CBSE NOTES CLASS 12 CHEMISTRY CHAPTER 5

SURFACE CHEMISTRY

Introduction to surface chemistry

Surface chemistry is the branch of chemistry which deals with the phenomena that occur on the surfaces or interfaces. Such phenomena include corrosion, catalysis, crystallisation, etc.

The interface or surface is represented by separating the bulk phases by a hyphen or a slash, e.g., solid-gas or solid/gas etc.

Due to complete miscibility, there is no interface between gases.

The bulk phases that we come across in surface chemistry may be pure compounds or solutions.

The interface is few molecules thick but its area depends on the size of the particles of bulk phases.

The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption.

The molecular species that accumulates at the surface is termed as adsorbate. For example, hydrogen gas, nitrogen gas etc. are some the adsorbates.

The material on the surface of which the adsorption takes place is called adsorbent.

For example, charcoal, silica gel, metals such as Ni, Cu, Ag, Pt and colloids are some adsorbents.

Desorption

The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption.

Occlusion

Adsorption of hydrogen over Pt is called occlusion.

1. If a gas like O2, H2, CO, Cl2, NH3 or SO2 is taken in a closed vessel containing powdered charcoal, it is observed that the pressure of the gas in the enclosed vessel decreases. The gas molecules concentrate at the surface of the charcoal, i.e., gases are adsorbed at the surface.

2. In a solution of an organic dye, say methylene blue, when animal charcoal is added and the solution is well shaken, it is observed that the filtrate turns colourless. The molecules of the dye, accumulate on the surface of charcoal, i.e., are adsorbed.

3. Aqueous solution of raw sugar, when passed over beds of animal charcoal, becomes colourless as the colouring substances are adsorbed by the charcoal.

4. Water vapours adsorbed by silica gel. The air becomes dry in the presence of silica gel because the water molecules get adsorbed on the surface of the gel.

1. In adsorption, the substance is concentrated only at the surface and does not penetrate through the surface to the bulk of the adsorbent, while in absorption, the substance is uniformly distributed throughout the bulk of the solid.

2. In case of 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.

3. The adsorption is rapid in the beginning and slows down near the equilibrium, while absorption occurs at the uniform rate

Adsorption arises due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. Inside the adsorbent all the forces 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 particles on its surface.

The extent of adsorption increases with the increase of surface area per unit mass of the adsorbent at a given temperature and pressure.

Why is adsorption an exothermic process?

During adsorption, there is always a decrease in residual forces of the surface, i.e., there is decrease in surface energy which appears as heat. Adsorption, therefore, is an exothermic process. In other words, ΔH of adsorption is always negative.

When a gas is adsorbed, the freedom of movement of its molecules becomes restricted. This means decrease in the entropy of the gas after adsorption, i.e., ΔS is negative. Adsorption is thus accompanied by decrease in enthalpy as well as decrease in entropy of the system.

For a process to be spontaneous, the thermodynamic requirement is that, at constant temperature and pressure, ΔG must be negative, i.e., there is a decrease in Gibbs free energy.

On the basis of equation,

ΔG = ΔH – TΔS,

ΔG can be negative if ΔH has sufficiently high negative value as –TΔS is positive. Thus, in an adsorption process, which is spontaneous, a combination of these two factors makes ΔG negative.

As the adsorption proceeds, ΔH becomes less and less negative, ultimately ΔH becomes equal to TΔS and ΔG becomes zero. At this state equilibrium is attained.

Sorption

It is a process in which both adsorption and absorption take place simultaneously.

Classification of adsorption depending upon the concentration

1. Positive adsorption: If the concentration of adsorbate is more on the surface as compared to its concentration in the bulk phase then it is called positive adsorption.

For example, when a concentrated solution of KCl is shaken with blood charcoal, it shows positive adsorption.

2. Negative adsorption: If the concentration of the adsorbate is less than its concentration in the bulk then it is called negative adsorption.

For example, when a dilute solution of KCl is shaken with blood charcoal, it shows negative adsorption.

1. Physical adsorption: If the forces of attraction existing between adsorbate and adsorbent are (weak) van der Waal’s forces, the adsorption is called physical adsorption or physisorption or van der Waal’s adsorption. It can be easily reversed by heating or decreasing the pressure.

2. Chemical adsorption: If the forces of attraction existing between adsorbate particles and adsorbent are almost of the same strength as chemical bonds, the adsorption is called chemical adsorption. This type of adsorption is also called as chemisorption or Langmuir adsorption. This type of adsorption cannot be easily reversed.

Characteristics of physisorption

1. Lack of specificity: A given surface of an adsorbent does not show any preference for a particular gas as the van der Waals’ forces are universal.

