}\,\,\text{power}\,\,\propto \,\,\frac{\text{1}}{\text{Gold}\,\,\text{number}}

 

Gold numbers of some hydrophilic substances

Hydrophilic substance Gold number Hydrophilic substance Gold number
Gelatin 0.005 – 0.01 Sodium oleate 0.4 – 1.0
Sodium caseinate 0.01 Gum tragacanth 2
Hamoglobin 0.03 – 0.07 Potato starch 25
Gum arabic 0.15 – 0.25

 

The protective colloids play very significant role in stabilisation of the non–aqueous dispersions, such as paints, printing inks etc.

(5)     Congo rubin number : Ostwald introduced congo rubin number to account for protective nature of colloids. It is defined as “the amount of protective colloid in milligrams which prevents colour change in 100 ml of 0.01 % congo rubin dye to which 0.16 g equivalent of KCl is added.”

(6)     Mechanism of sol protection 

(i)     The actual mechanism of sol protection is very complex. However it may be due to the adsorption of the protective colloid on the lyophobic sol particles, followed by its solvation. Thus it stabilises the sol via solvation effects.

(ii)    Solvation effects contribute much towards the stability of lyophilic systems. For example, gelatin has a sufficiently strong affinity for water. It is only because of the solvation effects that even the addition of electrolytes in small amounts does not cause any flocculation of hydrophilic sols. However at higher concentration, precipitation occurs. This phenomenon is called salting out.

(iii)   The salting out efficiency of an electrolyte depends upon the tendency of its constituents ions to get hydrated i.e, the tendency to squeeze out water initially fied up with the colloidal particle.

(iv)   The cations and the anions can be arranged in the decreasing order of the salting out power, such an arrangement is called lyotropic series.

Cations : Mg2+ > Ca2+ > Sr2+ > Ba2+ > Li+ > Na+ > K+ > NH4+ > Rb+ > Cs+

Anions : Citrate3+ > SO42− > Cl  > NH3 > I > CNS

Ammonium sulphate, due to its very high solubility in water, is oftenly used for precipitating proteins from aqueous solutions.

(v)    The precipitation of lyophilic colloids can also be affected by the addition of organic solvents of non-electrolytes. For example, the addition of acetone or alcohol to aqueous gelatin solution causes precipitation of gelatin. Addition of petroleum ether to a solution of rubber in benzene causes the precipitation of rubber.  

 

Emulsion.

“The colloidal systems in which fine droplets of one liquid are dispersed in another liquid are called emulsions the two liquids otherwise being mutually immiscible.”  

or

“Emulsion are the colloidal solutions in which both the dispersed phase and the dispersion medium are liquids.”

A good example of an emulsion is milk in which fat globules are dispersed in water. The size of the emulsified globules is generally of the order of 10−6 m. Emulsion resemble lyophobic sols in some properties.

(1)     Types of Emulsion : Depending upon the nature of the dispersed phase, the emulsions are classified as;

(i)     Oil-in-water emulsions (O/W) : The emulsion in which oil is present as the dispersed phase and water as the dispersion medium (continuous phase) is called an oil-in-water emulsion. Milk is an example of the oil-in-water type of emulsion. In milk liquid fat globules are dispersed in water. Other examples are, vanishing cream etc.

(ii)    Water-in-oil emulsion (W/O) : The emulsion in which water forms  the dispersed phase, and the oil acts as the dispersion medium is called a water-in-oil emulsion. These emulsion are also termed oil emulsions. Butter and cold cream are typical examples of this types of emulsions. Other examples are cod liver oil etc.

Note :     The emulsion can be inter converted by simply changing the ratio of the dispersed phase and dispersion medium. For example, an oil-in-water emulsion can be converted to water in oil emulsion by simply adding excess of oil in the first case.

 

(2)     Preparation of Emulsions

(i)     Emulsions are generally prepared by vigorously agitating a mixture of the relevant oil and water by using either a high speed mixer or by using ultrasonic vibrators.

(ii)    The emulsions obtained by simple mechanical stirring are unstable. The two components (oil and water) tend to separate out.

(iii)   To obtain a stable emulsion, a suitable stabilizing substance is generally added.

(iv)   The stabilizing substance is called emulsifier of emulsifying agent. The emulsifier is added along with the oil and water in the beginning. For Examples : substances which can act as emulsifiers are soaps, detergents, long chain sulphonic acid, lyophilic colloids like gelatin, albumin, casein etc.

 

(3)     Nature of emulsifier : Different emulsifiers may act differently in the case of a particular emulsion.

For example,

(a)    Sodium oleate is used to prepare oil-in-water (O/W) emulsions.

(b)    Magnesium and calcium oleates are used to prepare water-in-oil (W/O) emulsions. When calcium oleate is added to an emulsion stabilized by sodium oleate, the stability of the system decreases. At a certain ratio of Na+ : Ca2+, the oil-in-water emulsion becomes unstable. If the Ca2+ ions concentration is increased further very quickly, then the reversal of the emulsion type occurs, that is the oil-in-water emulsion gets converted into a water-in-oil type.

 

(4)     Identification of emulsions : Several methods are available to find out whether an emulsion is of the oil-in-water type or of the water-in-oil type emulsion. An emulsion can be identified as follows.

(i)     Dilute test : Add water to the emulsion. If the emulsion can be diluted with water this means that water acts as the dispersion medium and it is an example of oil-in-water emulsion. In case it is not diluted, then oil acts as dispersion medium and it is an example of water-in-oil emulsion.

