Surface Chemistry, Class 12, Chemistry Detailed Chapter Notes PDF Download

Classification and Phases of Colloids - Surface Chemistry, CBSE, Class 12, Chemistry

SURFACE CHEMISTRY

Surface chemistry deals with phenomena that occur at the surfaces or interfaces. Many important phenomena, noticeable amongst these being corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallisation occur at interfaces. The subject of surface chemistry finds many applications in industry, analytical work and daily life situations.

Particle size :

Phase of colloids :

A colloidal system is heterogeneous in character. It consists of two phases, namely a dispersed phase and a dispersion medium.

(a) Dispersed Phase (DP) : It is the component present in small proportion and is just like a solute in a true solution. For example, in the colloidal state of sulphur in water, the formeracts as a dispersed phase.

(b) Dispersion Medium (DM) : It is normally the component present in excess and is just like a solvent in a solution.

The particles of the dispersed phase are scattered in the dispersion medium in a colloidal system.

Classification of colloids :

Colloids can be can be classified in a number of ways based upon some of their important characteristics.

(1) Physical state of Dispersed Phase & Dispersion Medium:

Depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gaseous, eight types of colloidal system are possible. A gas mixed with another gas forms a homogeneous mixture and not a colloidal system. Typical examples of various type alongwith their characteristic names are given in table.

                                                                                      Common Colloidal System

(2) Based on dispersion medium

1. Water :        Hydrosols

2. Alcohol :     Alcosols

3. Gases :       Aerosols

4. benzene :   benzosol

5. solid :            gel

Some colloids, such as gelatin, can behave both as a sol and a gel. At high temperature and low concentration of gelatin, the colloid is a hydrosol. But at low temperature and high gelatin concentration, the hydrosol can change into a gel.

(3) Based on interaction or affinity of phases : On the basis of the affinity or interaction between the dispersed phase and the dispersion medium, the colloids may be classified into two types :

(i) Lyophilic Colloids : The colloidal system in which the particle of dispersed phase have great affinity for the dispersion medium, are called lyophilic (solvent-loving) colloids. In such colloids, the dispersed phase does not get easily precipitated and the sols are more stable. Such colloidal systems, even if precipitated, may be reconverted to the colloidal state by simply agitating them with the dispersion medium. Hence lyophilic colloids are reversible. When the dispersion medium is water, these are called hydrophilic colloids. Some common examples of lyophilic colloids are gum, gelatin, starch, rubber, proteins, etc.

(ii) Lyophobic colloids : The colloidal system in which the dispersed phase have no affinity for the dispersion medium are called lyophobic (solvent hating) colloids. They are easily precipitated (or coagulated) on the addition of small amounts of the electrolyte, by heating or by shaking. They are less stable and irreversible. When the dispersion medium is water, these are known as hydrophobic colloids. Examples of lyophobic colloids include sols of metals and their insoluble compounds like sulphides and oxides.

The essential differences between the lyophilic and lyophobic colloids are summarised in table.

                                                                                         Difference between Lyophilic and Lyophobic sols

(4) Based on type of particles of the dispersed phase : Depending upon the molecular size, the colloidal system has been classified into three classes :

(i) Multimolecular colloids : The multimolecular colloidal particles consists of aggregate of atoms of small molecules with diameters less than 10^-9 m or 1 nm.

For example, a sol. of gold contains particles of various sizes having several atoms. A sol. of sulphur consists of particles containing a thousand or so S₂ molecules. These particles are held together by vander Waal's forces. These are usually lyophobic sols for example gold sol.

(ii) Macromolecular colloids : The macromolecular colloidal particles themselves are large molecules. They have very high molecular weights varrying from thousands to millions. These substances are generally polymers. Naturally occurring macromolecules are such as starch, cellulose and proteins. Artificial macromolecules are such as polyethylene, nylon, polysyrene, dacron, synthetic rubber, plastics, etc. The size of these molecules are comparable to those of colloidal particles and therefore, their dispersion known as macromolecular colloids. Their dispersion also resemble true solutions in some respects. For example - Starch, cellulose, proteins and enzymes.

(iii) The associated colloids or miscelles : These colloids behave as normal electrolytes at low concentrations but colloids at higher concentrations. This is because at higher concentrations, they form aggregated (associated) particles called miscelles. Soap and synthetic detergents are examples of associated colloids. They furnish ions which may have colloidal dimensions.

RCOONa                                             RCOO^—  Na

Sod. Stearate soap                           (R = C₁₇H₃₅)

The long-chain RCOO^— ions associates or aggregate at higher concentrations and form miscelles and behave as colloids. They may contain 100 or more molecules.

Sodium stearate C₁₇H₃₅COONa is an example of an associated colloid. In water it gives Na  and sterate, C₁₇H₃₅COO^— ions. These ions associate to form miscelles of colloidal size.

Colloids which behave as normal electrolytes at low concentration, but exhibit colloidal properties at higher concentration due to the formation of aggregated particles called micelles are referred to as associated colloids. The micelles are formed by the association of dispersed particles above a certain concentration and certain minimum concentration is required for the process of aggregation to take place. The minimum concentration required for micelle formation is called micellisation concentration (CMC) and its value depends upon the nature of the dispersed phase. For soaps CMC is 10^-3 - 10^-4 M.