2. Nature of adsorbate: The amount of gas adsorbed by a solid depends on the nature of gas. In general, easily liquefiable gases (i.e., with higher critical temperatures) are readily adsorbed as van der Waals’ forces are stronger near the critical temperatures. Thus, 1g of activated charcoal adsorbs more sulphur dioxide (critical temperature 630K), than methane (critical temperature 190K) which is still more than 4.5 mL of dihydrogen (critical temperature 33K).

3. Reversible nature: Physical adsorption of a gas by a solid is generally reversible. Thus,

Solid + Gas ↔ Gas/Solid + Heat

More of gas is adsorbed when pressure is increased as the volume of the gas decreases and the gas can be removed by decreasing pressure. Since the adsorption process is exothermic, the physical adsorption occurs readily at low temperature and decreases with increasing temperature (Le Chateliers’ principle).

4. Surface area of adsorbent: The extent of adsorption increases with the increase of surface area of the adsorbent. Thus, finely divided metals and porous substances having large surface areas are good adsorbents.

5. Enthalpy of adsorption: Although physical adsorption is an exothermic process but its enthalpy of adsorption is quite low (20–40 kJ mol-1). This is because the attraction between gas molecules and solid surface is only due to weak van der Waals’ forces.

Characteristics of chemisorption

1. High specificity: Chemisorption is highly specific and it will only occur if there is some possibility of chemical bonding between adsorbent and adsorbate. For example, oxygen is adsorbed on metals by virtue of oxide formation and hydrogen is adsorbed by transition metals due to hydride formation.

2. Irreversibility: As chemisorption involves compound formation, it is usually irreversible in nature. Chemisorption is also an exothermic process but the process is very slow at low temperatures on account of high energy of activation. Like most chemical changes, adsorption often increases with rise of temperature. Physisorption of a gas adsorbed at low temperature may change into chemisorption at a high temperature. Usually high pressure is also favourable for chemisorption.

3. Nature of adsorbate: The amount of gas adsorbed by a solid depends on the nature of gas. Gases which react with the solid are adsorbed easily.

4. Surface area: Like physical adsorption, chemisorption also increases with increase of surface area of the adsorbent.

5. Enthalpy of adsorption: Enthalpy of chemisorption is high (80-240 kJ mol-1) as it involves chemical bond formation.

Distinction between physisorption and chemisorption

 Physisorption (van der Waal's adsorption) Chemisorption (Langmuir adsorption) It arises because of van der Waals’ forces. It is caused by chemical bond formation. It is not specific in nature. It is highly specific in nature. It is reversible. Solid + Gas ↔ Gas/Solid + Heat It is irreversible It depends on the nature of gas. More easily liquefiable gases are adsorbed readily. It also depends on the nature of gas. Gases which can react with the adsorbent show chemisorption. Low Entalpy of adsorption usually in range of 20-40 kJ/mol. Fast. High Enthalpy of adsorption in the range of 80-240 kJ/mol. Slow. Low temperature is favourable for adsorption. It decreases with increase of temperature. High temperature is favourable for adsorption. It increases with the increase of temperature. No appreciable activation energy is needed. High activation energy is sometimes needed. It depends on the surface area. It increases with an increase of surface area. It also depends on the surface area. It too increases with an increase of surface area. It results into multi molecular layers on adsorbent surface under high pressure It results into uni molecular layer.

Easily liquefiable gases e.g., CO2, NH3, Cl2 and SOetc. are adsorbed to a greater extent than the elemental gases e.g. H2, O2, N2, He etc. (while chemisorption is specific in nature.)

Porous and finely powdered solid e.g. charcoal, fullers earth, adsorb more as compared to the hard non-porous materials. Due to this property powdered charcoal is used in gas masks.

2. Surface area of the solid adsorbent

The extent of adsorption depends directly upon the surface area of the adsorbent, i.e. larger the surface area of the adsorbent, greater is the extent of adsorption.

Surface area of a powdered solid adsorbent depends upon its particle size. Smaller the particle size, greater is its surface area.

3. Effect of pressure on the adsorbate gas

An increase in the pressure of the adsorbate gas increases the extent of adsorption.

At low temperature, the extent of adsorption increases rapidly with pressure.

Small range of pressure, the extent of adsorption is found to be directly proportional to the pressure.

At high pressure (closer to the saturation vapour pressure of the gas), the adsorption tends to achieve a limiting value

4. Effect of temperature

As adsorption is accompanied by evolution of heat, so according to the Le Chatelier’s principle, the magnitude of adsorption should decrease with rise in temperature.

The plot between the extent of adsorption against the pressure of gas (P) at constant temperature (T) is called the adsorption isotherm.

A physical adsorption isotherm shows a decrease in $\frac{\mathrm{x}}{\mathrm{m}}$ (where ‘m’ is the mass of the adsorbent and x that of adsorbate) as the temperature rises.

The isotherm of chemisorption shows an increase in the beginning and then decrease as the temperature rises.

Freundlich, gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature.

