diffusion

Question 1

Define diffusion

Answer 1

The net movement of molecules from areas of high concentration to low concentration.
This is a spontaneous tendency of gas, liquid or dissolved solute to distribute itself uniformly over the space available to it.

Question 2

How is diffusion related to drug release?

Answer 2

Drugs are administered in formulations and must be release from these in order to act on the body.
Diffusion influences drug release from formulations as well as the rate of drug dissolution

Question 3

What is permeation? where can this occur?

Answer 3

Permeation is the process by which drugs are absorbed into, ditributed around, and eliminated from the body.
This occurs at e.g. the small intestine, in the sclera of the eye or in skin

Question 4

How is diffusion measured?

Answer 4

Diffusion is measured by quantifying molecular flux (J). This is the rate of molecular movement, and refers to how many molecules are travelling through a cross-sectional area per second.

Question 5

What is the formula for flux (J)?

Answer 5

J = dM/S x dt (g/cm^2)

• M: mass in g/mol
• S: cross sectional area in m^2
• T: time in s

Question 6

What is Fick’s first law and steady-state diffusion?

Answer 6

Diffusion under steady-state conditions means that concentration gradient does not change with time, but remains constant.
Fick’s first law is that flux is proportional to conc gradient.
J = -D (dC/dx)

Question 7

What is the stokes-einstein equation?

Answer 7

The Stokes-einstein equation tells us factors influencing the diffusion coefficient and hence flux, where;
D = kT/6πηr
k = boltzmann constant
T = temperature
η = viscosity
r = radius of molecule

Question 8

What factors effect the diffusion coefficient and hence diffusion?

Answer 8

• Intermolecular interactions of molecules will contribute to viscosity, so stronger interactions create a stronger barrier to diffusion. Bonds can be weakened by increasing temperature. As temperature increases, the diffusion coefficient increases. This means that flux will increase.
• As pressure increases, so does the viscosity of a system. This causes the D to decrease, hence decreasing flux
• chemical nature of the solute
• solute particle size; as size increases, the D decreases, and hence flux decreases.

Question 9

What is Fick’s second law and non-steady state diffusion?

Answer 9

Non-steady state diffusion means that concentration gradient is changing, meaning that flux will change.
Fick’s second law of diffusion is that the rate of conc change is proportional to the rate of conc gradient change.
dC/dt=D d^2 C/dx^2

Question 10

How does fick’s second law apply to steady state conditions?

Answer 10

Fick’s second law can describe steady-state and nonsteady state conditions. In steady state conditions, concentration gradient is constant so the rate of conc change is 0

Question 11

What is osmosis?

Answer 11

Osmosis is the diffusion of water (solvent) through a semi permeable membrane

Question 12

What happens in a hypotonic solutions?

Answer 12

This is a solution where there is a high conc of solute inside the cell compared to the surrounding environment, so water enters the cell and lyses it

Question 13

Describe an isotonic soltuion.

Answer 13

In an isotonic solution, there is an equal amount of solute inside the cell and in the solvent surrounding it. Water will flow in equal parts between the environment and the cell.

Question 14

Describe a hypertonic solution

Answer 14

This is a solution where there is a high conc of solute in the solvent compared to inside the cell. This causes water to leave the cell and enter the surrounding environment, leading to cell shrivelling

Question 15

What is dialysis in terms of diffusion

Answer 15

A seperation process based on unequal rates of passages of solutes and solvent through a semi-permeable membrane

Question 16

describe haemodialysis

Answer 16

A process where blood is filtered to exclude certain small molecules while preserving larger molecules (e.g. proteins).