Gibbs adsorption isotherm

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  • 1. Introduction Characteristics and factors affecting adsorption Adsorption isotherm Gibbs adsorption equation Discussions of gibbs adsorption equation Experimental results Conclusion References
  • 2.  “Adsorption is a technical term coined to denote the taking up of gas, vapour, liquid by a surface or interface”.  Adsorption is a surface phenomenon where the surface of a solid has a tendency to attract and to retain molecules of other species(gas or liquid) with which such surfaces come in contact.  Absorption is a bulk phenomenon in which the substance assimilated is uniformly distributed throughout the body of a solid or liquid to form a solution or a compound.  When adsorption and absorption takes place simultaneously is generally termed as sorption.  Water vapour is absorbed by anhydrous calcium chloride while it is absorbed by silica gel.  Ammonia is adsorbed by charcoal while it is adsorbed by water to form ammonium hydroxide.
  • 3. Characteristics • It is a spontaneous process and takes place in no time. • The phenomenon of adsorption can occur at all surfaces. • It is accompanied by a decrease in the free energy of the system. Factors Affecting • Nature of the adsorbent and adsorbate. • Surface area of the adsorbent. • The partial pressure of the gas in the phase. • Effect of temperature.
  • 4.  Adsorption isotherm (also A sorption isotherm) describes the equilibrium of the sorption of a material at a surface (more general at a surface boundary) at constant temperature.  It represents the amount of material bound at the surface (the sorbate) as a function of the material present in the gas phase and/or in the solution.  If the temperature is kept constant and pressure is changed, the curve between a and b is known adsorption isotherm. Where, a = amount of adsorbed p = pressure T = temperature a = f (P, T)
  • 5. This equation represents an exact relationship between the adsorption and change in surface tension of a solvent due to presence of a solute. This equation was derived by J. Willard Gibbs (1878) and afterwards independently by J.J Thomson , 1888. The dG for 2 comonent system is given by: dG = -SdT + Vdp + µ1dn1 + µ2dn2 + ϒdA ① Where, ϒ = Surface tension dA = Increase in surface area S = Entropy p= Pressure V = Volume
  • 6. dG = Change in Gibbs free energy Integrating equation no. ① at a constant temperature, pressure, surface tension and chemical potential of the component we obtain the expression G = µ1 n1 + µ2 n2 + ϒA ② Where, n1 = Number of moles of solvent. n2 = Number of moles of solute. Complete differential of Eq. ② dG = µ1 n1 + µ2 n2 + n1dµ1 + n2d µ2 + µdA + Adϒ ③ Comparing eq. no. ① & ③ the result is: SdT - Vdp + n1dµ1 + n2d µ2 + µdA + Adϒ = 0 ④
  • 7. At constant temperature and pressure equation no. ④ simplifies to n1dµ1 + n2d µ2 + Adϒ = 0 ⑤ We can imagine the system under consideration made of 2 portion: • Surface phase – It involves the portion of system affected by the surface process and therefore equation no. ⑤ holds true only for it. • Bulk Phase – The remainder of the solution which s unaffected by surface forces is known as bulk phase and therefore Gibbs Duehem equation holds for this only. This Equation is n1 0dµ1 + n2 0d µ2 = 0 ⑥
  • 8. Where On multiplying equation ⑥ by n1/n1 0 and subtracting from equation no. ⑤ we obtain the expression Adϒ + (n2 – n1n2 0/n1 0) dµ2 = 0 “Or” -dϒ/dµ = [n2 - n1n2 0/n1 0]/A ⑦ Where, n2 Represents no. of moles of solute associated with n1 moles of solvent in the surface phase and n1n2 0/ n1 0 is the corresponding quantity in the bulk phase. It therefore, follows that the quantity [n2 - n1n2 0/n1 0]/A is the excess concentration of the solute per unit area of the surface and is usually designated by the symbol ‘ᴦ’ Thus equation no. 7 becomes as ᴦ= -dϒ/ dµ2 ⑧
  • 9. Where ᴦis independent of n1 and is dependent only on the nature of surface phase and not on its amount. ᴦis also called the surface concentration of solute per unit area of interface . For a solution : µ2 = µ2 0 + RT ln a2 ⑨ Where, a2 is the activity of solute By differentiating equation no. ⑨ we get: dµ= RT d ln a2 ⑩ Assuming µ2 0 as a constant On substituting eq. no. ⑩ in eq. no. ⑧ we get, ᴦ= 1/ RT ᴦdϒ/d ln a2 or ᴦ= -a2/ RT ᴦdϒ/da2 [Since d ln a2 = da2/a2 ] ⑪ Equation no. ⑪ is known as Gibbs Adsorption Equation.
  • 10. A2 is proportional to c2 So , a2 = fc2 (f = Activity coefficient) From equation no. ⑪ we have: When dϒ/dc is negative, the adsorption is positive. It means that further addition of solute lowers the surface tension of the solution. When dϒ/dcis positive, the adsorption is negative. It means that further addition of the solute increases the surface tension of the solution.
  • 11. Surface tension for the interface between air and aqueous solutions generally display one of the three forms indicated schematically in the given figure
  • 12. • Curve 1 in the fig. is the type of behavior characteristic of most of the un-ionized compounds. The type of behavior indicated by curve 1 indicates positive adsorption of the solute. since dϒ/dc and dϒ/d ln c are negative, T must be positive. The curve 1 corresponds to relatively dilute solutions. • Curve 2 is typical of inorganic electrolytes and highly hydrated organic compounds. the positive slope for curve 2 indicates a negative surface excess or a surface depletion of the solute. Magnitude of negative adsorption is also less than that of positive adsorption. • The type of behavior indicated by Curve 3 is shown by soluble amphipathic species, especially ionic ones. The break in curve 3 is typical of these compounds ; however this degree of sharpness is observed only highly purified compounds. If impurities are present, the curve will display a slight dip at the point
  • 13. Gibbs adsorption isotherm for multi component system is an equation used to relate the changes in concentration of a component in contact with a surface with changes in the surface tension. This equation represents exact relationship between adsorption and change in surface tension of a solvent due to the presence of solute. This equation is corresponds to relatively solute solutions, highly hydrated organic compounds, amphipathic species.
  • 14. Source: • Advanced physical chemistry Gurdeep Raj, Krishna Prakashan Media, 2012. • Advanced physical chemistry Gurtu-Gurtu, Pragati Prakashan, Meerut, 2011. Net Source: •