Where, x is the mass of the gas adsorbed on mass m of the adsorbent at 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, n = 1,

At high pressure, n >> 1,

Taking logarithm of Eq. (i)

This equation is known as Freundlich’s Adsorption Equation for Gas.

Plotting log on y-axis and log p on x-axis, gives a straight line. This is known as Freundlich Adsorption Isotherm.

The slope of the line gives the value of = $\frac{1}{\mathrm{n}}$, and the intercept on the y-axis gives the value of log k.

Value of $\frac{1}{\mathrm{n}}$ can vary from 0 to 1

In solutions, the extent of adsorption

1. decreases with an increase in temperature.

2. increases with an increase of surface area of the adsorbent.

3. depends on the concentration of the solute in solution.

At a given temperature and pressure,

where, C is the equilibrium concentration. On taking log of the above equation, we have,

1. Production of high vacuum: The remaining traces of air can be adsorbed by charcoal from a vessel evacuated by a vacuum pump to give a very high vacuum.

2. Gas masks containing activated charcoal are used for breathing in coal mines. They adsorb poisonous gases.

3. Control of humidity: Silica and aluminium gels are used as adsorbents for removing moisture and controlling humidity.

4. Removal of colouring matter from solutions: Animal charcoal removes colours of solutions by adsorbing coloured impurities.

5. It is used in heterogeneous catalysis - Adsorption of reactants on the solid surface of the catalysts increases the rate of reaction. Eg. Manufacture of ammonia using iron as a catalyst, manufacture of H2SO4 by contact process and use of finely divided nickel in the hydrogenation of oils.

6. In separation of inert gas - Due to the difference in degree of adsorption of gases by charcoal, a mixture of noble gases can be separated by adsorption on coconut charcoal at different temperatures.

7. In curing diseases: A number of drugs are used to kill germs by getting adsorbed on them.

8. Froth floatation process: A low grade sulphide ore is concentrated by separating it from silica and other earthy matter by this method using pine oil and frothing agent

9. As adsorption indicators - Surfaces of certain precipitates such as silver halides have the property of adsorbing some dyes and thereby producing a characteristic colour at the ends.

10. In chromatographic analysis.

CATALYSIS

Substances, which accelerate the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction, are known as catalysts, and the phenomenon is known as catalysis.

2 KClO3 (s) $\stackrel{\mathrm{M}\mathrm{n}{\mathrm{O}}_{2}\left(\mathrm{s}\right)}{\to }$ 2 KCl (s) + 3 O2 (g)

Promoters

Promoters are the substances that enhance the activity of a catalyst.

Poisons

Poisons are the substances that decrease the activity of a catalyst.

For example, in Haber’s process for manufacture of ammonia, molybdenum acts as a promoter for iron which is used as a catalyst.

N2 (g) + 3 H2 (g) $\stackrel{\mathrm{F}\mathrm{e}\left(\mathrm{s}\right)+\mathrm{M}\mathrm{o}\left(\mathrm{s}\right)}{\to }$ 2 NH3 (g)

Types of catalysts depending on the type of action

Catalysis can be broadly divided into two groups, depending on the type of action.

A catalyst which increases the rate of a reaction is referred to as positive catalyst.

A catalyst which decreases the rate of a reaction is referred to as negative catalyst or inhibitors.

Types of catalysis based on the phase

Depending on the phase of the reactants and catalysts, catalysts are divided into two groups

1. Homogeneous catalysis

In this catalysis, the catalyst and reactants are in the same physical state [phase] as that of reactants.

Examples:

Oxidation of sulphur dioxide into sulphur trioxide with dioxygen in the presence of oxides of nitrogen as the catalyst in the lead chamber process,

Hydrolysis of methyl acetate is catalysed by H+ ions furnished by hydrochloric acid.

Hydrolysis of sugar is catalysed by H+ ions furnished by sulphuric acid.

2. Heterogeneous catalysis

In heterogeneous catalysis, catalyst is present in a different phase than that of reactants, e.g.,

Oxidation of sulphur dioxide into sulphur trioxide in the presence of Pt.

SO2(g) + O2(g) $\stackrel{\mathrm{P}\mathrm{t}\left(\mathrm{s}\right)}{\to }$ 2 SO3(g)

Combination between dinitrogen and dihydrogen to form ammonia in the presence of finely divided iron in Haber’s process.

N2 (g) + 3 H2(g) $\stackrel{\mathrm{F}\mathrm{e}\left(\mathrm{s}\right)+\mathrm{M}\mathrm{o}\mathrm{\left(s\right)}}{\to }$ 2 NH3(g)

Oxidation of ammonia into nitric oxide in the presence of platinum gauze in Ostwald’s process.

Hydrogenation of vegetable oils in the presence of finely divided nickel as catalyst.

3. Autocatalysis: When one of the products of a reaction acts as catalyst, the process is called autocatalysis. Examples are Haloform reactions, binding of oxygen with haemoglobin etc.