Mechanism of Micelle Formation :

Micelles are formed by surface active molecules called surfactants such as soaps and detergents. These molecules have lyophilic group at one end and a lyphobic group at the other end. Let us take the example of a soap (say sodium oleate, C₁₇H₃₃COO^—Na  ). The long hydrocarbon part of oleate radical (C₁₇H₃₃ -) is lyophobic end while COO^— part is lyophilic end. When the concentration of the solution is below its CMC, sodium oleate behaves as a normal electrolyte and ionises to give C₁₇H₃₃COO^— and Na  ions. When the concentration exceeds CMC, the lyophobic part starts receding away from the solvent and tends to approach each other. However, the polar COO^— ends tends to interact with the solvent (water). This finally leads to the formation of bigger molecules having the dimensions of colloidal particles. Thus 100 or more oleate ions are grouped together in a spherical way keeping their hydrocarbon parts inside and the -COO^— part remains projected in water.

List of surfactants and their critical micelle concentration (CMC)

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Methods of Preparation of Colloids - Surface Chemistry, CBSE, Class 12, Chemistry

Preparation of colloidal solutions

(1) Preparation of lyophilic sols : The colloidal solutions of lyophilic colloids like starch, glue, gelatin etc., can be readily prepared by dissolving these substances in water either in clod or on warming. Solutions of colloidal electrolytes such as soaps and dye stuffs can also be prepared similarly.

(2) Preparation of lyophobic sols : To get a substance in colloidal form either the substance in bulk is broken down into fine particles of colloidal dimension (1Å to 10³ Å ) or increasing the size of molecular particles as to form larger aggregates. In some cases, a third substance as to form larger aggregates. In some cases, a third substance is usually added to increase the stability of the sol. These substances are called stabilizers. Thus, there are two ways by which the lyophobic sols can be prepared :

(i) Dispersion methods : By splitting coarse aggregates of a substance into colloidal size.

(ii) Condensation methods : By aggregating very small particles (atoms, ions or molecules) into colloidal size.

S. No.     Dispersion methods                                                   Condensation methods

1.Mechanical dispersion                                                           1. Exchange of solvents

2. Electro-dispersion                                                                  2. Change of physical state

3. Ultrasonic dispersion                                                             3. Chemical methods :

4. Peptisation                                                                              (i) Double decomposition

                                                                                                       (ii) Oxidation

                                                                                                       (iii) Reduction

                                                                                                       (iv) Hydrolysis

Dispersion Methods

(1) Mechanical dispersion : Solid material is first finely ground by usual methods. It is then mixed with dispersion medium which gives a coarse suspension. The suspension is now introduced into the colloid mill. The simplest form of colloid mill consists of two metal discs held at a small distance apart from one another and capable of revolving at a very high speed (about 7000 revolutions per minute) in opposite directions. The particles are ground down to colloidal size and are then dispersed in the liquid . A stabilizer is often added to stabilize the colloidal solution. Colloidal graphite (a lubricant) and printing ink                                                                                                                      

are made by this method. Tannin is used as a stabilizer in the preparation of colloidal graphite and gum arabic in lampblack colloidal solution (Indian ink).

(2) Electro-dispersion (Bredig's arc method) : This method is suitable for the preparation of colloidal solutions of metals like gold, silver, platinum, etc. An arc is struck between the metal electrodes under the surface of water containing some stabilizing agents such as a track of KOH. The water is cooled by immersing the

container in an ice bath. The intense heat of the arc vaporises some of the metal which condenses under cold water.

Note : (1) This method is not suitable when the dispersion medium is an organic liquid as considerable charring occurs.

(2) This method comprises both dispersion and condensation.

(3) Ultrasonic dispersion : The sound waves of high frequency are usually called ultrasonic waves. These waves can be produced when quartz crystal discs are connected with a high frequency generator. The application of ultrasonic waves for the preparation of colloidal solutions was first introduced by wood and Loomis, in 1927. Various substances like oils, mercury, sulphur, sulphides and oxides of metals can be dispersed into colloidal state very easily with the help of ultrasonic waves.

(4) Peptization : The dispersion of a freshly precipitated material into colloidal solution by the action of an electrolyte in solution is termed peptization. The electrolyte used is called a peptizing agent.

A few examples of sols obtained by peptization are :

(i) Freshly prepared ferric hydroxide on treatment with a small amount of ferric chloride solution at once forms a dark reddish brown solution. Ferric chloride acts as a peptizing agent.

(ii) Freshly prepared stannic oxide on treatment with a small amount of dilute hydrochloric acid forms a stable colloidal solution of stannic oxide.

(iii) Freshly precipitated silver chloride can be converted into a colloidal solution by a small amount of hydrochloric acid.

(iv) Cadmium sulphide can be peptized with the help of hydrogen sulphide.

The process of peptization thus involves the adsorption of suitable ions (supplied by the electrolyte added -- particularly a common ion) and electrically charged particles then split from the precipitate as colloidal particles.

Important peptizing agents : Sugar, Gum, Gelatin & Electrolytes.

Example

Freshly prepared ferric hydroxide can be converted into colloidal state by shaking it with water containing Fe ³ or OH¯ or FeCl₃

                                                                           Fe(OH)₃  xFe ³ ¾ → Fe (OH)₃.xFe ³

Precipitate Peptizing agent colloid

Condensation Methods

(1) By exchange of solvents : If a solution of sulphur or phosphorus prepared in alcohol is poured into water, a colloidal solution of sulphur or phosphorus is obtained due to low solubility of sulphur or phosphorus is obtained due to low solubility in water. Thus, there are a number of substances whose colloidal solutions can be prepared by taking a solution of the substance in one solvent and pouring it into another solvent in which the substance is relatively less soluble.

