# Surface Chemistry

### What is Surface Chemistry?

Surface chemistry is the branch of chemistry in which we study about those chemical reactions or chemical changes which are taking place at the interface of two phases which can be solid – gas, solid – liquid, liquid – gas etc.

Surface Chemistry has various applications in analytical work, medicinal field, paint industry etc.

Adsorption is a surface phenomenon in which atoms, ions or molecules of liquids, gases or solids (dissolved) get accumulated at the surface of another substance (which is in different phase) through adhesive forces.

The substance, which concentrates or accumulates at the surface of another substance is termed as adsorbate and the material on the surface of which the adsorption takes place is called adsorbent.

In adsorption, molecules, atoms or ions of one substance get stick loosely on the surface of another substance of different kind.

## Difference between Absorption and Adsorption

 S.No. Absorption Adsorption 1. The process of absorption is a property of bulk matter as it deals with the matter in a whole.(Particles are uniformly present throughout the bulk matter)[Image] The process of adsorption mainly takes place on the surface of the matter and hence it is termed as a surface phenomenon. (Concentration of the particles at the surface is more than the bulk matter)[Image] 2. The absorbed substance is assimilated uniformly throughout the matter of the substance. The adsorb substance is accumulated at the surface of adsorbent only. 3. Absorption takes place at a constant rate. The initial rate of adsorption is rapid. The rate however decreases gradually till the equilibrium is attained. 4. Energy is absorbed during this process. Energy is given out in this process. 5. No visible effect of temperature on the process of absorption. Low temperature facilitates the process of adsorption.

The understanding of nature of forces existing between the adsorbate and the adsorbent molecules helps us to classify the process of adsorption in following two types:

1. The poisonous gases such as carbon monoxide, methane etc. can be adsorbed by activated charcoal which is widely used in gas masks.

2. Animal charcoal is used to adsorb the colouring matter from cane-sugar juice in the process of manufacture of sugar.

3. Ion exchange resin is used to remove hardness of water, since it can adsorb the salts of calcium and magnesium.

4. Chromatographic adsorption techniques are widely used to purify organic compounds.

5. To control humidity the mechanism of adsorption by silica gel is used.

6. Principle of adsorption is also used to explain the results of a number of catalysts, for e.g.: the spongy iron in the making of ammonia and Ni, Pt/Pd in the lowering of unsaturated hydrocarbons.

The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve termed as adsorption isotherm.

Freundlich adsorption isotherm – It is an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. It can be expressed mathematically as follows –

$\frac{x}{m}$ = k.$p^{\frac{1}{n}}$ (n > 1)

m = mass of solid adsorbent

p = pressure

k and n = Constants which depend on the following –

• Nature of the adsorbent and

• Nature of the gas at specific temperature

Logarithmic Equation of Freundlich Adsorption Isotherm -  log log$\frac{x}{m}$ =log log k + $\frac{1}{n}$log p

If 1/n = 0, x/m = constant then the adsorption is independent of pressure.

If 1/n = 1, x/m p, the adsorption varies directly with pressure.

### Catalysis

Well you will be amazed to know that long before chemists recognized catalysts and the process of catalysis, common people were already using it for a number of purposes such as fermenting wine to produce vinegar or raising bread. Uses of catalysts are countless. Catalysts are playing a vital role in many processes or manufacturing techniques on which industries are based on. It was late 18th century when chemists started to recognize the catalysis process. The concept of catalysis was invented by chemist Elizabeth Fulhame and was reported in her book in 1794. Although the term catalysis was first used by Jons Jakob Berzelius in 1835. In the 1880s, Wilhelm Ostwald investigated reactions catalyzed by the presence of acids and bases, for this he was awarded the Nobel Prize in Chemistry in 1909.

### What is Catalysis?

The process of increasing the rate of chemical reaction by adding a substance which does not take part in the reaction is called catalysis and the substance which is added and increases the rate of reaction is called a catalyst. A very small amount of catalyst is required to alter the rate of reaction. For example, in the reaction of converting hydrogen peroxide into water and oxygen gas, potassium permanganate is used as a catalyst which increases the rate of reaction.

2H2O2 Potassium permanganate→ 2H2O + O2

### Types of Catalysis

On the basis of phases of catalysts and reactants, catalysis can be divided into following two types –

• Homogeneous Catalysis

• Heterogeneous Catalysis

### What is Homogeneous Catalysis and Catalyst?

The catalyst who is present in the same phase as of the reactants in the reaction is called homogeneous catalyst and this type of catalysis process is called homogeneous catalysis.