Ester hydrolysis can be written as:

Ester + Water $\stackrel{{\mathrm{H}}^{+}}{\to }$ Acid + Alcohol

The acid produced in the reaction acts as a catalyst and makes the reaction faster. Acid here behaves as autocatalysts.

This theory explains the mechanism of heterogeneous catalysis.

The old theory, known as adsorption theory of catalysis, was that the reactants in gaseous state or in solutions are adsorbed on the surface of the solid catalyst. The increase in concentration of the reactants on the surface increases the rate of reaction. Adsorption being an exothermic process, the heat of adsorption is utilized in enhancing the rate of the reaction.

Another theory is that the catalytic action can be explained in terms of the intermediate compound formation

The modern adsorption theory is the combination of intermediate compound formation theory and the old adsorption theory. The catalytic activity is localised on the surface of the catalyst.

The mechanism involves five steps

1. Diffusion of reactants to the surface of the catalyst

2. Adsorption of reactant molecules on the surface of the catalyst.

3. Occurrence of chemical reaction on the catalyst’s surface through formation of an intermediate.

4. Desorption of reaction products from the catalyst surface.

5. Diffusion of reaction products away from the catalyst’s surface

Important features of solid catalysts

1. Activity of catalyst: The activity of a catalyst is its ability to increase the rate of a particular reaction. The activity of a catalyst depends upon the strength of chemisorption to a large extent. The adsorption should be reasonably strong but not so strong that they become immobile and no space is available for other reactants to get adsorbed. In case of Hydrogenation, the catalytic activity increases from Group 5 to Group 11 metals with maximum activity being shown by groups 7-9 elements of the periodic table,

2H2 (g) + O2 (g) 2H2O (l)

2. Selectivity of catalyst: The selectivity of a catalyst is its ability to direct a reaction to yield a particular product, e.g., starting with H2 and CO using different catalysts, we get different products.

CO (g) + 3H2 (g) CH4 (g) + H2O (g)

CO (g) + 2H2 (g) CH3OH (g)

CO (g) + H2 (g) HCHO (g)

That means, the action of a catalyst is highly selective in nature, i.e., a given substance can act as a catalyst only in a particular reaction and not for all the reactions.

Shape–selective catalysis by Zeolites

The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis.

Cracking isomerization of hydrocarbons in the presence of zeolites is an example of shape- selective catalysis. They are microporous aluminosilicates with three dimensional network of silicates in which some silicon atoms are replaced by aluminium atoms giving Al–O–Si framework.

An important zeolite catalyst used in the petroleum industry is ZSM-5. It converts alcohols directly into gasoline.

Enzyme catalysis

Enzymes are complex nitrogenous organic compounds which are produced by living plants and animals. They are actually protein molecules of high molecular mass and form colloidal solutions in water.

Numerous reactions that occur in the bodies of animals and plants to maintain the life process are catalysed by enzymes. They are also known as biochemical catalysts.

Examples of enzymatic reactions

 Enzyme Source Enzymatic reaction Invertase Yeast Sucrose → Glucose and fructose Zymase Yeast Glucose → Ethyl alcohol and carbon dioxide Diastase Malt Starch → Maltose Maltase Yeast Maltose → Glucose Urease Soyabean Urea → Ammonia and carbon dioxide Pepsin Stomach Proteins → Amino acids Lacto bacilli Curd Milk → Curd

Characteristics of enzyme catalysis

1. High efficiency: One molecule of an enzyme may transform one million molecule of reactant per minute

2. Highly specific nature: Each enzyme is specific for a given reaction, i.e., one catalyst cannot catalyse more than one reaction.

3. Highly active under optimum temperature Enzyme catalyst gives higher yield at optimum temperature i.e., at 298-310 K. Human body temperature, i.e., at being 310 K is suited for enzyme catalysed reactions.

4. Highly active under optimum pH: The rate of an enzyme catalysed reaction is the maximum at optimum pH range of 5 to 7.

5. Increasing activity in presence of activators and co-enzymes: Activators like ions such as Na+, Ca2+, Mn2+, Co2+, Cu2+, etc. help in the activation of enzymes which cannot act on their own strength.

Co-enzymes are the substance having nature similar to the enzyme and their presence increases the enzyme activity. When a small non-protein (vitamin) is present along with an enzyme, the catalytic activity is enhanced considerably.

6. Influence of inhibitors and poisons: Inhibitors slow down the rate of enzymatic reaction. The inhibitors or poisons interact with the active functional groups on the enzyme surface and often reduce or completely destroy the catalytic activity of the enzymes. The use of many drugs is based on enzyme inhibition action of those drugs in the body.

Mechanism of enzyme catalysis

There are a number of cavities present on the surface of enzymes. These cavities are of characteristic shape and possess active groups such as -NH2, -COOH, -SH (thiol), -OH, etc. The molecules of the reactant (substrate) with complementary shape; fit into

these cavities just like a key fits into a lock. An activated complex is formed which then decomposes to yield the products.