(2) By change of physical state : Colloidal solutions of certain elements such as mercury and sulphur are obtained by passing their vapour through cold water containing a stabilizer (an ammonium salt or a citrate)

(3) Chemical methods : The chemical methods involve chemical reactions in a medium in which the dispersed phase is sparingly soluble. A condition of supersaturation is produced but the actual precipitation is avoided. Some familiar reactions used are :

(a) Double decomposition : (i) Arsenious sulphide sol : A 1% solution of arsenious oxide is prepared is hot water. The solution is cooled, filtered and is then gradually in hot water saturated with hydrogen sulphide. This is continued till an intense yellow-coloured solution is obtained. Excess of H₂S is removed by bubbling hydrogen through the solution.

                                                                                         As₂O₃  3H₂S  As₂S₃  3H₂O

                                                                                                                            Yellow sol

(ii) Antimony sulphide sol : A 0.5% solution of potassium antimonyl tartarate is added drop by drop to water saturated with H₂S, whilst H2S is being passed through the solution. Orange coloured solution of antimony sulphide is obtained.

(b) Oxidation : A colloidal solution of sulphur is obtained by passing H₂O into a solution of sulphur dioxide.

                                                                           2H2O  

Sulphur sol can also be obtained when H₂S is bubbled through an oxidising agent (bromine water or nitric acid).

(c) Reduction : Colloidal solutions of metals like gold, silver, platinum, lead, etc., can be obtained when their salts solutions are acted upon by reducing agents.

                                                                          2AuCl₃  3SnCl₂  3SnCl₄   

Organic reducing agents such as formaldehyde, phenyl hydrazine, tannic acid, etc., can also be used.

                                                   AgNO₃  tannic acid  Silver sol

                                                   AuCl₃  tannic acid  Gold sol

(d) Hydrolysis : Colloidal solutions of some salts can be prepared by hydrolysis. A colloidal solution of ferric hydroxide is obtained by boiling a dilute solution of ferric chloride.]

                                                          FeCl₃ 3H2O  

The colloidal solution of silicic acid is also obtained by hydrolysis of dilute solution of sodium silicate with 4N hydrochloric acid which is added drop by drop with constant stirring.

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Purification of Colloidal Solutions - Surface Chemistry, CBSE, Class 12, Chemistry

PURIFICATION OF COLLOIDAL SOLUTIONS

Colloidal solutions prepared by above methods generally contain excessive amount of electrolytes and some other soluble impurities. The presence of traces of electrolyte is essential for the stability of the colloidal solution but larger quantities coagulate it. It is, therefore, necessary to reduce the concentration of these soluble impurities to a requisite minimum. The process used for reducing of these impurities to a requisite minimum is known as purification of colloidal solution. The purification of colloidal solution is carried out by the following methods.

(i) Dialysis* : It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through suitable membrane. Since, particles in true solution (ions or smaller molecules) can pass through animal membranes (bladder) or parchment paper or cellophane sheet but colloidal particles do not, the above can be used for dialysis. The apparatus used for this purpose is called dialyser. A bag of suitable membrane containing the colloidal solution is suspended in a vessel through which fresh water is continuously flown figure. The molecules and ions diffuse through the membrane into the outer water and pure colloidal solution is left behind.

(ii) Electro-dialysis : Ordinarily, the process of dialysis is quite slow. It can be made faster by applying an electric field if the dissolved substance in the impure colloidal solution is only electrolyte. The process is then named electro-dialysis. The colloidal solution is placed between two electrodes while pure water is taken in a compartment on each side. Electrodes are fitted in the compartment as shown in figure the ions present in the colloidal solution migrate out to the oppositely charged electrodes.

(iii) Ultrafiltration : Ultrafiltration is the process of separating the colloidal particles from the solvent and soluble solutes present in the colloidal solution by especially prepared filters, which are permeable to all substances except the colloidal particles.

Colloidal particles can pass through ordinary filter paper because the pores are too large. However, the pores of filter paper can be reduced in size by impregnating with collodion solution and subsequently hardened by soaking in formaldehyde. The usual colloidion is a 4% solution of nitro-cellulose in a mixture of alcohol and ether. An ultrafilter paper may be prepared by soaking the filter paper in a colloidion solution and hardened by formaldehyde and finally drying it. Thus, by using ultrafilter paper, the colloidal particles are separated from rest of the materials. Ultrafiltration is a slow process. To speed up the process, pressure or suction is used.

The colloidal particles left on the ultrafilter paper are then stirred with fresh dispersion medium (solvent) to get a pure colloidal solution.

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Properties of Colloidal Solutions - Surface Chemistry, CBSE, Class 12, Chemistry

Properties of Colloidal Solutions :

(1) Physical properties :

(i) Heterogeneity : Colloidal solutions are heterogeneous in nature consisting of two phases viz, the dispersed phase and the dispersion medium. Experiments like dialysis and ultra filteration clearly indicate the heterogeneous character of colloidal system. Recent investigations however, shown that colloidal solutions are neither obviously homogeneous nor obviously heterogeneous.

(ii) Filterability : Colloidal particles readily pass through orginary filter papers. It is because the size of the pores of the filter paper is larger than that of the colloidal particles.

(iii) Non-settling nature : Colloidal solutions are quite stable as the colloidal particles remain suspended in the dispersion medium indefinitely. Thus there is no effect of gravity on the colloidal particles.

(iv) Colour : The colour of the colloidal solution is not always the same as the colour of the substances in the bulk. The colour of the colloidal solution depends upon the following factors :

(a) Size and shape of colloidal particles.

(b) Wavelength of the source of light.

(c) Method of preparation of the colloidal solution.

(d) Nature of the colloidal solution.

(e) The way an observer receives the light, i.e., whether by reflection or by transmission.

(f) Stability : Colloidal solutions are quite stable. Only a few solutions of larger particles may settle but very slowly.

Examples :

(i) Finest gold is red in colour. As the size of particles increases, it becomes purple.