(ii)    Dye test : An oil soluble suitable dye is shaken with the emulsion. If colour is noticed on looking at a drop of the emulsion, it is oil-in-water type emulsion. In case the entire background is coloured, it is an example of water-in-oil type.

(iii)   Conductivity test : Add small amount of an electrolyte (e.g. KCl) to the emulsion. If this makes the emulsion electrically conducting, then water is the dispersion medium. If water is not the dispersed phase.

 

(5)     Properties of emulsion

(i)     Emulsions show all the characteristic properties of colloidal solution such as Brownian movement, Tyndall effect, electrophoresis etc.

(ii)    These are coagulated by the addition of electrolytes containing polyvalent metal ions indicating the negative charge on the globules.

(iii)   The size of the dispersed particles in emulsions in larger than those in the sols. It ranges from 1000 Å to 10,000 Å. However, the size is smaller than the particles in suspensioins.

(iv)   Emulsions can be converted into two separate liquids by heating, centrifuging, freezing etc. This process is also known as demulsification.

 

(6)     Applications of emulsions

(i)     Concentration of ores in metallurgy

(ii)    In medicine (Emulsion water-in-oil type)    

(iii)   Cleansing action of soaps.

(iv)   Milk, which is an important constituent of our diet an emulsion of fat in water.

(v)    Digestion of fats in intestine is through emulsification.

 

Gels.

(1)     “A gel is a colloidal system in which a liquid is dispersed in a solid.”

(2)     The lyophilic sols may be coagulated to give a semisolid jelly like mass, which encloses all the liquid present in the sol. The process of gel formation is called gelation and the colloidal system formed called gel.

(3)     Some gels are known to liquify on shaking and reset on being allowed to stand. This reversible sol-gel transformation is called thixotropy.

(4)     The common examples of gel are gum arabic, gelatin, processed cheese, silicic acid, ferric hydroxide etc.

(5)     Gels may shrink by loosing some liquid help them. This is known as synereises or weeping.

(6)     Gels may be classified into two types

(i)     Elastic gels : These are the gels which possess the property of elasticity. They readily change their shape on applying force and return to original shape when the applied force is removed. Common examples are gelatin, agar-agar, starch etc.

(ii)     Non-elastic gels : These are the gels which are rigid and do not have the property of elasticity. For example, silica gel.

 

 Application of colloids.

(1)     Purification of water by alum (coagulation): Alum which yield Al3+ ions, is added to water to coagulate the negatively charged clay particles.

(2)     In rubber and tanning industry (coagulation and mutual coagulation) : Several industrial processes such as rubber plating, chrome tanning, dyeing, lubrication etc. are of colloidal nature

(i)     In rubber platting, the negatively charged particles of rubber (latex) are made to deposit on the wires or handle of various tools by means of electrophoresis. The article on which rubber is to be deposited is made anode.

(ii)    In tanning the positively charged colloidal particles of hides and leather are coagulated by impregnating, them in negatively charged tanning materials (present in the barks of trees). Among the tanning agent chromium salts are most commonly used for the coagulation of the hide material and the process is called chrome tanning.

 

(3)     Artificial rains : It is possible to cause artificial rain by throwing the electrified sand or silver iodide from an aeroplane and thus coagulating the mist hanging in air.

 

(4)     Smoke precipitation (Coagulation) : Smoke is a negative sol consisting of carbon particles dispersed in air. Thus, these particles are removed by passing through a chamber provided with highly positively charged metallic knob.

 

(5)     Formation of deltas (coagulation) : River water consists of negatively charged clay particles of colloidal dimension. When the river falls into the sea, the clay particles are coagulated by the positive Na+, K+, Mg2+ ions etc. present in sea water and new lands called deltas are formed.

 

(6)     Blood consists of negatively charged colloidal particles (albuminoid substance). The colloidal nature of blood explains why bleeding stops by applying a ferric chloride solution to the wound. Actually ferric chloride solution causes coagulation of blood to form a clot which stops further bleeding.

 

(7)     Colloidal medicine : Argyrol and protargyrol are colloidal solution of silver and are used as eye lotions colloidal sulphur is used as disinfectant colloidal gold, calcium and iron are used as tonics.

 

(8)     Photographic plates : These are thin glass plates coated with gelatin containing a fine suspension of silver bromide. The particles of silver bromide are colloidal in nature. 

Note : 

  • Isoelectric point of the colloid : The hydrogen ion concentration at which the colloidal particles are neither positively charged nor negatively charged (neutral) is known as isoelectric point of the colloids. At this point, the lyophilic colloids are expected to have minimum stability because at this point particles have no charge or equal quantum of positively and negatively charge. For example, isoelectric point of gelatin is 4.7 (at pH 4.7 gelatin has no electrophoretic motion; at pH < 4.7, gelatin moves towards anode)                                 
  • Colloidal solution of graphite is called Aqua dug.
  • Ultrasonic dispersion : Various substances such as mercury, oils, sulphur, sulphides and oxide of metals can be dispersed into colloidal state very easily with the help of ultrasonic waves.
  • Stem technology: Colloidal particles in a sol are very small and most of them are not visible through an ultramicroscope or light microscope. Recently, new techniques have been developed to determine the size and shape of the colloidal particles. These are

(i)     Scanning Electron Microscope (SEM), (ii) Transmission Electron Microscope (TEM)

(ii)    A modified form of the above methods has also been developed. It is called Scanning Transmission Electron Microscope (STEM). All these techniques are superior to the light microscope because they have greater resolving power.

  • Bencroft rule : The phase in which the emulsifier is more soluble becomes outer phase of the emulsion.