Step 1: Binding of enzyme to substrate to form an activated complex.

E + S → ES

Step 2: Decomposition of the activated complex to form product.

ES → E + P

SOME INDUSTRIAL CATALYTIC PROCESSES

 Process Catalyst Haber’s process for the manufacture of ammonia N2(g) + 3H2(g) → 2NH3(g) Finely divided iron, molybdenum as promoter; conditions: 200 bar pressure and 723-773K temperature. Ostwald’s process for the manufacture of nitric acid. 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g) 2NO(g) + O2(g) → 2NO2(g) 4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq) Platinised asbestos; temperature 573K. Contact process for the manufacture of sulphuric acid. 2SO2(g) + O2(g) → 2SO3(g) SO3(g) + H2SO4(aq) → $\underset{\mathrm{O}\mathrm{l}\mathrm{e}\mathrm{u}\mathrm{m}}{\underset{⏟}{{\mathrm{H}}_{2}{\mathrm{S}}_{2}{\mathrm{O}}_{7}}}$(l) H2S2O7(l) + H2O(l) → 2H2SO4(aq) Platinised asbestos or vanadium pentoxide (V2O5); temperature 673-723K.

COLLOIDS

A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called dispersion medium.

The essential difference between a solution and a colloid is that of particle size. While in a solution, the constituent particles are ions or small molecules, in a colloid, the dispersed phase may consist of particles of a single macromolecule (such as protein or synthetic polymer) or an aggregate of many atoms, ions or molecules.

Colloidal particles are larger than simple molecules but small enough to remain suspended. Their range of diameters is between 1 and 1000 nm (10–9 to 10–6 m).

Colloidal particles have an enormous surface area per unit mass as a result of their small size.

This enormous surface area leads to some special properties of colloids.

Classification of colloids

Colloids are classified on the basis of the following criteria:

1. Physical state of dispersed phase and dispersion medium

2. Nature of interaction between dispersed phase and dispersion medium
1. Type of particles of the dispersed phase.

Classification of colloids based on physical state of dispersed phase and dispersion medium

Sols are solids in liquids, gels are liquids in solids and emulsions are liquids in liquids.

Types of colloidal systems

 Dispersed phase Dispersion medium Type of colloid Examples Solid Solid Solid sol Some coloured glasses and gem stones Solid Liquid Sol Paints, cell fluids Solid Gas Aerosol Smoke, dust Liquid Solid Gel Cheese, butter, jellies Liquid Liquid Emulsion Milk, hair cream Liquid Gas Aerosol Fog, mist, cloud, insecticide sprays Gas Solid Solid sol Pumice stone, foam rubber Gas Liquid Foam Froth, whipped cream, soap lather

Classification of colloids based on nature of interaction between dispersed phase and dispersion medium

Lyophilic colloids

The word ‘lyophilic’ means solvent attracting or liquid-loving. If water is the dispersion medium, the term used is hydrophilic. Colloidal sols directly formed by mixing substances like gum, gelatine, starch, rubber, etc., with a suitable liquid (the dispersion medium) are called lyophilic sols. These sols can be reconstituted by simply remixing with the dispersion medium. That is why these sols are also called reversible sols. These sols are quite stable and cannot be easily coagulated.

Lyophobic colloids

The word ‘lyophobic’ means solvent repelling or liquid-hating. If water is the dispersion medium, the term used is hydrophobic. Substances like metals, their sulphides, etc., when simply mixed with the dispersion medium do not form the colloidal sol. Their colloidal sols can be prepared only by special methods. These sols are readily precipitated (or coagulated) on the addition of small amounts of electrolytes, by heating or by shaking and hence, are not stable. Further, once precipitated, they do not give back the colloidal sol by simple addition of the dispersion medium. Hence, these sols are also called irreversible sols. Lyophobic sols need stabilizing agents for their preservation.

Classification of colloids based on type of particles of the dispersed phase

Multi-molecular colloids

On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having size in the colloidal range (diameter < 1 nm). For example, a gold sol may contain particles of various sizes having many atoms. Sulphur sol consists of particles containing a thousand or more of S8 sulphur molecules.

Macromolecular colloids

Macromolecules in suitable solvents form solutions in which the size of the macromolecules may be in the colloidal range. Such systems are called macromolecular colloids. These colloids are quite stable and resemble true solutions in many respects. Examples are starch, cellulose, proteins, enzymes, polythene, nylon, polystyrene, synthetic rubber, etc.

Associated colloids (Micelles)

There are some substances which at low concentrations behave as normal strong electrolytes, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated colloids.

The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions.

Surface active agents such as soaps and synthetic detergents belong to this class. For soaps, the CMC is 10–4 to 10–3 mol L–1. These colloids have both lyophobic and lyophilic parts. Micelles may contain as many as 100 molecules or more.

