Absorption of non-ionized drugs takes place through Passive diffusion.
What is Passive Diffusion?
It is also called as Non-ionized diffusion. It is the major process for absorption of more than 90 % of drugs. Driving force for this process is Concentration & Electrochemical gradients. It is defined as the difference in the drug concentration on the either side of the membrane. Drug movement is the result of kinetic energy of the molecules. Since no energy source is required , therefore the process called as passive diffusion. During passive diffusion of drug present in the aqueous solution at the absorption site partition & dissolve in the liquid material of the membrane & leaves it by dissolving again in the aqueous medium, this time at the inside of the membrane.
Passive diffusion is best explained by FICK’S First law of Diffusion, which states that drug molecule diffuse from higher conc. to one of the lower conc. untill equillibrium is attained & the rate of diffusion is directly proportion to the conc. gradient across the membrane. It can be mathemetically expressed by the following equation.
D is the diffusion coeffiicient or diffusibility.
x is the position in dimension of lengths.