Introduction:

This mainly illustrates the effect of surface on the chemical reactivity.

Surface, in the chemical sense, is a phase boundary. Geometrically a surface consists of an area however no thickness. Surface is the interface region where one phase ends and the other starts. Chemically, it is an area in which the properties differ from one phase to the other. The transition takes place over the distance of molecular dimensions. Therefore for a chemist, surface consists of a thickness that shrinks to zero in an ideal condition of a geometrical illustration. Here, we shall elaborate the properties of an interface which might be liquid or vapor, solid/liquid or solid/gas. We shall as well talk about adsorption of gases on solid surface. Such a discussion is significant since most of the chemical reactions in industry or in the biological systems occur on the interface.

Surface Tension of Solutions:

Liquids suppose a shape having minimum surface area. The bulk molecules have less energy than the molecules in the surface, as a molecule in the bulk interacts by larger number of molecules as compared to a molecule on the surface. We are aware that each and every system tries to suppose a state in which it consists of minimum energy. Therefore, a liquid strives to take a shape which consists of the least surface area. The shape supposed is spherical, as a sphere consists of the smallest surface to volume ratio. The force that opposes the rise in area of a liquid is termed to as surface tension. Surface tension is the force per unit length acting on the surface opposing the expansion of surface area. The other definition recommends that surface tension is the surface energy per unit area of the surface. Generally, the surface tension values reported are for the liquid-vapor interface in the presence of air.

Whenever we dissolve a solute in the solvent, then surface tension of the solution changes. A relationship could be derived to set up the fact that the concentration of the solute which lowers the surface tension would tend to be more on the surface of the solvent as compared to that in the bulk. This is the base of Gibbs adsorption isotherm, regarding which this part doesn't intend to provide a detailed discussion. Substances that form a remarkable lowering of interfacial tension are termed as 'surface active agents or surfactants'. We have observed one of the applications of the surfactants which are the cleaning action of detergents and soaps. The other application of surfactants is in the formation of surface films. A few insoluble substances like long-chain fatty acids and alcohols could spread on water surface to make a thin film. The formation of these surface films by employing long-chain alcohols has been useful in retarding the evaporation of water from the reservoirs.

Adsorption on Solids:

We are familiar that the molecules on the surface of a liquid undergo an inward pull. The liquid surface is therefore in a state of Unsaturation. The surface of a solid as well behaves in an identical way. In solid, the ions or the molecules at the surface of a crystal don't encompass all their valencies satisfied via union with other particles. These forces as well occur because of the fact that whenever a new solid surface is made by breaking a solid, a few inter-atomic bonds are broken and some of the valencies of surface atoms are left unsatisfied. As an outcome of such residual forces, the surface of the solid consists of a tendency to fascinate and retain available molecules and other particles towards it; such a condition is useful in reducing the surface energy of a solid. The molecules so fascinated are retained on the surface than in the bulk of the solid. This fact of higher concentration of a substance on the surface of a solid is known as adsorption. The substance attracted to the surface is known as adsorbate whereas the substance to which it is linked is known as adsorbent. For illustration, charcoal adsorbs acetic acid whenever kept in contact with it; here, acetic acid is adsorbate and charcoal is adsorbent.

Adsorption must be clearly differentiated from absorption. In case of absorption, the substance is not only retained on the surface however passes via the surface and is distributed al through the bulk of the solid. Therefore, anhydrous calcium chloride absorbs water to prepare a hydrate whereas acetic acid is adsorbed from its solution via charcoal. At times the term 'sorption' is utilized whenever there is a doubt whether a procedure is true adsorption or absorption.

Note: Porous substance is the substance including tiny opening via which fluids or air could pass.

The level of adsorption via charcoal can be raised by subjecting charcoal to a procedure of activation. It comprises heating of wood charcoal between 625K and 1275K in vacuum, steam, air, chlorine or carbon-dioxide. Throughout activation, hydrocarbons and other impurities are eliminated from charcoal leading thus to a big surface area for adsorption. The resultant substance is known as activated charcoal.

Adsorption of Gases by Solids:

The studies of adsorption of gases through solids are identical to those of the adsorption of liquids by solids. Here, we shall primarily study the adsorption of gases via solids.

However, all solids adsorb gases, the effects are not obvious unless the adsorbent is porous and consists of a very large area for a given mass. This is the reason why silica gel and charcoal, which encompass porous structure, are very efficient as adsorbing agents.

The amount of a gas adsorbed via a solid depends on:

a) The nature of gas and adsorbent

b) The surface area of the adsorbent

c) The pressure and temperature of the adsorbent-adsorbate system

The table illustrated below gives the volume of different gases adsorbed via 1.00 kg of charcoal at 288K. The volumes of gases have all been decreased to 273K and 1.013 x 10⁵ Pa pressure. This can be seen from the table that gases which can be liquefied simply are more readily absorbed.

Table: Adsorption of Gases on Charcoal

Gas     Volume adsorbed (m³)    Critical temperature (k)

H₂           4.7 x 10^-3                           33

N₂           8.0 x 10^-3                          126

CO          9.3 x 10^-3                          134

CO₂         4.8 x 10^-2                          304

HCI         7.2 x 10^-2                          324

H₂S         9.9 x 10^-2                          373

NH₃         1.8 x 10^-1                          406

The total amount of gas adsorbed rises by the surface area of the adsorbent. Throughout adsorption, equilibrium is established between the gas in contact by the solid and gas on the surface. An increase in temperature reduces the amount of gas adsorbed.