Mechanism of micelle formation

Soap is sodium or potassium salt of a higher fatty acid and may be represented as RCOONa+ (e.g., sodium stearate CH3(CH2)16COONa+, which is a major component of many bar soaps). When dissolved in water, it dissociates into RCOO and Na+ ions. The RCOO ions, however, consist of two parts — a long hydrocarbon chain R (also called non-polar ‘tail’) which is hydrophobic (water repelling), and a polar group COO (also called polar-ionic ‘head’), which is hydrophilic (water loving).

Cleansing action of soaps

The cleansing action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles. Since the polar groups can interact with water, the oil droplet surrounded by stearate ions is now pulled in water and removed from the dirty surface. Thus soap helps in emulsification and washing away of oils and fats. The negatively charged sheath around the globules prevents them from coming together and forming aggregates.

Preparation of colloids

1. Chemical methods: Colloidal solutions can be prepared by chemical reactions which form molecules by double displacement, oxidation, reduction or hydrolysis. These molecules then aggregate to form sols.

As2O3 + 3H2S As2S3 (sol) + 3H2O

SO2 + 2H2S 3S (sol) + 2H2O

2AuCl3 + 3 HCHO + 3H2O 2Au (sol) + 3HCOOH + 6HCl

FeCl3 + 3H2O Fe (OH)3 (sol) + 3HCl

2. Electrical disintegration or Bredig’s arc method: This process involves dispersion as well as condensation. Colloidal sols of metals such as gold, silver, platinum, etc., can be prepared by this method. In this method, electric arc is struck between electrodes of the metal immersed in the dispersion medium. The intense heat produced vapourises the metal, which then condenses to form particles of colloidal size.

1. Peptization: Peptization is defined as the process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this purpose is called peptizing agent.

During peptization, the precipitate adsorbs one of the ions of the electrolyte on its surface. This causes the development of positive or negative charge on precipitates, which ultimately break up into smaller particles of the size of a colloid.

Purification of colloidal solutions

Colloidal solutions when prepared; generally contain excessive amount of electrolytes and some other soluble impurities. While the presence of traces of electrolyte is essential for the stability of the colloidal solution, 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 the amount of impurities to a requisite minimum is known as purification of colloidal solution.

The purification of colloidal solution is carried out by the following mehods:

1. Dialysis: It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane.

The principle is based upon the fact that colloidal particles cannot pass through animal membrane (bladder) or parchment or cellophane membrane while the ions of the electrolyte can pass through it.

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 flowing. The impurities slowly diffused out of the bag leaving behind pure colloidal solution.

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

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

2. 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 an electrolyte. The process is then named electrodialysis.

The colloidal solution is placed in a bag of suitable membrane while pure water is taken outside.

Electrodes are fitted in the compartment. The ions present in the colloidal solution migrate out to the oppositely charged electrodes.

1. Ultra–filtration: Sol particles directly pass through ordinary filter paper because their pores are larger (more than 1μ) than the size of sol particles (less than 200 μ).

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.

1. Ultra–centrifugation: The sol particles are prevented from setting out under the action of gravity by kinetic impacts of the molecules of the medium.

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

1. Heterogeneous nature: Colloidal sols are heterogeneous in nature. They consist of two phases; the dispersed phase and the dispersion medium.

2. 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.

Stability of sols: Sols are thermodynamically unstable and the dispersed phase (colloidal particles), tend to separate out on long standing due to the van der Waal's attraction forces. However sols tend to exhibit some stability due to

1. Stronger repulsive forces between the similarly charged particles

2. Particle-solvent interactions: Due to strong particle-solvent (dispersion medium) interactions, the colloidal particles get strongly solvated.
1. Filterability: Colloidal particles are readily passed through the ordinary filter papers. However they can be retained by special filters known as ultra-filters (parchment paper).
1. Colligative properties: Due to formation of bigger aggregates, the number of particles in a colloidal solution is comparatively small as compared to a true solution. Hence, the values of colligative properties (osmotic pressure, lowering in vapour pressure, depression in freezing point and elevation in boiling point) are of smaller as compared to values shown by true solutions at same concentrations.

2. Tyndall effect: The property of colloids by virtue of which path of a beam of light is illuminated, is called Tyndall effect. The Tyndall effect is due to the fact that colloidal particles scatter light in all directions in space. This scattering of light illuminates the path of beam in the colloidal dispersion.

Tyndall effect is observed only when,

1. The diameter of the dispersed particles is not much smaller than the wavelength of the light used; and

2. The refractive indices of the dispersed phase and the dispersion medium differ greatly in magnitude.

1. Colour: The colour of colloidal solution depends on the wavelength of light scattered by the dispersed particles, size and nature of the particles and the manner which the observer receives the light. For example, a mixture of milk and water appears blue when viewed by the reflected light and red when viewed by the transmitted light. Finest gold sol is red in colour; as the size of particles increases, it appears purple, then blue and finally golden.
1. Brownian movement: The colloidal particles are moving at random in a zig–zag motion. This type of motion is called Brownian movement.

