Adsorption

Adsorption
Adsorption isotherms

1. Adsorption Dr. Hasan Tamuos Al- Azhar University of Gaza Faculty of Science Chemistry Department Preparation by: Mohammed H. Rida Supervised by: 23-3- 2015

2. 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 2

3. Adsorption and orientation at interfaces. ( Liquid-gas and liquid-liquid interfaces) 3

4. Liquid-gas and liquid-liquid interfaces Adsorption and orientation at interfaces: Adsorption of surface-active molecules as an orientated monolayer at air-water and oil-water interfaces. The circular part of the molecules represents the hydrophilic polar head group and the rectangular part represents the non-polar hydrocarbon tail. At the air-water interface, the hydrocarbon chains will tend to lie horizontally on top of the water surface at low coverage. 4

5. 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 interface favours an expansion of the interface; this must, therefore, be balanced against the tendency for the interface to contract under normal 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 5

6. 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 6

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8. The solid—gas interface. 8

9. 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. 9

10. Adsorbent and adsorbate The solid on the surface of which gas or a liquid molecules accumulate is called ‘ adsorbent ’. The substance (gas or liquid) whose molecules accumulate on the solid surface is called ‘ adsorbate ’. non-adsorbed gas is called ‘ adsorptive’ . The solid—gas interface. 10

11. - 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. 11

12. 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 -T  S it is evident that  H ads . must be negative - i.e. the adsorption of gases and vapours on solids is always an exothermic process. 12

13. Types of adsorption 1> Physical or Van der Waal’s adsorption 2> Chemical adsorption or chemisorption. The solid—gas interface. 13

14. 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 SO 2 and NH 3 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 of Adsorption 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 14

15. adsorption Solid adsorbent Absorbed gas molecules 15

16. The solid—gas interface. 16

17. 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 gravimetrically The volumetric method is mainly used for the purpose of determining specific surface areas of solids from gas (particularly nitrogen) 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 17

18. The solid—gas interface. 18

19. The solid—gas interface. The gravimetric method can be used for studying both gas and vapour sorption. Purified gas or vapour is introduced into an evacuated apparatus in which the sorbent sample is contained in a weighing pan, the pressure is noted and the extent of sorption is measured directly as the increase in the weight of the sorbent sample. An alternative gas-adsorption method , which does not require vacuum equipment, is that developed by Nelsen and Eggersten . a gas mixture containing the adsorbate (usually nitrogen) and a carrier gas ( usually helium ) is passed over the solid under test at room temperature. The gas flow into and out of the sample container is monitored by means of a pair of thermal conductivity detectors. When equilibrium has been established, the sample is cooled Owing to gas adsorption, the outlet stream is depleted for a time in adsorbate , the thermal conductivity detectors are thrown off balance and the amount of gas adsorbed can be measured in terms of the area under a peak on a recording potentiometer. 19

20. The solid—gas interface. ADSORPTION ISOTHERMS basics A dsorption 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 I n 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 20

21. Types of Adsorption Isotherms : 1- Freundlich Isotherm 2- Langmuir Isotherm 3- Temkin Isotherm 4- Brunauer , Emmett And Teller ( BET) Isotherm 21

22. 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 correctly established the relationship of adsorption with pressure at lower values, 22

23. Langmuir Adsorption isotherm Assumes independent sites, monolayer coverage The dynamic equilibrium is A(g ) + M(surface ) AM(surface )   23

24. Langmuir Adsorption isotherm 24

25. Temkin Isotherm takes into account all the indirect adsorbate adsorbate interactions on adsorption isotherms where c1 and c2 are constants, corresponds to supposing that the adsorption enthalpy changes linearly with pressure Temkin Isotherm 25

26. BET ISOTHERM Most 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 more than one molecule thick & which resembles a pure bulk liquid , V mon is the monolayer coverage vol., c is a constant - When c >> 1, the BET isotherm takes the simpler form: 26

27. The solid-liquid interface - Adsorption from solution 27

28. 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 between solute(s) and solvent or between the components of a liquid mixture for the adsorption sites. Consider , for example , a binary liquid mixture 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 . 28

29. 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) benzene from solution in methanol on to charcoal and, ( b) chloroform from solution in carbon tetrachloride on to charcoa 29

30. Isotherm equations, surface areas *In adsorption from solution, physical adsorption is far more common than chemisorption. However, chemisorption is sometimes possible; for example, fatty acids are chemisorbed from benzene solutions on nickel and platinum catalysts . * Solute adsorption is usually restricted to a monomolecular layer, since the solid-solute interactions, although strong enough to compete successfully with the solid-solvent interactions in the first adsorbed monolayer, do not do so in subsequent monolayers 30

31. 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 m of 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 experimentally less demanding than gas adsorption; however, the problems in interpretation are far greater. 31

32. References: 1- Introduction to Colloid and Surface Chemistry,4 th edition by Duncan J. Shaw 2- Physical Chemistry , 9 th edition by Peter Atkins 32