- Define Adsorption. Difference between physical and chemical adsorption.
- Explain how solid surfaces offer a site for adsorption of gases & dissoluted solute through concept of surface tension.
- Give thermodynamic considerations for adsorption process.
- Define adsorption isotherm. Explain Freundlich isotherm with equation & plot.
- What are the factors which affect adsorption of solute molecules from its solution?
- Comment : Adsorption is surface phenomenon & absorption is internal phenomenon.
- Explain positive and negative adsorption with example.
- Define desorption.
1. Define Adsorption. Difference between physical and chemical adsorption.
Weak van der Waals forces
Effect of temperature
Surface reaction proceeds
decreases as temperature increases Multilayer formation
above certain temperature
2. Explain how solid surfaces offer a site for adsorption of gases & dissolved solute through concept of surface tension.
3. Give thermodynamic considerations for adsorption process.
4. Define adsorption isotherm. Explain Freundlich isotherm with equation & plot.
In the study of adsorption, the amount of gas adsorbed per unit area or unit mass of solid is measured at different pressures of the gas. The study is usually conducted at constant temperature and graphs are plotted. These plots are called adsorption isotherms.
Freundlich isotherm : The relationship between pressure of the gas and amount adsorbed at constant temperature has been expressed by many equations. For Freundlich isotherm, the equation is:
|Adsorption isotherm for a gas on za solid. Freundlich adsorption isotherms. Curvilinear plot drawn as per equation|
The constants, ‘k’ and ‘n’, are evaluated from the results of the experiment and depend on the temperature and the nature of adsorbent and adsorbate. The constants can be easily obtained by converting the equation into logarithmic form
|Adsorption isotherm for a gas on za solid. Freundlich adsorption isotherms log-log plot drawn according to equation. Amount of ‘x’ of gas adsorbed per unit weight (m) adsorbent and ‘P’ is the equilibrium pressure.|
5. What are the factors which affect adsorption of solute molecules from its solution?
- Nature of adsorbent: The physicochemical nature of the adsorbent can have decisive impact on the rate and capacity for adsorption. Every solid material can be used as an adsorbent, but activated carbon and clays such as kaolin and bentonite have been used as particular adsorbents in pharmaceutical applications.
- Nature of adsorbate: The solubility of the adsorbate in the solvent from which adsorption takes place has an inverse relationship with the extent of adsorption (Lundelius’ rule). The forces between the adsorbate and solvent need to be broken for adsorption to occur. Thus, higher the solubility of the adsorbate in a solvent, the greater the forces and the smaller the extent of adsorption.
- Adsorbent—solute interaction: Adsorption of a solute from a dilute solution involves the breaking of bonds between the solute and the solvent molecules as well as the formation of bonds betweenthe solute and adsorbent molecules. As an example, the higher molecular weight solutes are usually more readily adsorbed than low molecular weight solutes. This is due to van der Waals forces of attraction, which increases with the size of molecules.
- Adsorbate concentration: The amount of adsorption increases with the increase in the concentration of solute at equilibrium until it reaches a limiting value. However, the relative amount of solute removed from the solution is greater in dilute solutions.
- Surface area of adsorbent: Adsorption is a surface phenomenon and the amount of solute adsorbed depends on the surface area available. Thus, reducing the particle Size of the adsorbent will increase the adsorption.
- Temperature: Physical adsorption is an exothermic process and thus a decrease in temperature will increase the extent of adsorption.
- Removal of adsorbed impurities: Removal of adsorbed impurities such as gases or moisture from the surface of solid adsorbent activates the active adsorption sites and increases the efficiency of adsorbents. This can be achieved by heating the adsorbent at high temperature (at 110°C for 1 hour).
- pH of the medium: pH of a solution influences the extent of adsorption since pH affects both the degree of ionization and the solubility of the adsorbate drug molecule. More ionized (i.e. polar) and soluble adsorbates adsorb much less than their unionized forms (i.e. lipophilic). Amphoteric adsorbates such as proteins are usually best adsorbed at the isoelectric point where the net charge of the adsorbate becomes zero, and at the lowest solubility.
6. Comment : Adsorption is surface phenomenon & absorption is internal phenomenon.
7. Explain positive and negative adsorption with example.
Certain molecules or ions when added to a liquid, they may modify the interface in different ways. When the added molecules move on their own accord to the interface, this process is called positive
adsorption or simply adsorption. Examples are surface active agents such as sodium lauryl sulfate, Tweens and triethanolamine. They partition in favour of interface and reduce the interfacial (or surface) tension.
|Influence of added substance on the surface tension of water|
Certain molecules prefer to remain in the bulk of solution. Such process is referred to as negative adsorption. Though they remain in the bulk, still they affect the interfacial properties. For example, when inorganic electrolytes (sodium chloride) are added to water, they marginIlly enhance be surface tension. The electrolytes remain in in the bulk and pull the solvent molecules on the surface through electrostatic interactions.