The molecules of the dispersion medium are constantly colloiding with the particles of the dispersed phase. The impacts of the dispersion medium particles are unequal, thus causing a zig-zag motion of the dispersed phase particles.

The Brownian movement helps in providing stability to colloidal sols by not allowing them to settle down.

2. Diffusion: The sol particles diffuse from higher concentration to lower concentration region. However, due to bigger size, they diffuse at a lesser speed.

3. 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.
1. Electrical Properties
1. Charge on colloidal particles: Colloidal particles always carry an electric charge. The nature of this charge is the same on all the particles in a given colloidal solution and may be either positive or negative.

The charge on the sol particles is due to one or more reasons, viz., due to electron capture by sol particles during electro-dispersion of metals, due to preferential adsorption of ions from solution and/or due to formulation of electrical double layer.

2. Electrophoresis: The phenomenon of movement of colloidal particles under an applied electric field is called electrophoresis.

If the sol particles accumulate near the negative electrode, the charge on the particles is positive.

If the sol particles accumulate near the positive electrode, the charge on the particles is negative.

When electrophoresis of a sol is carried out without stirring, the bottom layer gradually becomes more concentrated while the top layer which contains pure solution may be decanted.

The process of transferring the clear liquid without disturbing the sediments is called decantation.

This is called electro-decanation and is used for the purification as well as for concentrating the sol.

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.

1. Electrical double layer theory

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

Example 1: When silver nitrate solution is added to potassium iodide solution, the precipitated silver iodide adsorbs iodide ions from the dispersion medium and negatively charged colloidal solution results.

AgI/I (Negatively charged)

However when KI solution is added to AgNO3 solution, positively charged sol results due to adsorption of Ag+ ions from dispersion medium.

AgI/Ag+ (Positively charged)

Example 2: If FeCl3 is added to excess of hot water, a positively charged sol of hydrated ferric oxide is formed due to adsorption of Fe3+ ions.

Fe2O3.xH2O/Fe3+

However, when ferric chloride is added to NaOH a negatively charged sol is obtained with adsorption of OH- ions.

Fe2O3.xH2O/OH

• Having acquired a positive or a negative charge by selective adsorption on the surface of a colloidal particle as stated above, this layer attracts counter ions from the medium forming a second layer, e.g.,

AgI/I-K+ or AgI/Ag+I-

• Combination of the two layers of opposite charges around the colloidal particle is called Helmholtz electrical double layer.

• The ion preferentially adsorbed is held in fixed part and imparts charge to colloidal particles.

• 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.

• The existence of opposite sign on fixed and diffused parts of double layer leads to appearance of a difference of potential, known as zeta potential or electrokinetic potential.

• When electric field is employed the particles move (electrophoresis).

Electro-osmosis

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.

The movement of the dispersed particles is prevented from moving by semipermeable membrane.

The reverse of electro-osmosis is called streaming. When liquid is forced through a porous material or a capillary tube, a potential difference is setup between the two sides, which is called streaming potential.

Coagulation or Flocculation or Precipitation

The process of settling of colloidal particles is called coagulation or precipitation of the sol.

The stability of the lyophobic sols is due to the presence of charge on colloidal particles. If, somehow, the charge is removed, the particles will come nearer to each other to form aggregates (or coagulate) and settle down under the force of gravity.

Coagulation of lyophobic sols

1. By electrophoresis: The colloidal particles move towards oppositely charged electrodes, get 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. For example mixing of hydrated ferric oxide (+ve sol) and arsenious sulphide (–ve sol) bring them in the precipitated forms. 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: When excess of an electrolyte is added, the colloidal particles are precipitated. The reason is that colloids interact with ions carrying charge opposite to that present on themselves. This causes neutralization leading to their coagulation.

The ion responsible for neutralisation of charge on the particles is called the coagulating ion or flocculating ion. A negative ion causes the precipitation of positively charged sol and vice versa.

Hardy schulze rule: The coagulation capacity of different electrolytes is different. It depends upon the valency of the flocculating ion. According to Hardy Schulze rule,

greater the valency of the active ion or flocculating ion, greater will be its coagulating power”

For example to coagulate negative sol, the coagulation power of cations is in the order Al3+ > Mg2+ > Na+.

Similarly, to coagulate a positive sol such as Fe(OH)3 , the coagulating power of different anions is in the order [Fe(CN)6]4- > PO43- > SO42- > Cl-

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.”

Coagulation of lyophilic sols

There are two factors which are responsible for the stability of lyophilic sols. These factors are the charge and solvation of the colloidal particles.

When these two factors are removed, a lyophilic sol can be coagulated. This is done (i) by adding electrolyte (ii) and by adding suitable solvent.

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.