Examples of Homogeneous Catalysis and Catalysts – 1. Hydrolysis of Sugar – In hydrolysis of sugar reactants sugar (sucrose solution) and water are used in liquid states and the catalyst sulfuric acid is also used in the liquid state. Reaction is given below –

C12H22O11(l) + H2O(l) H2SO4(l) C6H12O6(l) + C6H12O6(l)

Sucrose                                            Glucose      Fructose

2. Hydrolysis of the Ester – In hydrolysis of the ester, ester is taken in liquid state with water (liquid) for the reaction in presence of catalyst hydrochloric acid which is also taken in liquid state. Reaction is given below –

CH3COOCH3(l) + H2O(l) HCl(l)→ CH3COOH(l) CH3OH(l)

### What is Heterogeneous Catalysis and Catalysts?

The catalyst whose phase differs from that of the reactants in the reaction is called heterogeneous catalyst and this type of catalysis process is called heterogeneous catalysis.

Examples of Heterogeneous catalysis and catalysts – 1. In Haber’s process of formation of ammonia, nitrogen and hydrogen are used in gaseous forms while catalyst iron is used in solid form.

N2(g) + 3H2(g) Fe(s)→ 2NH3

2. Formation of Sulfuric Acid – In this process sulfur dioxide (gas) is oxidized to sulfur trioxide (gas) by heterogeneous catalysis in presence of solid V2O5 catalyst. Then sulfur trioxide is hydrolyzed to sulfuric acid.

SO2(g) + O2(g) V2O5(s) 2 SO3(g)

### What is Adsorption Theory of Heterogeneous Catalysis?

Modern Adsorption theory of heterogeneous catalysis is the mixture of moderate compound hypothesis and the old adsorption hypothesis or old adsorption theory. Old adsorption theory lacked specificity so there was a need for modern adsorption theory.

According to adsorption theory of heterogeneous catalyst, there are free valancies in the catalyst on which reactant molecules get attached. The mechanism of adsorption theory of heterogeneous catalysis involves following steps –

• Step 1. Diffusion of reactant molecules

• Step 3. Intermediate complex formation

• Step 4. Desorption

• Step 5. Diffusion of product molecules

Step 1. Diffusion of Reactant Molecules – In this step reactant molecules get diffused towards the surface of the catalyst.

Step 2. Adsorption – In this step reactant molecules get adsorbed on the surface of the solid catalyst or form loose bonds with the free valancies of the catalyst.

Step 3. Intermediate Complex Formation – In this step adsorbed reactant molecules on the surface of the catalyst react with each other and form new stronger bonds with each other which leads to the formation of an intermediate.

Step 4. Desorption – In this step intermediate converts into product as it loses its affinity towards the catalyst. The product molecule gets desorbed from the surface of the catalyst.

Step 5. Diffusion of Product Molecules – In this step desorbed product molecules from the surface of the catalyst get diffused away from the catalyst.

### What is Shape Selective Catalysis by Zeolites?

Shape selective catalysis is that type of catalysis which depends upon the pore structure of the catalyst and size of the reactant and product molecules. Zeolite is a porous solid made up of silicon, aluminium and oxygen and a good shape selective catalyst. It has cavities in its structure where ions or atoms or small molecules can reside.

### What is Enzyme Catalysis?

Many enzymes also act as catalysts for many reactions. As enzymes catalyze many reactions which occur in our body, plants and animals. So, they are known as biochemical catalysts and the phenomenon is called biochemical catalysis.

### Characteristics of Enzyme Catalysis

• Highly efficient

• Very specific nature

• They perform best at optimum temperature and optimum pH

• Activity of enzymes is increased in the presence of activators and coenzymes

• Inhibitors can destroy or reduce the activity of enzymes

## What are Colloids?

In chemistry, colloids are heterogeneous mixtures of two substances in which minute particles of one substance are dispersed in another substance. The substance whose minute particles are suspended in another substance is called the dispersed phase while the substance in which it is suspended is called dispersion medium. For example, in fog dispersed phase is water (liquid) and dispersion medium is different gases. We can’t see particles of dispersed phase in colloids by naked eyes as they are very small in size.

Examples of Colloids

We see many colloidal solutions around us. Many food items such as cake, milk, bread, butter, ice cream fruit juices, whipped cream etc. are examples of colloids. Apart from these fog, mist clay etc. are also examples of colloids. We are providing list of examples of colloids with their dispersed phase and dispersing medium below –

 Dispersed Phase Dispersing Medium Examples Liquid Gas Fog, mint Solid Gas Smoke, automobile exhaust Gas Liquid Shaving cream Liquid Liquid Milk Solid Liquid Mud Gas Solid Foam, rubber Liquid Solid Jelly, butter Solid Solid Garnet, citrine

### Properties of Colloids

Colloids show following properties –

• It is a heterogeneous mixture.

• The size of colloidal particles is very small. Their particle size ranges between 1-1000 nanometers.

• It shows a tyndall effect. It means it scatters the beam of light and shows its path through itself.

• They don’t settle down when left undisturbed for some time. it means colloidal solutions are quite stable.

