Surface Chemistry B.Sc. 2nd Year Notes

Surface Chemistry B.Sc. 2nd Year Notes

Surface Chemistry

Adsorption is a surface phenomenon where particles (i.e. molecules, atoms or even ions) of a gas, liquid or solid in a dissolved state are attached(accumulate) at the top layer(surface) of solid material.
The substance whose molecules get adsorbed at the surface is called the adsorbate. while the substance on whose surface the process takes place is called the adsorbent.

Types of Adsorption
Depending upon the nature of forces which hold the molecules of the adsorbate on the surface of the adsorbent, the adsorption is classified into two types.
1. Physical adsorption
2. Chemical adsorption

1. Physical Adsorption
When the particles of the adsorbate are held to the surface of the adsorbent by the van der Waals forces, the adsorption is called physical adsorption or Physisorption.
For example- H2 and N2 gases adsorb on coconut charcoal.
The attractive forces are weak so, these forces can be easily overcome either by increasing the temperature or by decreasing the pressure. Physical adsorption can be easily reversed or decreased.
For example- Formation of iron nitride on the surface when the iron is heated in N2 gas at 623 K.>
Characteristics of physical adsorption or Physisorption
1. Physisorption is caused by physical forces.
2. Physisorption is a weak surface phenomenon.
3. Physisorption is a multi-layered process.
4. Physisorption is not specific and takes place all over the adsorbant.
5. Surface area, temperature, pressure, nature of adsorbate effects physisorption.
6. Energy for activation is low (20 – 40 kg/mol).

2. Chemical adsorption
When the molecules of the adsorbate are held to the surface of the adsorbent by the chemical forces or chemical bonds the adsorption is called chemical adsorption or chemisorption.
Characteristics of chemical adsorption or chemisorption
1. chemisorption is caused by chemical forces.
2. chemisorption is a very strong process.
3. chemisorption is almost a single-layered phenomenon.
4. Chemisorption is highly specific and takes place at reaction centres on the adsorbant.
5. Surface area, temperature, nature of adsorbate effects chemisorption.
6. Energy of activation is very high 40 – 400 kJ/mol.

Postulates or Assumptions of Langmuir Adsorption Isotherm
The main postulates or assumptions of Langmuir Adsorption Isotherm are as follows-
1. Adsorption of adsorbate molecules takes place only on fixed number of adsorption sites that are available on the surface of solid adsorbent.
2. Adsorption is a process of sticky collision. This means when a gas molecule hits a vacant site on the adsorbent surface, it will get adsorbed; however, if it hits an occupied site, an elastic collision occurs and the gas molecule gets rebound. This implies that adsorption takes place only on a vacant site, where each such site can hold only one gas molecule.
3. All active sites on the adsorbent surface are energetically equivalent, i.e., they involve constant heat of adsorption.
4. The surface of the solid adsorbent is assumed to be completely flat and uniform on microscopic dimensions.
5. Under conditions of low pressure and moderately high temperature, a monomolecular layer of adsorbate molecules is formed on the adsorbent surface.
6. There are no interactions between the gas molecules that are getting adsorbent on the adsorbent surface; adsorption of gas molecules takes place independent of the occupation of the neighboring sites. These gaseous molecules are thus assumed tobehave ideally.
7. Dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules. Adsorption takes place on vacant sites and desorption takes place from occupied sites, till a state of equilibrium is attained.
At equilibrium, Rate of adsorption = rate of desorption.

Langmuir Adsorption Isotherm
Langmuir derived an adsorption isotherm on the basis of kinetic theory of gases. He considered that the gas molecules strike a solid surface and then adsorbed. Some of these molecules are evaporated or desorbed fairly rapidly. Thus eventually a dynamic equilibrium is set up between the two opposing processes.
The rate of adsorption of the gas on the adsorbent surface will be proportional to the rate at which gas molecules strike the surface.
According to the kinetic theory of gas, the rate of striking of gas molecules is proportional to the pressure of the gas (P) at constant temperature. If θ is the fraction of the surface covered by adsorbed molecules at any instance, then the fraction of uncovered surface is (1- θ). Therefore-
Rate of adsorption ∝ (1- θ)P
or, Rate of adsorption = K1 (1- θ)P
Rate of desorption ∝ θ
or, Rate of desorption = K2 θ
where K1 and K2 are rate constant
at equilibrium-
Rate of adsorption = Rate of desorption
or, K1 (1- θ)P = K2 θ
or, θ = K1P/(K1P + K2)
The mass w of the gas adsorbed per unit mass of adsorbent is directely proportional to fraction of surface covered(θ).
That is- w = K3 θ
where K3 is proportionality constant.
w = K1 K3P/(K1P + K2)
or, w = (K1 K3/K2)P/(K1/ K2)P +1
or, w = AP/(BP + 1)
where A and B are constant.
This relation is Langmuir Adsorption Isotherm.
and is written as-
P/w = 1/A + (B/A)P
Langmuir Adsorption Isotherm

