Adsorption

Dr. Hasan Tamuos

Al-Azhar University of Gaza

Faculty of Science

Chemistry Department

Preparation by:

Mohammed H. Rida

Supervised by:

23-3- 2015

Contents

Adsorption and orientation at interfaces.

(Liquid-gas and liquid-liquid interfaces)

The solid—gas interface.

- Adsorption of gases and vapours on solids

- Differentiation between adsorption and absorption

- Physical adsorption and chemisorption

- Measurement of gas adsorption

- Adsorption Isotherms basics

- Adsorption Isotherms types

The solid-liquid interface

- Adsorption from solution

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Adsorption and orientation

at interfaces.

(Liquid-gas and liquid-liquid interfaces)

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Liquid-gas and liquid-liquid interfaces

Adsorption and orientation at interfaces:Adsorption of surface-active molecules as an orientated monolayerat air-water and oil-water interfaces. The circular part of themolecules represents the hydrophilic polar head group and therectangular part represents the non-polar hydrocarbon tail. At theair-water interface, the hydrocarbon chains will tend to liehorizontally on top of the water surface at low coverage.

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The strong adsorption of such materials at surfaces or interfaces in the form of an orientated monolayer is termed surface activity.

Surface-active materials (or surfactants):consist of molecules containing both polar and non-polar parts.

The tendency for surface-active molecules to pack into an interfacefavours an expansion of the interface; this must, therefore, bebalanced against the tendency for the interface to contract undernormal surface tension forces.

If is the expanding pressure (or surface pressure) of an adsorbed layer of surfacant, then the surface (or interfacial) tension will be lowered to a value = o -

Liquid-gas and liquid-liquid interfaces

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the effect of lower members of the homologous series of normal fatty alcohols on the surface tension of water.

The longer the hydrocarbon chain, the greater is the tendency for the alcohol molecules to adsorb at the air-water surface and, hence,lower the surface tension.

Liquid-gas and liquid-liquid interfaces

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The solid—gas interface.

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Adsorption Absorption

adsorption is a process in which a substance takes another substance on its surface , i.e. they remain on the surface

Absorption is a process in which one substance penetrates into another substance, i.e. the molecules penetrate into the body of the solid.

The process is fast in the beginning and gradually becomes slower

It happens at an uniform rate.

At the end, molecules of the adsorbate are found only on the surface

At the end, absorbed molecules penetrate into the body of the solid.

The solid—gas interface.

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Adsorbent and adsorbate

• The solid on the surface of which gas or a liquidmolecules accumulate is called ‘adsorbent’.

• The substance (gas or liquid) whose moleculesaccumulate on the solid surface is called‘adsorbate’.

• non-adsorbed gas is called ‘adsorptive’.

The solid—gas interface.

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- Any solid is capable of adsorbing a certain amount of gas, the extent of adsorption at equilibrium depending on:temperature, the pressure of the gas and the effective surface area of the solid.

- The most notable adsorbents are, therefore, highly porous solids, such as charcoal and silica gel (which have large internal surface areas) and finely divided powders.

- The relationship at a given temperature between the equilibrium amount of gas adsorbed and the pressure of the gas is known as the adsorption isotherm

The solid—gas interface.

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The solid—gas interface.

When adsorption takes place, the gas molecules are restricted to two-dimensional motion. Gas adsorption processes are, therefore, accompanied by a decrease in entropy.Since adsorption also involves a decrease in free energy,

then, from the thermodynamic relationship,G = H-TS it is evident that Hads. must be negative - i.e. the adsorption of gases and vapours on solids is always an exothermic process.

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Types of adsorption

1> Physical or Van der Waal’s adsorption

2> Chemical adsorption or chemisorption.

The solid—gas interface.

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The solid—gas interface.Physical adsorption Chemical adsorption

Surface area of the adsorbent

The extent of the adsorption of gases on the surface is directly proportional to the surface area of the solid

The extent of the adsorption of gases on the surface is directly proportional to the surface area of the solid.

Nature of the gas Since the forces that bind the molecule are Van der Waal’s forces, is easily liquefiable gases like SO2 and NH3 are readily adsorbed .

Chemical adsorption is highly specific and is formed if only the adsorbed molecules are capable of forming a chemical bond with the surface.

Effect of Pressure Pressure is directly proportional to extent of adsorption.

Pressure is directly proportional to extent of adsorption only until the surface gets saturated ,after that pressure has no effect

Heat ofAdsorption

Since the Van der Waal’s forces is weak, the heat of adsorption is low

Since the chemical bond is much stronger, the heat of adsorption is much higher

Reversible Nature At high pressure adsorption takes place whereas decrease in pressure causes desorption

Decrease in pressure does not cause desorption as molecules are strongly bound to the surface.

Effect of temperature

Van der Waal’s force of attraction is weak at high temperature. Hence physical adsorption is favored at low temperatures

As any chemical reaction is fast at high temperature, chemisorption is favored at high temperature.

Thickness of the adsorbed layer

At low pressure unimolecular thick layer is formed on the surface. However at high pressure the multi-molecular thick layer is formed.

In chemisorption , there is only a unimolecular thick layer on the surface

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adsorption

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The solid—gas interface.

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The solid—gas interface.

