Diffusion layer theory

Diffusion layer theory is a concept used to describe the process of drug dissolution from solid forms, like tablets or granules, into solution. It is particularly important to understand how drugs become available for absorption in the body after oral administration. The theory explains how a drug dissolves in a solvent (usually water or gastrointestinal fluids) and moves from the surface of the solid drug into the bulk solution.

Key Concepts of Diffusion Layer Theory:

1. Diffusion Layer Formation:

   • When a drug comes into contact with a solvent, a thin layer of saturated solution forms around the surface of the drug particle. This layer is called the diffusion layer or stagnant layer.
   • The drug molecules in this layer dissolve into the solvent, creating a concentration gradient between the diffusion layer and the surrounding bulk solution.

2. Concentration Gradient:

   • Within the diffusion layer, the concentration of the drug is highest at the surface of the solid drug and decreases as it moves further into the solvent.
   • This gradient drives the diffusion of drug molecules from the surface to the bulk solution, where the drug concentration is lower.

3. Dissolution Process:

   • Drug molecules move from the solid surface to the diffusion layer, and then they diffuse across this layer into the bulk solution.
   • The rate at which this process occurs is referred to as the dissolution rate.

4. Fick's First Law of Diffusion:

   • The dissolution process is governed by Fick’s First Law, which states that the flux of drug molecules is proportional to the concentration gradient across the diffusion layer.

5. Factors Influencing Dissolution:

   • The thickness of the diffusion layer (thicker layers slow down dissolution).
   • Solubility of the drug in the solvent.
   • Agitation or stirring (which can reduce the thickness of the diffusion layer).
   • Surface area of the drug (smaller particles have a larger surface area, increasing the rate of dissolution).

Mathematical Representation:

The Noyes-Whitney equation describes the rate of dissolution:

$$\text{dC/dt}=\frac{DA(C_s-C)}{h}$$

Where:

• dC/dt = rate of dissolution,
• D = diffusion coefficient of the drug,
• A = surface area of the drug,
• C_s = saturation concentration of the drug at the surface,
• C = concentration of drug in the bulk solution,
• h = thickness of the diffusion layer.

Application:

Understanding diffusion layer theory helps in designing drug formulations that optimize drug absorption, improving bioavailability and therapeutic effectiveness.