(ii) Dilute milk gives a bluish tinge in reflected light whereas reddish tinge in transmitted light.

(2) Mechanical Properties :

(a) Brownian movement : Colloids particles exhibit a ceaseless random and swarming motion. This kinetic activity of particles suspended in the liquid is called Brownina movement.

Robert Brown first observed this motion with pollen grains suspended in water.

Cause of movement : Brownian movement is due to bombardment of the dispersed particles by molecules of the medium. The Brownian movement (figure) depends upon the size of sol. particles. With the increase in the size of the particle, the chance of unequal bombardment decrease, and the Brownial movement too disappears. It is due to the fact that the suspension fails to exhibit this phenomenon.

It should be noted that Brownian movement does not change with time but changes with temperatures.

Importance :

(i) Brownian movement is a direct demonstration of the assumption that the molecules in a gas or solution are in a state of constant ceaseless motion. Thus it confirms kinetic theory.

(ii) Brownian movement does not allow the colloidal particles to settle down due to gravity and thus is responsible for their stability.

(iii) Brownian movement helps to calculate the Avogadro's number (Detail beyond the scope of the book).

(b) Sedimentation : Heavier sol. particle tend to settle down very slowly under the influence of gravity. This phenomenon is called sedimentation.

(3) Optical Properties (Tyndal Effect) :

When a strong and converging beam of light is passed through a colloidal solution, its path becomes visible (bluish light) when viewed at right angles to the beam of light (figure). This effect is called Tyndall effect. The light is observed as a bluish cone which is called Tyndall cone.

The Tyndall effect is due to scattering of light by the colloidal particles. The scattering of light cannot be due to simple reflection, because the size of the particles is smaller than the wave, length of the visible light and they are unable to reflect light waves. In fact, colloidal particles first absorb light and then a part of the absorbed light is scattered from the surface of the colloidal particles as a light of shorter wavelength. Since maximum scattering of light takes place at right angles to the place of incident light, it becomes visible when seen from that direction.

The Tyndall effect is observed under the following conditions :

(i) The diameter of the dispersed particles must not be much smaller than the wavelength of light employed.

(ii) The refractive indices of the dispersed phase and the dispersion medium must differ widely. This condition is fulfilled by lyophobic colloids.

It is important to note that Tyndall effect is not shown by true solutions as their particles are too small to cause scattering. Tyndall effect has been used in devising ultramicroscope and in determining the number of colloidal particles in a colloidal solution.

(4) Electrical Properties :

Origin of charge : Various reasons have been given regarding the original of charge on the colloidal particles. These are given below :

(i) Frictional electrification : It is believed to be frictional due to the rubbing of the dispersed phase particles with medium molecules.

(ii) Dissociation of the surface molecules : It leads to electric charge on colloidal particles. For example, an aqueous solution of a soap (sodium palmitate) dissociates into ions.

                          C₁₅H₃₁COONa  →   C₁₅H₃₁COO^—  Na

                          sod. palmitate

The Na  ions pass into the solution while C₁₅H₃₁COO^— ions have a tendency to form aggregates due to weak attractive forces present in the hydrocarbon chains. Thus, the anions which are of colloidal size bear negative charge.

(iii) Preferential adsorption of ions from solution : The charge on the colloidal particles is generally acquired by preferentially adsorbing positive or negative ions from the electrolyte. Thus AgCl particles can adsorb Cl^— ions from chloride solutions and Ag  ions from excess Ag  ions solutions; the sol. will be negatively charged in the first case and positively charged in the second case.

(iv) Capture of electron : It is from air during preparation of sol. by Bredig's arc method.

(v) Dissociation of molecular electrolytes on the surface of particles : H₂S molecules get adsorbed on sulphides during precipitation. By dissociation of H₂S, H  ions are lost and colloidal particles become negatively charged.

Electrical charged sols.

Positively charged sols                                                          Negatively charged sols

1. Ferric hydroxide, aluminium hydroxide                             Metals such as Pt, Au, Ag, Metals sulphides,

                                                                                                        e.g. arsenius sulphide.

2. Basic dyes such as methylene blue                                   Starch, clay, silicic acid.

3. Haemoglobin                                                                           Acid dyes, such as eosin.

The two electrical properties of colloidal solutions are :

(a) Electrophoresis or Cataphoresis and (b) Electro-osmosis

(a) Electrophoresis or Cataphoresis : In a colloidal solution, the colloidal particles are electrically charged and the dispersion medium has equal but opposite charge. Thus colloidal solution on the whole is electrically neutral. When an electric current is passed through a colloidal solution, the charged particles move towards the oppositely charged electrode where coagulate due to loss of charge.

The phenomenon involving the migration of colloidal particles under the influence of electric field towards the oppositively charged electrode, is called electrophoresis or cataphoresis.

This phenomenon is used to determine the charge on the colloidal particles. For example, when a sol. of ferric hydroxide is taken in a U-tube and subjected to electric field, the ferric hydroxide (sol.) particles get accumulated near the cathode (figure). This shows that ferric hydroxide sol. particles are positively charged.

The sol. particles of metals and their sulphides are found to be negatively charged while those of metal hydroxides are positively charged. Basic dyes such as methylene blue haemoglobin are positively charged while acid dyes like are negatively charged.

(b) Electro-osmosis : The phenomenon involving the migration of the dispersion medium and not the colloidal particles under the influence of an electric field is electro-osmosis.