The adsorption of a gas or vapour can be measured by admitting a known amount of the adsorbate into an evacuated, leak-free space containing the outgassed adsorbent. The extent of adsorption can then be determined either volumetrically or gravimetricallyThe volumetric method is mainly used for the purpose ofdetermining specific surface areas of solids from gas (particularlynitrogen) adsorption measurements. The gas is contained in a gas burette, and its pressure is measured with a manometer All of the volumes in the apparatus are calibrated so that when the gas is admitted to the adsorbent sample the amount adsorbed can be calculated from the equilibrium pressure reading. The adsorption isotherm is obtained from a series of measurements at different pressures.

Measurement of gas adsorption

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The solid—gas interface.

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The solid—gas interface.

The gravimetric methodcan be used for studying both gas and vapour sorption.Purified gas or vapour is introduced into an evacuated apparatus inwhich the sorbent sample is contained in a weighing pan, thepressure is noted and the extent of sorption is measured directly asthe increase in the weight of the sorbent sample.An alternative gas-adsorption method, which does not requirevacuum equipment, is that developed by Nelsen and Eggersten .a gas mixture containing the adsorbate (usually nitrogen) and acarrier gas (usually helium) is passed over the solid under test atroom temperature. The gas flow into and out of the samplecontainer is monitored by means of a pair of thermal conductivitydetectors. When equilibrium has been established, the sample iscooled Owing to gas adsorption, the outlet stream is depleted fora time in adsorbate, the thermal conductivity detectors are thrownoff balance and the amount of gas adsorbed can be measured interms of the area under a peak on a recording potentiometer. 19

The solid—gas interface.

ADSORPTION ISOTHERMS basics

Adsorption process is usually studied through graphs known as: Adsorption Isotherm. The amount of adsorbate on the adsorbent as a function if its pressure (gas) or concentration (liquid) at constant T

In the given Adsorption Isotherm , after saturation pressure Ps adsorption does not increases

Vacancies on the surface of the adsorbent are limited, at Ps a stage is reached when all the sites are occupied and further increase in pressure does not increases adsorption extent

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Types of Adsorption Isotherms :

1- Freundlich Isotherm

2- Langmuir Isotherm

3- Temkin Isotherm

4- Brunauer, Emmett And Teller (BET) Isotherm

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Empirical equation for representing the isothermal variation of adsorption of a quantity of gas adsorbed by unit mass of solid adsorbent with pressure is Freundlich Adsorption Isotherm

Freundlich Adsorption Isotherm

x/m = adsorption per gram of adsorbent which is obtained by dividing the amount of adsorbate (x) by the weight of the adsorbent (m).P is Pressure, k and n are constants whose values depend upon adsorbent and gas at particular temperature .

Taking the logarithms of Freundlich Isotherm

• Freundlich Isotherm correctlyestablished the relationship ofadsorption with pressure at lowervalues,

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Langmuir Adsorption isotherm

Assumes independent sites, monolayer coverage

The dynamic equilibrium is A(g) + M(surface)⇌ AM(surface)

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Langmuir Adsorption isotherm

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Temkin Isotherm takes into account all the indirect adsorbateadsorbate interactions on adsorption isotherms

where c1 and c2 are constants, corresponds to supposing that the adsorption enthalpy changes linearly with pressure

Temkin Isotherm

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BET ISOTHERMMost widely used isotherm dealing with multilayer adsorption is BET Isotherm-In physical adsorption , if initial adsorbed layer can act as substrate instead of the isotherm levelling off to some saturated value at high pressures, it can be expected to rise indefinitely- Under the condition of high P & low T , thermal energy of gaseous molecules decreases & more & more gaseous molecules would be available per unit surface area of adsorbent and this leads to multilayer adsorption

p* is the vapour pressure above a layer of adsorbate that is morethan one molecule thick & which resembles a pure bulk liquid ,Vmon is the monolayer coverage vol., c is a constant-When c >> 1, the BET isotherm takes the simpler form:

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The solid-liquid interface

- Adsorption from solution

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The theoretical treatment of adsorption from solution, however, is,in general, more complicated than that of gas adsorption, since adsorption from solution always involves competition betweensolute(s) and solvent or between the components of a liquid mixturefor the adsorption sites. Consider, for example, a binary liquidmixture in contact with a solid. Zero adsorption refers to uniform mixture composition right up to the solid surface. If the proportion of one of the components at the surface is greater than its proportion in bulk, then that component is positively adsorbed and, consequently, the other component is negatively adsorbed.

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apparent adsorption isotherms are, therefore, calculated from changes in solution concentration.Examples of apparent adsorption isotherms for binary liquid mixtures are given in the next Figure.

Composite (surface excess) isotherms for the adsorption of (a) benzenefrom solution in methanol on to charcoal and,

(b) chloroform from solution in carbon tetrachloride on to charcoa

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Isotherm equations, surface areas

*In adsorption from solution, physical adsorption is far more commonthan chemisorption. However, chemisorption is sometimes possible;for example, fatty acids are chemisorbed from benzene solutions onnickel and platinum catalysts.

* Solute adsorption is usually restricted to a monomolecular layer,since the solid-solute interactions, although strong enough tocompete successfully with the solid-solvent interactions in the firstadsorbed monolayer, do not do so in subsequent monolayers

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The Langmuir and Freundlich equations are frequently applied to adsorption from solution data, for which they take the form:

respectively, where x is the amount of solute adsorbed by a mass mof solid, c is the equilibrium solution concentration, and a, k and n are constants , (x/m)max monolayer capacity .

Adsorption from solution has the merit of being experimentallyless demanding than gas adsorption; however, the problems ininterpretation are far greater.

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References:

1- Introduction to Colloid and Surface Chemistry,4th edition by Duncan J. Shaw

2- Physical Chemistry , 9th edition by Peter Atkins

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