Protection of colloids

Lyophilic sols are more stable than lyophobic sols. This is due to the fact that lyophilic colloids are extensively solvated, i.e., colloidal particles are covered by a sheath of the liquid in which they are dispersed.

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

Lyophilic colloids can prevent the coagulation of any lyophobic colloids. When a lyophilic sol is added to the lyophobic sol, the lyophilic particles form a layer around lyophobic particles and thus protect the latter from electrolytes. Lyophilic colloids used for this purpose are called protective colloids.

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

EMULSIONS: liquid-liquid colloids

“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.”

If a mixture of two immiscible or partially miscible liquids is shaken, a coarse dispersion of one liquid in the other is obtained which is called emulsion.

Types of emulsion

Depending upon the nature of the dispersed phase, the emulsions are classified as;

1. 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 and vanishing cream are examples of the oil-in-water type of emulsion. In milk liquid fat globules are dispersed in water.

1. 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 emulsions are also termed oil emulsions. Butter, cream and cod liver oil, are examples of this type of emulsion.

Properties of emulsion

• Emulsions of oil in water are unstable and sometimes they separate into two layers on standing. For stabilisation of an emulsion, a third component called emulsifying agent is added. The emulsifying agent forms an interfacial film between suspended particles and the medium. The principal emulsifying agents for O/W emulsions are proteins, gums, natural and synthetic soaps, etc., and for W/O, heavy metal salts of fatty acids, long chain alcohols, lampblack, etc.

• Emulsions can be diluted with any amount of the dispersion medium. On the other hand, the dispersed liquid when mixed forms a separate layer.

• The droplets in emulsions are often negatively charged and can be precipitated by electrolytes, containing polyvalent metal ions indicating the negative charge on the globules.

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

• Emulsions can be broken into constituent liquids by heating, freezing, centrifuging, etc. This process is also known as demulsification.

• The size of the dispersed particles in emulsions is 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.

Examples of colloids in nature

1. Blue colour of the sky: Dust particles along with water suspended in air scatter blue light which reaches our eyes and the sky looks blue to us.

2. Fog, mist and rain: When a large mass of air containing dust particles, is cooled below its dewpoint, the moisture from the air condenses on the surfaces of these particles forming fine droplets. These droplets being colloidal in nature continue to float in air in the form of mist or fog. Clouds are aerosols having small droplets of water suspended in air. On account of condensation in the upper atmosphere, the colloidal droplets of water grow bigger and bigger in size, till they come down in the form of rain. Sometimes, the rainfall occurs when two oppositely charged clouds meet.

Artificial rain is caused by throwing electrified sand or spraying a sol carrying charge opposite to the one on clouds from an aeroplane.

3. Food articles: Milk, butter, halwa, ice creams, fruit juices, etc., are all colloids in one form or the other.

4. Blood: It is a colloidal solution of an albuminoid substance. The styptic action of alum and ferric chloride solution is due to coagulation of blood forming a clot which stops further bleeding.

5. Soils: Fertile soils are colloidal in nature in which humus acts as a protective colloid. On account of colloidal nature, soils adsorb moisture and nourishing materials.

6. Formation of delta: River water is a colloidal solution of clay. Sea water contains a number of electrolytes. When river water meets the sea water, the electrolytes present in sea water coagulate the colloidal solution of clay resulting in its deposition with the formation of delta.

Applications of colloids

1. Electrical precipitation of smoke: Smoke is a colloidal solution of solid particles such as carbon, arsenic compounds, dust, etc., in air. The smoke, before it comes out from the chimney, is led through a chamber containing plates having a charge opposite to that carried by smoke particles. The particles on coming in contact with these plates lose their charge and get precipitated. The particles thus settle down on the floor of the chamber. The precipitator is called Cottrell precipitator.

2. Purification of drinking water: Alum is added to water to coagulate the suspended impurities and make it fit for drinking purposes.

3. Medicines: Most of the medicines are colloidal in nature. For example, argyrol is a silver sol used as an eye lotion. Colloidal antimony is used in curing kalaazar. Colloidal gold is used for intramuscular injection. Milk of magnesia, an emulsion, is used for stomach disorders.

Colloidal medicines are more effective because they have large surface area and are therefore easily assimilated.

1. Tanning: Animal hides are colloidal in nature. When a hide, which has positively charged particles, is soaked in tannin, which contains negatively charged colloidal particles, mutual coagulation takes place. This results in the hardening of leather. This process is termed as tanning. Chromium salts are also used in place of tannin.
1. Cleansing action of soaps and detergents: As already discussed.
• Photographic plates and films: Photographic plates or films are prepared by coating an emulsion of the light sensitive silver bromide in gelatin over glass plates or celluloid films.
1. Rubber industry: Latex is a colloidal solution of rubber particles which are negatively charged. Rubber is obtained by coagulation of latex.
1. Industrial products: Paints, inks, synthetic plastics, rubber, graphite lubricants, cement, etc., are all colloidal solutions.