Take the pure solvent (dispersion medium) in two limbs of U-tube. In the lower middle portion of 
U-tube, a porous diaphragm containing the colloidal system is present which divides the U-tube in two sections. In each section of U-tube, an electrode is present, as shown in figure. When the electrode potential is applied to the electrodes, the solid phase of sol. (colloidal system) cannot move but the solvent (dispersion medium) moves through the porous diaphragm towards one of the electrodes. The direction of migration of dispersion medium due to electro-osmosis determines the charge on sol. particles e.g., if the dispersion medium moves towards the cathode (negative electrode), the sol. particles are also negatively charged because the dispersion medium is positively charged as on the whole colloidal solution is neutral.

(c) Coagulation : the colloidal sols are stable due to the presence of electric charges on the colloidal particles. Because of the electrical repulsion, the particles do not come close to one another to form precipitates. The removal of charge by any means will lead to the aggregation of particles and hence precipitation will occur immediately.

This process by means of which the particles of the dispersed phase in a sol. are pecipitated is known as coagulation.

If the coagulated particles instead of settling at the bottom of the container, float on the surface of the dispersion medium, the coagulation is called flocculation.

Most of the sols are coagulated by adding an electrolyte of opposite sign. This is due to the fact that the colloidal particles take up the ions of electrolyte whose charges are opposite to that on colloidal particles with the result that charge on the colloidal particles is neutralized. Thus coagulation takes place. For example, arsenius sulphide sol. (negatively charged) precipitated by adding barium chloride solution. It is due to the fact that the negatively charged particles of the sol. take up barium ions and get neutralized which lower the stability. As a result precipitation takes place.

It is observed that different amounts of different electrolytes is required to bring coagulation of a particular solution.

The minimum amount of an electrolyte required to cause precipitation of one litre of a colloidal solution is called coagulation value or flocculation value of the electrolyte for the sol.

The reciprocal of coagulation value is regarded as the coagulating power.

For example, the coagulation values of NaCl, BaCl₂ and AlCl₃ for arsenic sulphide sol. are 51, 0.69 and 0.093 millimoles/litre respectively. Thus their coagulating powers are ,  and  i.e., 0.0196, 1.449 and 10.75 respectively.

The coagulation values of a few electrolytes for negatively charged arsenic sulphide and positively charged ferric hydroxide sol. are given in table given below. The valency of the coagulation ion (the ion whose charge is opposite to that of the colloidal particles) is also give.

                                                                                Coagulation values of different electrolytes

From the above table, it is clear that the coagulating power of Al³  ions in precipitating the arsenic sulphide sol. is approximately 550 times more than that of sodium (Na ) or potassium (K ) ions. Again, it is observed that the negatively charged arsenic sulphide sol. is coagulated by cations while positively charged ferric hydroxide sol. is coagulated by anions.

Hardy-Schulz rules : H. Schulze (1882) and W.B. Hardy (1900) suggested the following rules to discuss the effect of electrolytes of the coagulation of the sol.

(1) Only the ions carrying charge opposite to the one present on the sol. particles are effective to cause coagulation, e.g., the negative charged sol. is best coagulated by cations and a positive sol. is coagulated by anions.

(2) The charge on coagulating ion influences the coagulation of sol.

In general, the coagulating power of the active ion increases with the valency of the active ion.

After observing the regularities concerning the sing and valency of the active ion, a law was proposed by Hardy and Schulz which is termed as Hardy-Schulze law which is stated as follows:

"Higher is the valency of the active ion, greater will be its power to precipitate the sol."

Thus, coagulating power of cations is in the order of Al³  > Ba²  or Mg²  > Na  or K .

Similarly, to coagulating the positively charged sol. the coagulating power of anion is in the order of [Fe(CN)₆]^4_ > PO₄^3_ > SO₄^2_ > Cl^_

Some other methods of coagulation :

Apart from the addition of electrolyte, coagulation can also be carried out by following methods:

(i) By persistent dialysis : It has been observed that traces of electrolytes are associated with the solution due to which it is stable. If the solution is subjected to prolonged dialysis, the traces of electrolytes are removed and coagulation takes place.

(ii) By mutual coagulation of colloids : When two sols of oppositively charges are mixed together in a suitable proportion, the coagulation takes place. The charge of one is neutralized by the other. For example, when negatively charged arsenic sulphide sol. is added to positively charged ferric hydroxide sol., the precipitation of both occurs simultaneously.

(iii) By electrical method : If the electrical charge of lyophobic sol. is removed by applying any electric field such as in electrophoresis, they also precipitate out.

(iv) By excessive cooling or by excessive heating.

(5) Colligative properties : Colloidal solutions too exhibit colligative properties such as osmotic pressure, lowering of vapour pressure, depression in freezing point and elevation in boiling point. But the effect of colloidal particles on colligative properties except osmotic pressure is very small. This is due to the large size of colloidal particles. The number of colloidal particles produced by a given mass of colloid is much less than the number produced in a molecular solution, containing the same mass of solute. Hence the colligative effect in colloidal solutions is too less.

Protective colloids :

Lyophilic sols are more stable than the lyophobic sols. This is because, lyophilic colloids are extensively hydrated and these hydrated particles do not combine to form large aggregates.

Lyophobic sols are more easily coagulated by the addition of suitable electrolyte. To avoid the precipitation of lyohobic sol. by the addition of electrolyte, some lyophilic colloid is added to it. Such lyophilic colloid is called protective colloid and the action of lyophilic colloid by the electrolytes is known as protective anion. The substances commonly used as protective colloids are gelating, albumin, gum arabic, casein, starch, glue etc. A gold sol. containing a little gelatin as protective colloid needs a very large amount of sodium chloride to coagulate the sol.

Explanation : The particles of the protective colloid get adsorbed on the particles of the lyophobic colloid, thereby forming a protective layer around it (figure). The protective layer prevents the precipitating ions from coming in contact with the colloidal particles.

