For example: Is "oxygen-hydrogen" binary mass diffusion coefficient affected by N2 gas existing in the same volume?
Kemalettin, the general answer is 'yes'. For an "n component" system there are n(n-1)/2 independent diffusion coefficients. Thus, for a 3 component system there are 3 independent diffusivities. The diffusion process for multicomponent systems are usually written by analogy to the binary case - that is, the flux of a given component is due to the concentration gradients of that component plus those of the other independent components. For a 3 component system, the flux of component "1" is then
J1 = -D11delc1-D12delc2. D11 is referred to as the primary diffusion coefficient and D12 is called a cross diffusion coefficient. Normally, D11 is similar in value to the binary diffusion coefficient while D12 is usually a smaller value, though this is not always true. Often, the cross-diffusion coefficients are set to zero for simplicity. There is also another formulation of multicomponent diffusion by Stefan and Maxwell.
There is a lot of information in the literature on this topic. You might start by googling Stefan-Maxwell.
I fully agree with the answer provided by Zoubair Boulahia. The diffusion coefficient depends on the density of the gas, its composition (collisional cross-sections of species...). However the diffusion coefficient for a given species in a mixture of two other gases will not in general be lower than in a the case of a binary diffusion. This depends on the collision cross section of the added gas and wether the third species is added in a way that the particle density of the components increases or not...A pragmatic approach to this kind of problems is provided by the book of Poling, Prausnitz...:
http://rushim.ru/books/physchemie/the-properties-of-gases-and-liquids.pdf
Using the book is somewhat tedious, but very practical.
Good luck, Mico
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