Adsorption

5) Adsorption at solid-liquid interfaces. Factor affecting adsorption and pharmaceutical application of adsorption process.

5.1 Adsorption at solid-liquid interfaces

Adsorption is a process that occurs when gas or liquid solutes accumulates on solid or liquid surfaces and forming a molecular or atomic film. Solid or liquid surfaces that allow the accumulation of solutes on them is called as adsorbent whereas molecular or atomic film that form during the adsorption process called adsorbate.

Adsorption occurs due to bond deficiency on the surface, because the atoms are not wholly surrounded by other atoms. Adsorption can be classified into two, which are physisorption and chemisorptions. Physisorption or physical adsorption refers to the adsorption of solute molecules on the surface by Van der Waals interaction. Chemisorption or chemical adsorption is a type of adsorptions in which molecules adheres on the surface by chemical bond.

Surfactants also involved in adsorption at solid-liquid interface. Adsorbed surfactants form aggregates on the surfaces of solid. This can change the properties of interfaces. Adsorption of surfactant on solid surfaces can be used to increase dispersions and wetting.

Adsorption at solid-liquid interfaces can be further explains by using Langmuir isotherm. For liquids (adsorbate) adsorbed on solids (adsorbent), the Langmuir isotherm can be expressed as

m=Amaxk.c

1+ kc

where,

m= amount of adsorbate adsorbed per gram (or kg) of the adsorbent (mol.g-1 or mol.kg-1)

Amax= maximal amount of adsorbate per gram (or kg) of the adsorbent (mol.g-1 or mol.kg-1)

k= adsorption constant (mol-1.dm3

c= concentration of adsorbate in liquid (mol.dm-3)

5.2 Factors affecting adsorption

Almost all solid adsorb liquid molecules to some extent. However, there are several factors that could affect the amount of liquid molecules adsorb on solid surfaces. The factors that affect adsorption...