According to a recent view, the increase in stability of the lyophobic colloid is due to the mutual adsorption of the lyophilic and lyophobic colloids. It is immaterial which is adsorbed on which. In fact the smaller particles, whether of the protective colloid or the lyophobic colloid, are adsorbed on the bigger particles.

Gold number of a protective colloid is a minimum weight of it in milligrams which must be added to 10 ml of a standard red gold sol so that no coagulation of the gold sol. (i.e. change of colour from red to blue) takes place when 1 ml of 10 % sodium chloride solution is rapidly added to it. Obviously, smaller the gold number of a protective colloid, the greater is the protective action.

Protective colloid Gold number

Geltain : 0.005

Haemoglobin : 0.03

Albumin : 0.15

Starch : 2.5

Isoelectric Point of Colloid :

The hydrogen ion concentration at which the colloidal particles are neither positively charged nor negatively charged (i.e. uncharged) is known as isoelectric point of the colloid. At this point lyophilic colloid is expected to have minimum stability because at this point particles have no charge. The isoelectric point of gelatin is 4.7. This indicates that at pH = 4.7, gelating has no electrophoretic motion. Below 4.7, it moves towards the cathode and above 4.7 it moves forwards the anode. It is not always true, e.g., silicic acid has been found to have maximum stability at the isoelectric point.

Electric double layer :

The surface of a colloidal particle acquires a positive or a negative charge by selective adsorption of ions carrying ve or _ ve charges respectively. The charged layer attracts counter ions from the medium which forms a second layer. Thus, an electrical double layer is formed on the surface of the particles i.e., one due to absorbed ions and the other due to oppositely charged ions forming a diffused layer. This layer consists of ion of both the signs, but its net charge is equal and opposite to those absorbed by the colloidal particles. The existence of charges of opposite signs on the fixed and diffused parts of the double layer creates a potential between these layers. This potential difference between the fixed charge layer and diffused layer of opposite change is called electrokinetic potential or zeta potential.

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Emulsions and their Properties - Surface Chemistry, CBSE, Class 12, Chemistry

Emulsions

An emulsion is a colloidal solution of a liquid. It may be defined as a heterogeneous system consisting of more than one immiscible liquids dispersed in one another in the form of droplets whose diameter, in general, exceeds 0.1 m.

For example, milk is an emulsion in which small drops of liquid fat are dispersed in aqueous medium. Cod liver oil is an emulsion in which the water drops are dispersed in the oil. This means in most of the emulsions one of the liquid is water and the other liquid is oil. Here the term 'oil' is used to represent all organic substances which are soluble in water.

The emulsion are classified as :

(1) Oil in water type emulsion (O/W): In this emulsion, oil is the dispersed phase and water is the dispersion medium. It is denoted by O/W or O in W. For example, milk (liquid fat dispersed in water), vanishing cream, etc.

(2) Water in oil type : In this emulsion, water is the dispersed phase and oil is the dispersion medium. It is denoted by W/O or W in O. For example, butter, cod liver oil, cold cream, etc.

The type of emulsion obtained by agitating two immiscible liquids depends upon the relative amounts of two components liquids. The liquid that is in excess forms the dispersion medium. Thus, the two types of emulsions can be interconverted into each other by changing the concentration of one of the liquids.

Distinction between two types of emulsions : the two types of emulsions may be distinguished from each other in a number of ways.

(1) Dye test : It involves the addition of oil soluble dye to the emulsion under experiment. If the emulsion acquires the colour of the dye readily, it is water-in-oil type emulsion and it the emulsion remains colourless, it is oil-in-water type emulsion.

(2) Conductivity test : It involves the addition of electrolyte to the emulsion under experiment. If the conductivity of the emulsion increases appreciably with the addition of electrolyte, it is
oil-in-water type emulsion and it conductivity is very small, it is water-in-oil type emulsion.

(3) Dilution test : As a general rule, an emulsion can be diluted with the dispersion medium while the addition of the dispersed phase forms a separate layer. Thus, if an emulsion can be diluted with oil, it is water-in-oil type.

Preparation of emulsion (Emulsification) : Emulsification is the process which involves the preparation of emulsion. Generally, an emulsion is prepared by subjecting a mixture of the immiscible liquid to a distinct layers upon standing. The oil globules rise to form an upper layer while aqueous medium forms lower layers. To prevent the separation of layers and to get the stable emulsion, a small quantity of the third substance is added. This substance which stabilizes the emulsion is called emulsifier or emulsifying agent. The commonly used emulsifying agents are soaps, detergents and lyophilic colloids. Casein, a lyophilic colloid present in milk, acts as an emulsifier as it forms a protective layer around fat molecules dispersed in water. Hence milk is a fairly stable emulsion.

Function of emulsifier : The main function of emulsifier or emulsifying agents is to lower the interfacial tension between oil and water and thus helps the intermixing of two liquids. For example, a molecule of a soap or detergent (emulsifier) gets concentrated at the interface between oil and water. The polar end of the emulsifier is in water and non-polar end is in oil as shown in figure.

In a soap, RCOONa, R is the non-polar end, whereas COO^— Na  is the polar end.

Properties of emulsion :

(i) The size of particles of the dispersed phase of an emulsion is usually larger than in sols.

(ii) Like colloidal particles, emulsions exhibit properties such as Tyndall effect, Brownian movement (provided the particles are not too large), electrophoresis, coagulation, etc.

Demulsification : The process which involves the breaking of an emulsion into two separate liquid layers is called demulsification. The following methods may be used to bring demulsification:

(1) Chemical Methods : An emulsion may be demulsified by adding a chemical substance whose action on the dispersed phase and the dispersion medium is opposite to that of the original emulsifying agent used to produce the stable emulsion.

