Review questions answers

Review questions

8.1 Which of the following is NOT true for gas adsorption on a solid?

A.      Chemical adsorption is reversible

B.      Physical adsorption is based on weak van der Waals forces

C.      Chemical adsorption may require activation energy

D.      Chemical adsorption is specific to the substrate

E.       All of the above

8.2 What is the difference between absorption and adsorption? Compare physical and chemical adsorption.

8.3 What is adsorption isotherm? What are the types of adsorption iso-therms? What is the BET equation used for? What are its inherent assumptions in terms of nature of adsorption (physical or chemical) and molecules adsorbed (monomolecular or multimolecular)?

8.4 Why it is easy to measure the amount of adsorption of a pure gas but difficult to measure the adsorption of a pure liquid?

8.5 What is a wetting agent? What are the types of wetting agents used for formulation of pharmaceutical suspension?

8.6 Calculate the surface tension of a 2% w/v solution of a wetting agent that has a density of 1.008 g/cm³ and that rises 6.60 cm in a capillary tube having an inside radius of 0.02 cm.

8.7 The surface tension of an organic liquid is 25 ergs/cm², the surface tension of water is 72.8 ergs/cm², and the interfacial tension between the two liquids is 30 ergs/cm² at 20°C. What is the work of cohesion of the organic liquid and the work of adhesion between the liquid and water at 20°C?

Answers:

8.1 A. Chemical absorption is an irreversible specific process and may require activation energy, whereas physical adsorption is revers-ible and associated with van der Waals forces.

8.2 Adsorption is different from absorption, which implies penetration through organs and tissues. Physical adsorption is associated with van der Waals forces and is reversible. Removal of the adsorbate from the adsorbent is known as desorption. Physical adsorption is rapid, relatively weak, and nonspecific. Chemical adsorption (also known as chemisorption) is irreversible and in this the adsorbate is attached to the adsorbent by chemical bonds. Chemical adsorption is specific. It may require activation energy and therefore be slow, and only a monomolecular chemisorbed layer is possible.

8.3 The relationship between the amount of gas physically adsorbed on a solid and the equilibrium pressure or concentration is known as the adsorption isotherm. The isotherms are classified into five types. Both Freundlich and Langmuir isotherms are of type I, whereas BET is a type II isotherm. Type I isotherms show a fairly rapid rise in the amount of adsorption with increasing pressure, and adsorption is restricted to a monolayer. Type II isotherms are frequently encountered, and repre-sent multilayer physical adsorption on nonporous solids. They are often referred to as sigmoid isotherms. Isotherm IV is typical of adsorption onto porous solids. Types III and V isotherms are produced in a rela-tively few instances in which the heat of adsorption of the gas in the first layer is less than the latent heat of condensation of successive layers.

8.4 Because gases have negligible intermolecular attractions, whereas liquids have significant attractive forces between the liquid molecules.

8.5 A wetting agent lowers the contact angle and aids in displacing an air phase at the surface and replacing it with a liquid phase. There are three types of wetting agents used in suspension formulations: (1) surfactants, (2) hydrophilic colloids, and (3) solvents.

8.6 According to = 1/2 rhρg , γ = 1/2 ∗ 0 . 02 ∗ 6.60 ∗ 1.008 ∗ 981 = 65.3 dyn/cm

8.7 W_c = 2 γ_L = 2 ∗ 25 = 50 erg/cm²

W_a = γ _L + γ_S − γ_LS = 25 + 72. 8 − 30 = 67 .8 erg/cm²