Published on May 18, 2010
BASICS OF DIFFUSION : BASICS OF DIFFUSION Presented by:- Gurpreet Arora M.Pharm.-1st Sem. Pharmaceutics M090400003 Chitkara University Baddi.(H.P) What is Diffusion : What is Diffusion Movement of molecules from an area of high concentration to an area of low concentration. Movement from one side of a membrane to another. Diffusion is a time-dependent process. Movement is based on kinetic energy (speed), charge, and mass of molecules Slide 4: Other types of diffusion Atomic diffusion, in solids. Eddy diffusion, incoarse-grained description of turbulent flow . Effusion of a gas through small holes. Electronic diffusion, resulting in an electric current called the diffusion current. Facilitated diffusion, present in some organisms. Slide 5: Heat equation, diffusion of thermal energy Gaseous diffusion, used for isotope separation Knudsen diffusion of gas in long pores with frequent wall collisions Momentum diffusion, ex. the diffusion of the hydrodynamic velocity field Osmosis is the diffusion of water through a cell membrane. Slide 6: Reverse diffusion, against the concentration gradient, in phase separation Rotational diffusion, random reorientions of molecules Diffusion gradient : A diffusion gradient The molecules are more densely packed on the left and so they tend to diffuse into the space on the right. This is a diffusion gradient Diffusion gradient Steady state diffusion : Steady state diffusion System said to be at steady state , if the conditions do not vary with time. dc/dt or dm/dt should be constant for diffusion. Fick’s First law : Fick’s First law Flux (J) → Flow / area / time [Atoms / m2 / s] Flux is directly proportional to the concentration gradient. Applied in the design of sustained and controlled release systems. Explain drug diffusion across the biomembranes. Slide 10: Fick’s I law No. of atoms crossing area Aper unit time Cross-sectional area Concentration gradient Matter transport is down the concentration gradient Diffusion coefficient/ diffusivity A Flow direction Slide 11: Fick’s first law Slide 12: Types of Transport Kirkendall effect : Kirkendall effect Materials A and B welded together with Inert marker and given a diffusion anneal Usually the lower melting component diffuses faster (say B) Higuchi’s equation for diffusion controlled drug release : Higuchi’s equation for diffusion controlled drug release Q= [De/T(2A-eCs)]Cst½ Where , Q= Amount of drug released per unit surface are at time t. D= Diffusion coefficient of drug in the membrane. Cs= Solubility of drug in the release medium. Slide 15: T= Tortuosity of matrix. e= Porosity of the matrix. A= Concentration of drug in matrix per unit volume. The assumptions made in deriving the above equation are: 1. A pseudo steady state is maintained during release 2. A>>Cs i.e excess solute is present Slide 16: 3. C=0 in solution at all times (perfect sink) 4. Drug particles are much smaller than those of the matrix 5. Diffusion coefficient remains constant 6. No interaction occurs between drug and matrix Osmosis : Osmosis Diffusion of water through a semi-permeable membrane Semi-permeable: permeable to solvents (WATER), but not to large molecules. High [water] to low [water] Dissolved molecules (i.e. glucose, starch) are called solutes. REMEMBER: Water = solvent Glucose, Starch = solutes Slide 19: Hypotonic Solution - One solution has a lower concentration of solute than another. Hypertonic Solution - one solution has a higher concentration of solute than another. Isotonic Solution - both solutions have same concentrations of solute. Applications : Applications Release of drugs from dosage forms diffusion controlled like sustained and controlled release products. Moleculer weight of polymers can be estimated from diffusion process. The transport of drugs from gastrointestinal tract, skin etc. can be predicted from principal of diffusion. Slide 21: Processes such as dialysis, microfiltration, ultrafiltration, haemodialysis, osmosis etc. use the principal of diffusion. Diffusion of drugs into tissues and excretion through kidney can be estimated through diffusion studies. Refrences : Refrences ‘SINKO .J PATRICK’ , “Martin’s physical pharmacy and pharmaceutical sciences” , 5th edition , pp no.301 to 337. ‘SUBRAMANYAM.C.V.S’ , “A text book of physical pharmaceutics” , pp no.-110 to 127.