(2) Centrifugation : Cream is separated from milk by the centrifugal method.

(3) Cooling : Fat can be removed from milk by keeping it in a refrigerator for a few hours.

Demulsification :

Besides the above noted methods of demulsification, the following methods have also been developed:

(i) Suitable centrifugal action-milk cream is separated from milk by centrifugation.

(ii) Application of electric field-electrophoresis.

(iii) Addition of an electrolyte having multivalent opposite charge than that on the dispersed phase.

(iv) Chemical destruction of stabiliser.

(v) Distilling off of one of the components, usually water.

(vi) Addition of demulsifiers like alcohol, phenol etc.

Oil in water type emulsion (O/W) Use of emulsion :

(1) Many pharmaceutical preparations-medicines, ointments, creams and various lotions are emulsions. It is believed that medicines are more effective and easily assimilated by the body tissues when they are in colloidal form i.e., emulsion.

(2) All paints are emulsions.

(3) The digestion of fat in the intestines is helped by emulsification. A little of the fat forms a medium soap (emulsifier) with the alkaline solution of the intestine and this soap emulsifier the rest of the fats, thus making it easier for the digestive enzymes to do their metabolic functions.

(4) Soaps and detergents remove dust and dirt from the dirty piece of cloth by making an oil in water emulsion.

(5) Milk is an emulsion of liquid fats in water.

(6) In the process of metallurgy, one of the important steps is the concentration of ore which is usually done by froth floatation process in which an oil is added to the finely-divided ore taken in water. The particles of ore go on the surface due to formation of foams while the other impurities are left at the bottom of the vessel.

(7) The emulsion of asphalt in water is used in road making and building.

Gels

Colloidal system in which liquids are the dispersed phase and solid act as the dispersion medium are called gels. The common examples are : boot polishes, jelly, gum arabic, agar agar, processed cheese and silicic acid.

When the gels are allowed to stand for a long time, they give out small quantities of trapped liquids with accumulate on its surface. This action of gels is known as Synresis or Weeping. Some gels such as silica, gelatin and ferric hydroxide liquify on shaking and reset on allowing to stand. This phenomenon of Sol-gel transformation is called thixotropy.

Gels are divided into two categories i.e. elastic gels and non elastic gels. The two categories differ from their behaviour towards dehydration and rehydration as under.

                              Elastic gels                                                                                         Non-elastic gels

1. They change to solid mass on dehydration                                                          1. They change to solid mass on dehydration

     which can be changed back to original                                                                     which cannot be changed back to original

    form by addition of water followed by warming.                                                          form with water.

2. They absorb water when placed in it with                                                               2. They do not exhibit imbibation.

simultaneous swelling. This phenomenon is

called imbibation.

Uses of Colloids :

(1) Medicines : The medicines containing gold, silver or calcium etc. in colloidal form are more effective and easily assimilated by the human systems.

(2) Dyes : In dyeing, mordants colloidal substances are used in textile dyeing industry to fasten dyes.

(3) Rubber industry : Latex is a colloidal solution of negatively charged particles. The article to be rubber plated is made the anode. Under the influence of electric field the rubber particles get deposited on the anode and the article gets rubber plated.

(4) Smoke screens : Smoke screens which consist of titanium dioxide dispersed in air are used in warfare for the purpose of concealment and camouflage.

(5) Formation of delta : The river waver carries with it charged clay particles and many other substances in the form of colloidal solution. When the sea water comes in contact with these particles, the colloidal particles in river water are coagulated by the electrolytes present in sea water to form deltas.

(6) Purification of water : The turbidity in water is due to the presence of negatively charged clay particles. The addition of potash alum, i.e., Al³  ions neutralizes the negative charge on the colloidal particles and thus causes their coagulation. The coagulated matter settles down and thus becomes clear.

(7) Artificial rain : Artificial rain can be caused by throwing electrified sand on clouds which are colloidal solutions or charged particles of water in air.

(8) Smoke precipitation : Smoke coming out of the chimney is industrial area is a nuisance and health hazard. It is a colloidal particles are charged particles and thus they are removed from fuel gases by electrical precipitation (Cottrell Precipitator).

In cottrell precipitator, the smoke is made to pass through chambers fitted with highly electrically charged plates which precipitate the carbon and dust particles leaving in the gases to escape through chimney (figure).

(9) Sewage disposal : Sewage water consists of particles of dirt, rubbish, mud, etc., which are of colloidal dimensions and carry an electric charge and thus do not settle down easily. These particles can thus be removed by cataphoresis. A system of two tanks fitted with metallic electrodes is used for this purpose. When electric field is created, then the dust particles are coagulated on he oppositely charged electrodes. The deposit may be utilized as a manure.

(10) Cleansing action of soap and detergent : Soap solution may be used to wash off the dirt sticking to the fabric, in the presence

(i) If forms a collodial solution in water forms (miscelles), removes dirt by simple adsorption of oily substance and thus washes away.

(ii) It decreases the interfacial tension between water and grease, and it causes the emulsification of grease in water. By mechanical action such as rubbing, the dirt particles are also detached alongwith the only material.

(11) In Photography : Various colloidal system are used in photographic process. In the preparation of photographic plates, the silver bromide in gelatin is coated on thin glass plates. In developing and fixation, various colloidal substances are used. In different kinds of colour printing, gelatin and other colloidal mixtures are used.

(12) Blue colour of the sky : Colloidal particles scatter only blue light and the rest of is absorbed. In sky there are a number of dust and water particles. They scatter blue light and, therefore, sky looks bluish. If there were no scattering, the sky would have appeared totally dark.