If pressure (P) is very low-
BP/A may be ignored and the above equation may be written as-
w = AP
If pressure (P) is very high-
1/A may be ignored and the above equation may be written as-
w = A/B
Hence at low pressure, the amount of gas adsorbed is directely proportional to pressure(P) while at high pressure the mass adsorbed (w) reached a constant value (A/B) when the surface is completely covered with one molecule thick layer of gas. At this stage, adsorption is independent of pressure. Thus-
at low pressure-
w = KP'
and high pressure-
w = KPo
and at intermediate pressure-
w = KPn
where n = 0 and 1

Limitations of Langmuir Adsorption Equation
1. The Langmuir Equation is valid under low pressure only. The adsorbed gas has to behave ideally in the vapor phase. This condition can be fulfilled at low pressure conditions only.
2. Langmuir Equation assumes that adsorption is monolayer. But, monolayer formation is possible only under low pressure condition. Under high pressure condition the assumption breaks down as gas molecules attract more and more molecules towards each other.
3. Another assumption was that all the sites on the solid surface are equal in size and shape and have equal affinity for adsorbate molecules i.e. the surface of solid if homogeneous. But we all know that in real solid surfaces are heterogeneous.
4. Langmuir Equation assumed that molecules do not interact with each other. This is impossible as weak force of attraction exists even between molecules of same type.
5. The adsorbed molecules have to be localized i.e. decrease in randomness is zero (ΔS = 0). This is not possible because on adsorption liquefaction of gases taking place, which results into decrease in randomness but the value is not zero.

Hardy Schulze Rule
Hardy Schulze rule states that the amount of electrolyte required for the coagulation of a definite amount of a colloidal solution is dependent on the valency of the coagulating ion(i.e.Coagulating ion is the ion which has the charge opposite to the charge of the colloidal particles).
Greater the valency of the flocculating or the coagulating ion, the greater its power to facilitate the coagulation.
Coagulation power order of Al3+, Na+ and Ba2+ is Al3+ > Ba2+ > Na+.

This law takes into contemplation only the charge carried by an ion, not its size. The lesser the size of an ion, the more will be its polarizing control. Thus, Hardy-Schulze law can be modified in terms of the polarizing power of the flocculating ion. Thus, the modified Hardy-Schulze law can be stated as the greater the polarizing power of the flocculating ion added, the greater is its power to cause precipitation.

Preparation of Colloidal Solutions
Colloidal Solutions can be prepared by a variety of methods involving physical, chemical as well as some dispersion methods.
Chemical Methods of Preparation of Colloids
1. Oxidation Method
Colloidal solution of Sulphur (Sulphur Sol) is made to pass through an aqueous solution of sulphur dioxide. It can also be obtained by passing the gas through a solution of an oxidizing agent such as Br2-H2O as well as HNO3.
SO2 + 2H2S → 2H2O + 3S
H2S + 2HNO3 → 2H2O + 2NO2 + S
2. Double Decomposition Technique
When hydrogen sulphide(H2S) is passed through a solution of arsenious oxide(As2O3) in distilled water, we get a colloidal solution of arsenious chloride.
As2O3 + 3H2S → As2S3 + 3H2O
Physical Methods of Preparation of Colloids
These methods are employed for obtaining colloidal solutions of metals like Au, Ag, Pt etc.
1. Bredig’s Arc Method
An electric arc is struck between the two metallic electrodes placed in a container of water. The intense heat of the arc converts the metal into vapours, which are condensed immediately in the cold water bath. This results in the formation of particles of colloidal size. We call it as metal sol. e.g. gold sol.
Bredig’s Arc Method

2. Peptisation
Peptisation is the process of converting a freshly prepared precipitate into colloidal form by the addition of a suitable electrolyte. The electrolyte is called peptising agent.
For example when ferric chloride is added to a precipitate of ferric hydroxide, ferric hydroxide gets converted into reddish brown coloured colloidal solution because preferential adsorption of cations of the electrolyte by the precipitate.
When FeCl3 is added to Fe(OH)3, Fe3+ ions from FeCl3 are adsorbed by Fe(OH)3 particles. Thus the Fe(OH)3 particles acquire +ve charge and they start repelling each other forming a colloidal solution.

Purification of Colloidal Solution
When a colloidal solution is prepared it contains certain impurities. These impurities are mainly electrolytic in nature and they tend to destabilise the colloidal solutions. Therefore colloidal solutions are needed to purify. Colloidal solutions are purified by-
1. Dialysis
2. Electrodialysis
1. Dialysis
Dialysis is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane (parchment or cellophane membrane).
The process of dialysis is based on the fact that colloidal particles cannot pass through parchment or cellophane membrane while the ions of the electrolyte can. The colloidal solution is taken in a bag of cellophane which is suspended in a tub full of fresh water.
The impurities diffuse out leaving pure coloidal solution in the bag as shown in figure. This process of separating the particles of colloids from impurities by means of diffusion through a suitable membrane is called dialysis.
2. Electrodialysis
Generally dialysis process is a slow process so to speed up its rate, it is carried out in the presence of an electrical field. When the electric field is applied through the electrodes, the ions of the electrolyte present as impurity diffuse towards oppositely charged electrodes at a fast rate. The dialysis carried out in the presence of electric field is known as electrodialysis.