• They cannot be separated by a filtration process.

• They can be separated by centrifugation.

• Colloidal particles show Brownian movement.

### Classification of Colloids

On the basis of physical state of dispersed phase and dispersion medium – Colloids can be divided into 8 types on the basis of physical state of dispersed phase and dispersion medium. All eight types of colloids are listed below –

 Types of colloid Dispersed phase Dispersing medium Aerosol Liquid Gas Aerosol Solid Gas Foam Gas Liquid Emulsion Liquid Liquid Sol Solid Liquid Solid sol Gas Solid Gel Liquid Solid Solid sol Solid Solid

On the basis of nature of interaction between dispersed phase and dispersion medium – Colloids can be divided into following two types on the basis of nature of interaction between dispersed phase and dispersion medium –

• Lyophilic colloids or lyophilic sols

• Lyophobic colloids or lyophobic sols

### What are Lyophilic Sols?

Lyophilic means ‘liquid loving’. Those sols in which dispersed phase and dispersion medium (water) have strong attraction between them are called lyophilic sols. For example, colloidal solution formed by dissolving starch in water. In this colloidal solution dispersion medium is water and dispersed phase is starch. This sol can be prepared by heating water at 100 and dissolving starch in it. It is a stable sol and cannot be separated easily due to strong attraction between dispersed phase and dispersion medium. Egg albumin sol is another example of lyophilic sol.

### What are Lyophobic Sols?

Lyophobic means ‘Liquid hating’. Those sols in which dispersed phase and dispersion medium (water) have very less attraction or no interaction between them are called lyophobic sols. In these sols dispersed phase particles show very less or no affinity for dispersion medium. These are not stable sols and can be separated easily. If a small quantity of an electrolyte is added to these types of sols, then dispersed phase and dispersion medium are easily get separated. To make them stable sols, we need to add stabilizers while preparing them. These are also known as hydrophobic sols. Example of a lyophobic sol is ferric hydroxide sol. It is prepared by hydrolysis of ferric chloride. In this boiling water is used. In this process hydrochloric acid is also produced which is removed from the sol as it makes the sol unstable. It is removed by dialysis of the ferric hydroxide sol (lyophobic sol).

## Difference between Lyophilic and Lyophobic Sols

 S. No. Lyophilic Sol Lyophobic Sol 1. In these sols, dispersion phase has strong affinity for dispersion medium. In these sols, dispersion phase has very less or no affinity for the dispersion medium. 2. They are more stable. They are less stable. 3. They are reversible. They are irreversible. 4. Colloidal particles have no charge. Colloidal particles carry either positive or negative charge. 5. They need no stabilizers during preparation. They need stabilizers during preparation. 6. They are solvent loving colloids. They are solvent hating colloids. 7. They are highly viscous sols. They possess the same viscosity as the solvent. 8. When water is taken as solvent, it is called hydrophilic sol. When water is taken as solvent, it is called hydrophobic solvent. 9. Examples – Starch sol, egg albumin sol etc. Examples – Ferric hydroxide sol, aluminium hydroxide sol etc.

On the basis of type of particles of the dispersed phase – Colloids can be classified into following three types on the basis of type of particles of the dispersed phase –

• Multimolecular colloids

• Macromolecular colloids

• Associated colloids

Multimolecular Colloids – In these colloids, particles (atoms or small molecules) of dispersed phase aggregate together to form species which have a size of 1-1000nm range. These types of colloids are called multimolecular colloids. Example – In Sulphur sol, generally Sulphur particles consist of 8 Sulphur atoms.

Macromolecular Colloids – Those colloidal solutions which are formed by the dispersion of macromolecules in the suitable dispersion medium are called macromolecular colloids. Example – starch sol.

Associated Colloids – Some substances at higher concentrations show colloidal behavior due to the formation of aggregates or micelles. These are also known as micelles 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). Micelles change into individual ions on dilution. Associated colloids exhibit both lyophilic and lyophobic parts. Example – soap solution.

### Preparation of Colloids

Colloids can be prepared by following methods –

• Chemical Methods – Double decomposition, hydrolysis, oxidation and reduction are the various chemical reactions which are used in the preparation of colloids through chemical methods.

Example – SO2 + 2H2S Double decomposition→ 3S + 2H2

• Electrical Disintegration or Bredig’s Arc Method – In this method metal electrodes are immersed in the dispersion medium and intense heat is produced which converts the metal into vapor. Vapor of metal get condensed into particles of colloidal size. Example – gold sol, silver sol etc.

• Peptization – Peptization is 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.

Purification of Colloidal Solution – Colloidal solutions generally contain some impurities like extra amount of electrolytes etc. The process used for reducing the amount of impurities to a requisite minimum is known as purification of colloidal solution. Dialysis, electrodialysis, ultrafiltration are the main methods which are used in the purification of colloids.

