Pharmacy Notes: Solubilization And Detergency In Physical Pharmaceutics

Adsorption onto the solid surface of molecules dissolved in solution may occur if solute molecules and solid surface have chemical groups capable of interacting. In addition, non-specific or general adsorption may also occur if solute is surface active.

Adsorption from solution is usually more complex than from a gas because of solvent and other components present in the system. Adsorption from solutions may be measured by separation of solid and solution and estimating the amount of adsorbate adhering to solid or the loss in concentration of adsorbate from solution phase.

The solvent can complete for the site of adsorption, it would also determine. escaping tendency of solute i.e., more polar the molecules, less is the adsorption.

 The degree of adsorption of gas by solid depends on the chemical nature of

adsorbent (material used to adsorb the gas) and the adsorbate (substance being adsorbed), the surface area of the adsorbent, the temperature and partial pressure of the gas.

This type of adsorption called as physical adsorption is associated with Van der Waals forces. It is reversible and removal of adsorbate from adsorbent is called as desorption.

A physically adsorbed gas may be desorbed from a solid by increasing temperature and pressure. Chemiosorption is a process where adsorbate is attached to adsorbent by primary chemical bonds.

The relationship between the amount of gas physically adsorbed on a solid and equilibrium pressure or concentration at constant temperature yields an adsorption isotherm.

An isotherm is defined as a plot.at constant temperature. The number of moles, gm or ml 'x of gas adsorbed on 'm' grams of adsorbent at STP (standard temperature and pressure) is plotted on the vertical axis against the equilibrium pressure of the gas in mm of Hg on horizontal axis.

Adsorption data can be measured by an apparatus called as 'Quantasorb'. It essentially consists of a balance contained within a vacuum system. The solid previously degassed is placed on the pan and known amount of gas at a known pressure is allowed to enter.

The increase in weight at the corresponding equilibrium gas pressure is recorded. This may be achieved by extension of a calibrated quartz spring used to suspend the pan containing the sample.