Properties of Colloidal Solutions
Optical Properties of Colloidal Solutions
Tyndall Effect
When a beam of light is passed through a colloidal solution(homogeneous solution) kept in dark, the path of the beam gets illuminated with blue colour. This effect was first observed by Faraday and later studied in detail by Tyndall and is termed as Tyndall effect and the path of the beam is known as the Tyndall cone. The Tyndall effect occurs due to the scattering of light by colloidal particles. This effect is not exhibited by a true solution due to the particles in the solution are too small to scatter light. So, Tyndall effect is used to distinguish between a colloidal solutions and true solutions.
Tyndal Effect
Electrical Properties of Colloidal Solutions
The colloidal particles are electrically charged and carry the same type of charge, either negative or positive. The dispersion medium has an equal and opposite charge. The colloidal particles therefore repel each other and do not cluster together to settle down.
When electric potential is applied across two platinum electrodes dipping in a colloidal solution, the colloidal particles move towards one or the other electrode. 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 as shown in figure.
When electrophoresis(i.e. movement of particles) is prevented by some suitable means, it is observed that the dispersion medium begins to move in an electric field. This phenomenon is termed electroosmosis.
Q. Which phenomenon occurs when an electric field is applied to a colloidal solution and electrophoresis is prevented
a. Reverse osmosis takes place
b. Electroosmosis takes place
c. Dispersion medium begins to move
d. Dispersion medium becomes stationary
Q. Movement of dispersion medium under the influence of electric field is known as
a. Electrodialysis
b. Electrophoresis
c. Electroosmosis
d. Cataphoresis

Gels are the type of colloids in which the dispersed phase is a liquid and the dispersion medium is a solid. A gel is a semi-solid that can have properties ranging from soft and weak to hard and tough and is defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state, although the liquid phase may still diffuse through this system. Cheese, jelly, boot polish are common examples of gel. Most of the commonly used gels are hydrophilic colloidal solution in which a dilute solution, under suitable conditions set as elastic semi solid masses. For example 5% aqueous solution of gelatin in water on cooling forms the jelly block.
Gels may shrink on keeping by loosing some of its liquid. This is known as syneresis or resetting on standing.
Gels are divided in two categories i.e. Elastic Gels and Non Elastic Gels.
Elastic gels are reversible. When partly dehydrated on loosing water, they change back into the original form on addition of water. The non elastic gels are not reversible.
Gels are useful in many ways. Silica, cheese, jelly, boot polish, curd are commonly used gels. Solidified alcohol fuel is a gel of alcohol in calcium acetate.

Emulsions are colloidal solutions in which both the dispersed phase and dispersion medium are liquids. However, the two liquids are immiscible.
Emulsions contain both a dispersed and a continuous phase, with the boundary between the phases called the "interface". Emulsions tend to have a cloudy appearance because the many phase interfaces scatter light as it passes through the emulsion. Emulsions appear white when all light is scattered equally. If the emulsion is dilute enough, higher-frequency (low-wavelength) light will be scattered more, and the emulsion will appear bluer – this is called the "Tyndall effect".
If the emulsion is concentrated enough, the color will be distorted toward comparatively longer wavelengths, and will appear more yellow. This phenomenon is easily observable when comparing skimmed milk (contains about 0.1% fat), which contains little fat, to cream, which contains a much higher concentration of milk fat. One example would be a mixture of water and oil.
Emulsion are of two types
1. Oil-in-water emulsion
In this type of emulsion, the dispersed phase is oil while the dispersion medium is water. Milk is an example of this type of emulsion, as in milk liquid fats are dispersed in water. Vanishing cream is another example.
2. Water-in-oil emulsion
In this type of emulsion, dispersed phase is water and dispersion medium is oil. Butter, cod- liver oil, cold creams are examples of this type.
The liquids forming emulsion i.e. oil and water will separate out on keeping as they are immiscible. Therefore an emulsifying agent or emulsifier is added to stabilise the emulsion. Soap is a common emulsifier.
The preparation of emulsion in the presence of an emulsifier is called emulsification.
Q. How does an emulsifier work ?
It is believed that an emulsifier gets concentrated at the interface between oil and water i.e. the surface at which oil and water come in contact with each other. It acts as a binder between oil and water.
Applications of Emulsions
Emulsions play a very important role in our daily life. Some of the common applications are given below-
1. The cleansing action of soap and synthetic detergents for washing clothes, bathing etc is based upon the formation of oil in water type emulsion.
2. Milk is an emulsion of fat in water. Milk cream and butter are also emulsions.
3. Various cold creams, vanishing creams, body lotions etc. are all emulsions.
4. Various oily drugs such as cod liver oil are administered in the form of emulsion for their better and faster absorption. Some ointments are also in the form of emulsions.
5. The digestion of fats in the intestine occurs by the process of emulsification.
6. Emulsions are used for concentrating the sulphide ores by froth flotation process. Finely powdered ore is treated with an oil emulsion and the mixture is vigorously agitated by compressed air when the ore particles are carried to the surface and removed.

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. 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).
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.