Physical Pharmacy

Dr.yasser.A.El. AleemLecturer of pharmaceutics and industrial pharmacy

Diffusion and Dissolution

N.B The transport of particles ( molecules, atoms or ions )

continues until equilibrium is reached and there is a uniform concentration through the material. This occur through a barrier such as a polymeric membrane.

The difference between the region of high concentration andlow concentration is called concentration gradient.

Examples of Diffusion only for understand

1. Distribution of perfume where it is sprayed in one part of a room, yet soon you can smell it everywhere.

2. Making tea, where molecules from the tea cross the tea bag and spread out into the cup of water.

3. Shaking salt into water. The salt dissolves and the ions move until they are evenly distributed.

4. Lighting a cigarette, where the smoke spreads to all parts of a room.

5. A sugar cube in water, the sugar will dissolve and evenly sweeten the water without having to stir it.

Diffusion of drugs in solution

The rate of drug diffusion may be calculated from:

Fick's laws of diffusion

describe diffusion and were derived by Adolf Fick in 1855. They can be used to solve for the diffusion coefficient, D.

Fick's first law can be used to derive his second law which in turn is identical to the diffusion equation.

o Ficks first law

Gives ( shows ) the amount of mass transported across unit area

of a barrier in unit time).

The rate of diffusion of a solute molecules through a barrier is

proportional to the concentration gradient.

Rate of diffusion =

Where

dM/dt = rate of diffusion D: diffusion coefficient (constant for each solute at certain

temperature. Its units is cm2 /sec) A: surface area x: barrier thickness (Ch -Cl): concentration difference between higher drug

concentration (Ch) and lower drug concentration (Cl). Ficks first low of diffusion can express as follow

J= dm / dt = - D dC/dx

Where J = flux of a component

N.B

The negative sign indicate that the diffusion occurs from higher to lower concentration.

The values of D is affected by

temperature

pressure,

solvent properties

and the chemical nature of diffusing solute.

Diffusion increases with increasing temperature (as molecules move more rapidly),

and decreases with increasing pressure (which packs more molecules in a given volume, making it harder for them to move).

Ficks second low (gives the change in concentration).

dc/dt = D. d2 C/ dx2

The change in concentration with respect to time at a

particular region is proportional to

The change in the concentration gradient at that point

in the system.

Steady state diffusion

Ficks first low of diffusion;

The rate of diffusion of a solute molecules through a barrier

is proportional to the concentration gradient.

J= dm / dt = - D dC/dx

Where J = flux of a component

During diffusion,

the solute molecules diffuse from donor compartment ---

through barrier membrane ---to reach receptor

compartment, which is kept under sink conditions by---

constantly replacing the solution with fresh solvent to keep

the concentration in receptor compartment at low level.

Under these conditions, the diffusion concentration

falls in the left compartment and rises in the right

compartment ---until the system attains an

equilibrium based on the rate of removal of the

diffusion from the sink and the nature of the barrier.

After sufficient time,

the conc in the solutions at left and right of barrier

become constant with respect to time and the rate of

change of conc, dc/dt becomes zero

dc/dt = D.d2c/dx2 = 0

Thank you for listening!