### Applications of Colloids

Colloids have various applications in many fields. Some uses of colloids are listed below –

• Colloids are used in foods and food industries at a large level. Many foods which we consume are actually colloidal in nature. Such as milk, cheese etc.

• Colloids have various applications in the medicinal field as well. Many medicines which we use are in the form of emulsions. Antibiotics such as penicillin and streptomycin are given in the form of colloidal solutions so that they can be absorbed by the human body easily.

• Colloids are used in water purification.

• Sewage water contains impurities like dirt, stool, urine etc. which are dispersed in water. Thus, forms a colloidal system. These can be removed by electrophoresis.

• Smoke is also a colloidal system of carbon particles in air. This can also be purified by electrophoresis.

• These are used in artificial rain as well.

• Rubber is obtained by colloidal solution called latex through coagulation.

• Treatment of skin of animals to get leather is called tanning. In the process of tanning colloids are used.

### Emulsions: What is Emulsion?

A mixture of two or more liquids that are normally immiscible is called Emulsion. Generally, students get confused between the terms - emulsion and colloid. The term emulsion is specifically used for mixtures in which dispersed phase and dispersion medium both are liquids. Although emulsion is a type of colloid, but it does not mean that all colloids are emulsions only. In colloidal solution it is not necessary that both dispersed phase and dispersion medium will always be liquids. For example, aerosol (Examples – Clouds, Fog etc.) is also a colloidal solution in which dispersion medium is gas and dispersed phase is liquid. The term emulsion comes from Latin word ‘emulgere’ which means to “to milk out”.

Examples of Emulsions – Milk, mayonnaise, hand creams (lotions), Latex, cutting fluid, vinaigrettes etc. are examples of emulsions.

### What is Emulsification?

The process of formation of emulsions is called emulsification. In this process, one immiscible liquid is dispersed in another immiscible liquid. Thus, we can say to emulsify two immiscible liquids is called emulsification. For example, in oil – water cutting fluid emulsion used for metalworking is formed by emulsifying oil in the water medium.

### Mechanism of Emulsification

Many different chemical and physical processes and mechanisms can be involved in the process of emulsification. Mechanism of emulsification can be based on following three theories –

• Surface Tension Theory – According to surface tension theory, emulsification takes place by reduction of interfacial tension between dispersed phase and dispersion medium.

• Repulsion Theory – According to repulsion theory repulsion force between the particles of the dispersed phase cause them to remain dispersed in the dispersion medium. The emulsifying agent makes a film over one phase which makes globules of that phase and these globules repel each other.

• Viscosity Modifications – Some emulsifying agents increase the viscosity of the medium. Due to increase in the viscosity of the medium, globules of dispersed phase remain dispersed in dispersion medium.

### Properties of Emulsions

Properties of emulsions are listed below –

• Emulsions contain both a dispersed phase and a dispersion medium.

• The boundary between the dispersion phase and dispersed medium is called “interface”.

• They have cloudy appearance.

• They show various colors depending on the dilution. Such as, emulsion appears white if it scatters the light equally. If it is diluted it will appear blue while if it is concentrated, then it will appear yellow.

• It shows the Tyndall effect.

• Particle size of dispersed phase in emulsions may vary.

• Generally, emulsions are inherently unstable, exposure to energy and power ultrasound is needed to form a stable emulsion.

• Emulsion particles form dynamic inhomogeneous structures on a small length scale.

• Both the phases of emulsion may get separated, If it is kept undisturbed for a longer period of time or in absence of an emulsifying agent.

### Types of Emulsions

Broadly, Emulsions can be divided into two types –

• Simple Emulsions

• Complex Emulsions

Simple Emulsions – Simple emulsions are those emulsions which are formed by either dispersing oil in water or water in oil. Simple emulsions can be divided into following two types –

• Water in oil emulsion

• Oil in water emulsion

Water in Oil Emulsion – If dispersed phase is water and dispersion medium is oil in the emulsion, then these types of emulsions are called water in oil emulsions. It is also called W/O types of emulsions.

In these types of emulsions, water is an internal phase and oil is an external phase. Cold cream, butter etc. are examples of water in oil emulsions.

Oil in Water Emulsion – If dispersed phase is oil and dispersion medium is water in the emulsion, then these types of emulsions are called oil in water emulsions. It is also called O/W types of emulsions.

Complex Emulsions – Complex emulsions are also called multiple emulsions. In these types of emulsions, a complex system exists in which both oil in water and water in oil emulsion exist together and are stabilized by surfactants. These can be divided into following types –

• Water - in – oil – in - water emulsion

• Oil – in – water – in – oil emulsion

Water - in – Oil – in - Water Emulsion – These are also called W/O/W emulsion. In these types of emulsions oil droplets enclosing water droplets are dispersed in water. These are actually double emulsions of O/W emulsion and W/O emulsion.