Type of adsorption- Pharmaeutical Physical Chemistry

Adsorption
Adsorption versus absorption, Desorption
Types of adsorption: Physisorption and Chemisorption
Factors affecting adsorption
Adsorption isotherms: Freundlich and Langmuir
Gibbs adsorption isotherm
Bet equation and its use in surface area determination
Applications
ADSORPTION

Adsorption is the process in which matter is extracted from one phase and concentrated at the surface of a second phase. (Interface accumulation). This is a surface phenomenon as opposed to absorption where matter changes solution phase, e.g. gas transfer. This is demonstrated in the following schematic.

1. Adsorption BY- SANCHIT DHANKHAR

2. Contents Adsorption Adsorption versus absorption, Desorption Types of adsorption: Physisorption and C hemisorption Factors affecting adsorption Adsorption isotherms: Freundlich and Langmuir Gibbs adsorption isotherm Bet equation and its use in surface area determination Applications 2

3. ADSORPTION A d sorption is the process in which matter is extracted from one phase and concentrated at the surface of a second phase. (Interface accumulation). This is a surface phenomenon as opposed to absorption where matter changes solution phase, e.g. gas transfer. This is demonstrated in the following schematic. 3

4. d 4

5. Adsorption The phenomenon of higher concentration of any molecular species at the surface than in the bulk 5

6. Adsorbent The substance on the surface of which adsorption takes place is called adsorbent Adsorbate The substance which is being adsorbed on the surface of another substance. 6

7. Activated Carbon Activated Alumina Silica Gel Molecular Sieves ( Zeolites ) Polar and Non-polar adsorbents Adsorbent Materials 7

8. Made from nutshells, wood, and petroleum, bituminous coal by heating in the absence of oxygen to dehydrate and carbonize (remove volatile components), "Activation" is the process that produces the porous structure essential for effective adsorption by oxidation of carbon with water vapor or CO 2 . Activated carbon attracts non-polar molecules such as hydrocarbons. Typical surface areas are 300 to 1500 m 2 /g. Activated carbon 8

9. Adsorption vs Absorption 9

10. Types of Adsorption Positive adsorption occurs when the concentration of adsorbate is higher on the surface of adsorbent than in the bulk. Negative adsorption occurs when the concentration of adsorbate is less on the surface of adsorbent than in the bulk. 10

11. Types of Adsorption Physical adsorption Chemical adsorption 11

12. Factors Affecting Adsorption Effect of adsorbate: The easily liquifiable gases like NH 3 , HCl, CO 2 etc. are adsorbed to a greater extent than the permanent gases such as H 2 , O 2 , N 2 , etc. Effect of specific area of the absorbent: The greater the specific area of the solid, the greater would be its adsorbing capacity. Effect of temperature : adsorption decreases with increase in temperature. Effect of pressure: An increase in pressure causes an increase in the magnitude of adsorption of an adsorbent. 12

13. Adsorption On Solid Surface Five types of physisorption isotherms are found over all solids Type I is found for porous materials with small pores e.g. charcoal. It is clearly Langmuir monolayer type, but the other are not Type II for non-porous materials Type III porous materials with cohesive force between adsorbate molecules greater than the adhesive force between adsorbate molecules and adsorbent Type IV staged adsorption (first monolayer then build up of additional layers) Type V porous materials with cohesive force between adsorbate molecules and adsorbent being greater than that between adsorbate molecules 13 I II III IV V relative pres. P/P 1.0 amount adsorbed

14. Over a narrow range of p Freundlich Isotherm A graph between the amount (x/m) adsorbed by an adsorbent and the equilibrium pressure of the adsorbate at constant temperature is called adsorption isotherm At low pressure the graph is nearly straight line At high pressure x/m becomes independent of p 14

15. Freundlich Isotherm 15

16. Rate of adsorption Rate of desorption At equilibrium, r a = r d ; Mono-layer coverage m : mass of adsorbate adsorbed per unit mass of adsorbent f : fraction of surface area covered f 1- f p : partial pressure of the adsorbate Langmuir Adsorption Isotherm 16

17. Langmuir adsorption isotherm: The values of constants ‘a’ and ‘b’ depend upon the nature of adsorbate, nature of solid adsorbent and temperature. a = k a x k a ’/k d b = k a /k d Combining equations (1) and (2): 17

18. Summary of adsorption isotherms Easy to fit adsorption data Chemisorptions and physisorption Freundlich Useful in analysis of reaction mechanism Chemisorption and physisorption Langmuir Note Application Isotherm equation Name 18

19. Adsorption Isobar Graph between the amount adsorbed(x/m) and temperature at a constant equilibrium pressure of adsorbate gas is known as adsorption isobar Chemisorption isobar shows an initial increase with temperature and then expected decrease .The initial increase is because of the fact that the heat supplied acts as activation energy required in chemisorption. 19

20. Adsorption On Solid Surface BET (Brunauer-Emmett-Teller) isotherm Many physical adsorption isotherms were found, such as the types II and III, that the adsorption does not complete the first layer (monolayer) before it continues to stack on the subsequent layer (thus the S-shape of types II and III isotherms) Basic assumptions the same assumptions as that of Langmuir but allow multi-layer adsorption the heat of ads. of additional layer equals to the latent heat of condensation based on the rate of adsorption=the rate of desorption for each layer of ads. the following BET equation was derived Where P - equilibrium pressure P - saturate vapour pressure of the adsorbed gas at the temperature P/P is called relative pressure V - volume of adsorbed gas per kg adsorbent V m - volume of monolayer adsorbed gas per kg adsorbent c - constant associated with adsorption heat and condensation heat Note: for many adsorption systems c =exp[( H 1 - H L )/ RT ], where H 1 is adsorption heat of 1st layer & H L is liquefaction heat, so that the adsorption heat can be determined from constant c . 20

21. Adsorption On Solid Surface Comment on the BET isotherm BET equation fits reasonably well all known adsorption isotherms observed so far (types I to V) for various types of solid, although there is fundamental defect in the theory because of the assumptions made (no interaction between adsorbed molecules, surface homogeneity and liquefaction heat for all subsequent layers being equal). BET isotherm, as well as all other isotherms, gives accurate account of adsorption isotherm only within restricted pressure range. At very low (P/P <0.05) and high relative pressure (P/P >0.35) it becomes less applicable. The most significant contribution of BET isotherm to the surface science is that the theory provided the first applicable means of accurate determination of the surface area of a solid (since in 1945). Many new development in relation to the theory of adsorption isotherm, most of them are accurate for a specific system under specific conditions. 21

22. Adsorption On Solid Surface Use of BET isotherm to determine the surface area of a solid At low relative pressure P/P = 0.05~0.35 it is found that Y = a + b X The principle of surface area determination by BET method: A plot of against P/P will yield a straight line with slope of equal to ( c -1)/( cV m ) and intersect 1/( cV m ). For a given adsorption system, c and V m are constant values, the surface area of a solid material can be determined by measuring the amount of a particular gas adsorbed on the surface with known molecular cross-section area A m , * In practice, measurement of BET surface area of a solid is carried out by N 2 physisorption at liquid N 2 temperature; for N 2 , A m = 16.2 x 10 -20 m 2 22 P/P V m - volume of monolayer adsorbed gas molecules calculated from the plot, L V T,P - molar volume of the adsorbed gas, L/mol A m - cross-section area of a single gas molecule, m 2

23. Application of Adsorption In clarification of sugar In gas masks In catalysis In adsorption indicators In chromatographic analysis In softening of hard water In preserving vacuum In paint industry In removing moisture from air in the storage of delicate instruments 23

24. Summary 24

25. THANKYOU 25