- Surface chemistry deals with phenomena that occur at surfaces or interfaces.
- The subject of surface chemistry finds many applications in industry, analytical work, and daily life situations.
- In this Unit, we will be studying some important features of surface chemistry such as adsorption, catalysis, and colloids including emulsions and gels.
- Surface of a solid has the tendency to attract and retain the molecules of the phase with which it comes into contact. These molecules remain only at the surface and do not go deeper into the bulk.
- The accumulation of molecular species at the surface rather 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 the surface of which the adsorption takes place is called adsorbent.
- Solids which have large surface area acts as good adsorbents. therefore, charcoal, silica gel, alumina gel, clay, colloids, metals in finely divided state, etc. act as good adsorbents.
Adsorption in action
(i) 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.
(ii) 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, thus, accumulate on the surface of charcoal, i.e., are adsorbed.
(iii) The air becomes dry in the presence of silica gel because the water molecules get adsorbed on the surface of the gel.
- The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption.
- Distinction between Adsorption and Absorption
- 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.
For example, when a chalk stick is dipped in ink, the surface retains the colour of the ink due to adsorption of coloured molecules while the solvent of the ink goes deeper into the stick due to absorption. On breaking the chalk stick, it is found to be white from inside.
- In other words, 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.
Both adsorption and absorption can take place simultaneously also. The term sorption is used to describe both the processes.
Mechanism of Adsorption :
Mechanical Approach: 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.
Thermodyanamical Approach: This approach is based on the heat of adsorption.
- 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, is an exothermic process. Therefore DH of adsorption is always negative. When a gas is adsorbed, the freedom of movement of its molecules become restricted. This amounts to decrease in the entropy of the gas after adsorption, i.e., DS is negative.
For a process to be spontaneous, DG must be negative, On the basis of equation,
DG = DH – TDS,
DG can be negative if DH has sufficiently high negative value as – TDS is positive.
As the adsorption proceeds, DH becomes less and less negative ultimately DH becomes
equal to TDS and DG becomes zero. At this state equilibrium is attained.
Types of Adsorption: There are mainly two types of adsorption of gases on solids.
For comparative study between Physisorption and Chemisorption please go through the pdf given above.
A catalyst is a substance which increases the rate of a reaction without itself undergoing any permanent chemical change.
- It is also found that a catalyst does not change the equilibrium constant of a reaction rather, it helps in attaining the equilibrium faster.
- Catalysis can be broadly divided into two groups (a) Homogeneous catalysis (b) Heterogeneous catalysis
Homogeneous catalysis When the reactants and the catalyst are in the same phase (i.e., liquid or gas), the process is said to be homogeneous catalysis.
Heterogeneous catalysis The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis.
Adsorption Theory of Heterogeneous Catalysis:
- This theory explains the mechanism of heterogeneous catalysis. This theory states that the reactants in gaseous state or in solutions, are adsorbed on the surface of the solid catalyst.
- As there is increase in concentration of the reactants on the surface, this increases the rate of reaction.
- Adsorption being an exothermic process, the heat of adsorption is utilised in enhancing the rate of the reaction. And 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:
(i) Diffusion of reactants to the surface of the catalyst.
(ii) Adsorption of reactant molecules on the surface of the catalyst.
(iii) Occurrence of chemical reaction on the catalyst’s surface through
formation of an intermediate.
(iv) Desorption of reaction products from the catalyst surface, and thereby, making the surface available again for more reaction to occur.
(v) Diffusion of reaction products away from the catalyst’s surface.
- The surface of the catalyst has free valencies which provide the seat for chemical forces of attraction.
- When a gas comes in contact with such a surface, its molecules are held up there due to loose chemical combination.
- If different molecules are adsorbed side by side, they may react with each other resulting in the formation of new molecules.
- Thus, formed molecules may evaporate leaving the surface for the fresh
- This theory explains why the catalyst remains unchanged in mass
and chemical composition at the end of the reaction and is effective even in small quantities.
For detailed explanation of Catalysis please refer the pdf.
- 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).
Lyophilic colloids: The word ‘lyophilic’ means liquid-loving. Colloidal sols directly formed by mixing substances like gum, gelatine, starch, rubber, etc., with a suitable liquid (the dispersion medium) are called lyophilic sols. An important characteristic of these sols is that if the dispersion medium is separated from the dispersed phase (say by evaporation), the sol can be reconstituted by simply remixing with the dispersion medium. That is why these sols are also called reversible sols.
Lyophobic colloids: The word ‘lyophobic’ means liquid-hating. 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. Such sols are called lyophobic sols.
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.
- 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: Let us take the example of soap solutions. Soap is sodium or potassium salt of a higher fatty acid and may be represented as RCOO–Na+. 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).
- The RCOO– ions are, therefore, present on the surface with their COO– groups in water and the hydrocarbon chains R staying away from it and remain at the surface
- But at critical micelle concentration, the anions are pulled into the bulk of the solution and aggregate to form a spherical shape with their hydrocarbon chains pointing towards the centre of the sphere with COO– part remaining outward on the surface of the sphere.
- An aggregate thus formed is known as ‘ionic micelle’. These micelles may contain
as many as 100 such ions. Similarly, in case of detergents, e.g., sodium laurylsulphate, CH3(CH2)11SO4 –Na+, the polar group is –SO4 – along with the long hydrocarbon chain.
- Hence, the mechanism of micelle formation here also is same as that of soaps.
- Charge on colloidal particles: Colloidal particles always carry an electric charge.
- The charge on the sol particles is due to one or more reasons, due to electron capture by sol particles during electrodispersion of metals, due to preferential adsorption of ions from solution and/or due to formulation of electrical double layer.
- Preferential adsorption theory states that the common ion which is present in both dispersed phase and dispesed medium, gets adsorbed on the particles of dispersed phase and thus the charge is exhibited.
- This can be explained by taking the following example:
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.
- However, when KI solution is added to AgNO3 solution, positively charged sol results due to adsorption of Ag+ ions from dispersion medium.
- 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.
- The combination of the two layers of opposite charges around the colloidal particle is called Helmholtz electrical double layer.
- According to modern views, the first layer of ions is firmly held and is termed fixed layer while the second layer is mobile which is termed diffused layer.
- Since separation of charge creates potential, the charges of opposite signs on the fixed and diffused parts of the double layer results in a difference in potential between these layers. This potential difference between the fixed layer and the diffused layer of opposite charges is called the electrokinetic potential or zeta potential.
Electrophoresis: The movement of colloidal particles under an applied electric potential is called electrophoresis. Positively charged particles move towards the cathode while negatively charged particles move towards the anode.
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 an electrolyte and
(ii) by adding a suitable solvent. When solvents 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.
Following are the interesting and noteworthy examples of colloids:
- 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.
- Food articles: Milk, butter, halwa, ice creams, fruit juices, etc., are all colloids in one form or the other.
- 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.
Applications of colloids
- Colloids are widely used in the industry. Following are some examples:
Industrial products: Paints, inks, synthetic plastics, rubber, graphite lubricants, cement, etc., are all colloidal solutions.
- Colloids are also widely used in Cleansing action of soaps and detergents.
- 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.
- Medicines: Most of the medicines are colloidal in nature. For example, argyrol is a silver sol used as an eye lotion.