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Adsorption and Absorption - Surface Chemistry, CBSE, Class 12, Chemistry

Adsorption

This tendency of accumulation of molecular species at the surface than in the bulk of a solid (or liquid) is termed adsorption. The molecular species or substance which concentrates or accumulates at the surface is termed adsorbate and the material on whose surface the adsorption has taken place is called adsorbent.

The reverse process i.e. removal of adsorbed substance from the surface is calleddesorption.

The adsorption of gases on the surface of metals is called occlusion.

The term sorption is employed when adsoption as well as absorption take place simultaneously.

Distinction Between Adsorption And Absorption

In adsorption the concentration of the adsorbate increases only at the surface of the adsorbent, while in absorption the concentration is uniform throughout the bulk of the solid.

Adsorption is due to the fact that the surface particles of the adsorbent are in different state than the particles inside the bulk. Inside the adsorbent all the force acting between the particles are mutually balanced but on the surface the particles are not surrounded by atoms or molecules of their kind on all sides and hence they possess unbalanced or residual attractive forces. These forces of the adsorbent are responsible for attracting the adsorbate particle on its surface.

Adsorption is a surface phenomenon, whereas absorption is a bulk phenomenon. Adsorption occurs only at the surface of adsorbent, whereas absorption occurs throughout the body of the material.

Types of adsorption

(A) Positive and negative adsorption

When the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk, it is called positive adsorption.

When the concentration of the adsorbate is less relative to its concentration in the bulk, it is called negative adsorption.

(B) physi-sorption and chemisorption

When a gas is adsorbed at the surface of a solid by week forces (Vander Waal's forces), it is called physical adsorption.

When a gas is held on the surface of a solid by forces similar to those of a chemical bond, it is called chemical adsorption or chemiosorption. The chemical bonds may be covalent or ionic in nature. Chemisorption has a rather high energy of activation and is, therefore, often referred to as activated adsorption.

Sometimes these two processes occur simultaneously and it is not easy to ascertain the type of adsorption. A physical adsorption at low temperature may pass into chemisorption as the temperature is increased. For example, hydrogen is first adsorbed on nickel by van der Walls' force. Molecules of hydrogen then dissociate and hydrogen atoms are held on the surface by chemisorption.

Potential energy mape of physi-sorption and chemisorption

Comparison of physi-sorption and chemisorption

Physical adsorption                                                                                       Chemical adsorption

1. It is caused by intermolecular van der Walls' forces                             It is caused by chemical bond formation.

2. It is not specific.                                                                                            It is highly specific.

3. It is reversible.                                                                                               It is irreversible.

4. It depends on the nature of gas. More easily                                           It depends on the nature of gas. Gases which

form com liquefiable gases are adsorbed readily.                                      pounds with the adsorbent exhibit chemi-sorption.

5. Heat of adsorption is low.                                                                              Heat of adsorption is high.

6. Low temperature is favourable. It decreases                                            High temperature is favourable. It increases with increase                                                   with increase of temperature.                                                                           of temperature.

7. No appreciable activation energy is involved.                                           High activation energy is involved.

8. High pressure is favourable.                                                                         Decrease of High pressure is favourable. Decrease of pressure                                              does pressure causes desorption.                                                                not cause desorption.

9. It depends on the surface area. It increases                                              It also depends on the surface area. It increases                                                                         with increase of surface area.                                                                         with increase of surface area.

10. It forms multilayers on adsorbent surface                                                It forms unimolecular layer.

under high pressure.

Characterstic of adsorption

Molecules at the surface of a solid, a metal, or a liquid experience in net inward force of attraction with free valencies.

Adsorption is accompanied by evolution of heat. The amount of heat evolved when one mole of a gas is adsorbed on a solid, is known as molar heat of adsorption. Its magnitude depends upon the nature of the gas.

The magnitude of gaseous adsorption depends upon temperature, pressure, nature of the gas and the nature of the adsorbent.

Adsorption decreases with increase in temperature, since it is accompanied by evolution of heat.

The adsorption increases with increase in pressure, since adsorption of gas leads to decrease in pressure.

More readily soluble and easily liquefiable gases HCl, Cl₂ , SO₂ and NH₃ are adsorbed more than the so called permanent gases such as H₂ , O₂ , N₂ etc. because Vander Waal's forces involved in adsorption are much predominant in the former gases than the latter category of gases.

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Adsorption Isotherms - Surface Chemistry, CBSE, Class 12, Chemistry

Adsorption Isotherm

The variation of adsorption with pressure at a constant temperature is called adsorption isotherm.

(1) Freundlich adsorption isotherm

Freundlich, in 1909, gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. The relationship can be expressed by the following equation.

                                                 At constant temperature x/m = k · P^1/n

where `x' is the mass of the gas adsorbed on a mass `m' of the adsorbent at a pressure P. k and n are constants which depend on the nature of the adsorbent and the gas at a particular temperature.

At low pressure, the amount of the gas adsorbed per unit quantity of adsorbent is proportional to the pressure. At high pressure, the amount of adsorbed gas is independent of pressure. At intermediate pressures, Freundlich adsorption isotherm is expected to hold

Langmuir Adsorption Isotherm :-

According to Langmuir —

(a) There is adsoption of gas molecules on the surface of the solid.

(b) There is desorption of the adsorbed molecules from the surface of the solid.

(c) There is formation of unimolecular layer and thus it is chemisorption

(e) A dynamic equilibrium is attained when rate of adsorption = rate of desorption.

(f) Based on the above facts, langmuir adsorption isotherm is represented as