What is the difference between intradiffusion and interdiffusion? How one can differentiate between the intradiffusion and interdiffusion? What is the criteria which separates them? As both suggest the molecular motion under the concentration gradient.
 A: 
Interdiffusion (mutual diffusion) smooths out gradients in chemical composition by allowing random thermal motions to interchange different chemical substances.  During interdiffusion in an aqueous sucrose solution, for example, sucrose and water move in opposite directions.  Intradiffusion, by contrast, refers to the interchange of labeled and unlabeled species under conditions of uniform chemical composition.  An example would be the intradiffusion of sucrose when a 1.00 mol/L aqueous sucrose solution is brought into contact with another 1.00 mol/L aqueous sucrose solution in which a portion of the sucrose molecules are tagged with 14C.  Whereas inter diffusion is important in a wide range of of practical mass transport processes, intradiffusion coefficients provide fundamental information about solvation and the structure and dynamics of solutions.

--Simultaneous Measurement of Mutual Diffusion and Intradiffusion by Taylor Dispersion Derek G. Leaist and Ling Hao J. Phys. Chem. 1994, 98(17) pp 4702-4706.