Adsorption Isotherms:

The amount of a substance adsorbed through an adsorbent temperature depends on the concentration or pressure of the adsorbate. For adsorption of a substance (that is, adsorbate) present in a solution via a solid adsorbent, Freundlich provide or proposed an empirical equation. This equation illustrates the relationship between the mass of the adsorbate (x) adsorbed via a particular mass (m) of the adsorbent and the equilibrium concentration of the adsorbate (c) in the solution at a specific temperature as illustrated below:

(x/m) = K_C^1/n

Here, 'K' and 'n' are constants. The above equation is a form of Freundich adsorption isotherm. By plotting log x/m against log c (figure described below), we can assess K and n. The values of K and n mainly depend on:

a) Nature of the adsorbate

b) Nature and particle size of adsorbent

c) Temperature

This is worth mentioning that as the particles size becomes smaller, the surface area rises very much. This raises the adsorbing capacity of an adsorbent.

The above equation could be slightly tailored to deduce the adsorption of a gas via a solid as illustrated below:

x/m = Kp^1/n

In this equation, p denotes the pressure of the gas adsorbate; other terms have the similar importance as illustrated in equation (x/m) = K_C^1/n

Fig: Graphical Representation of Freundlich Adsorption Isotherm

It will be noted that Freundlich isotherm ((x/m) = KC^1/n or x/m = Kp^1/n) is applicable only if the concentration or pressure of the adsorbate is low.

Langmuir Adsorption Isotherm:

Langmuir acquired a relationship for the adsorption of a gas through a solid. Langmuir began with the suppositions illustrated below:  

a) The adsorbed gas behaves preferably in the vapor phase; there are no repulsive or attractive forces among the gas molecules.

b) The surface of a solid is homogeneous and there is fixed number of adsorption sites. Each and every site consists of the similar attraction for the gas molecules.

c) Each adsorption site can adsorb just one molecule. A solid surface can't form a layer more than a single molecule in depth. In another words, the adsorption of a gas could lead merely to the making of Unimolecular layer on the solid.

d) There acquired an equilibrium between the condensation of gas molecules on the adsorbent and their desorption from it. The initial rate of condensation of gas molecules on the solid surface is high and it reduces as the surface available for the adsorption reduces. The desorption can take place via thermal agitation and the rate of desorption will based on the amount of solid surface covered through gas molecules. This will rise as the surface becomes more and more saturated. At a certain phase, the rates of condensation and desorption become equivalent and an equilibrium is established.

e) By using the above suppositions, Langmuir equation for adsorption can be derived as:

Note:  

Desorption is the procedure of discharge of the adsorbed molecules. Desorption might as well be known as the evaporation of the adsorbed molecules.

Suppose the fraction of the net surface covered by gas molecules is 0; then the surface available for adsorption is 1 to 0. According to the kinetic theory of gases, the rate at which the molecules strike the unit area of surface is proportional to the pressure of the gas. Whenever p is the equilibrium gas pressure, then, the rate of condensation is represented as:

Rate of condensation = a (1 - 0) p  

Or rate of condensation = k₁ (1 - 0) p

Here, k₁ is a constant of proportionality.

Rate of evaporation from the surface will be proportional just to the fraction of the surface that has adsorbed gas molecules on it. Therefore,

Rate of evaporation = K₂0

K₂ is as well proportionality constant.

At equilibrium, the rates of condensation and evaporation are equivalent.

Therefore,

K₁ (1 - 0) p = K₂0

Reorganizing the above equation, we can obtain,

0 = K₁P/(K₂ + K₁P) = (K₁/K₂)p/[1 + (K₁/K₂)p] (Dividing the numerator and Denominator by K₂)

0 = Kp/(1 + Kp) (where K₁/K₂ = K, the other constant)

The equation above could be altered to determine a relationship between the amount of gas adsorbed and the gas pressure. The amount of gas adsorbed (y) at a pressure 'p' is proportional to the fraction of the net surface covered (0) through the gas molecules.

That is, y α 0

Therefore, y = y_m0 or y/y_m = 0

Here, y_m is the proportionality constant and is equivalent to the amount of the gas molecules needed to make a Unimolecular layer; that is, y = y_m if 0 = 1

By using both the equations 0 = K_p/(1 + K_p) and y = y_m0 or y/y_m = 0

y/y_m = K_p/(1 + K_p)

y = y_m K_p/(1 + K_p)

Or p/y = (1 + K_p)/y_m K = 1/y_m K = p/y_m

The equation above is termed as Langmuir adsorption isotherm.

Whenever the gas pressure is low, (p/y_m­) is small whenever compared to (1/y_mK) therefore p/y = 1/y_m K or p/y = constant, as y_m and K are constants.

Or p α y

This signifies that at low pressures, the amount of gas adsorbed is proportional to the gas pressure.

If the gas pressure is high, p/y_m is much bigger than 1/y_mk. Therefore,

The equation can be represented as:

p/y = p/y_m

Or y = y_m that signifies that at high pressures, the amount of gas adsorbed is adequate to form a Unimolecular layer. A way to confirm Langmuir adsorption isotherm is to plot p/y against p. A straight line should be acquired.

Fig: Graphical representation of Langmuir Adsorption Isotherm

It is noticed that straight line plots are acquired if the surfaces are smooth and non-porous, and if the pressures are not too high. Under such conditions, equation (p/y = p/y_m) is followed. Deviations from the Langmuir adsorption isotherm are seen if;

a) Surface is porous (that is, a good adsorbent)

b) Pressure is extremely high.

Under such conditions, gas molecules give increase to multilayer adsorption on the solid surface that accounts for derivations from the equation (p/y = p/y_m). For describing multilayer adsorption, Brunauer, Emmet and Teller have introduced a model that is termed as BET isotherm.

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