IS there a dependency of diffusion coefficient on equivalence ratio?

I have an assignment where I'm supposed to find Lewis number of deficit species for different equivalence ratios ( Ф ) of say H2 - air mixture. Lewis number is given by: Le= α/D where α is the thermal diffusivity and D is the mass diffusivity (or mass diffusion coefficient). Now I know for a fact that α doesn't depend on Ф. For some values of Ф air will be the deficit species and for some values H2 will be deficit according to that I'll either use the value of α for H2 or Air. But then D must depend on Ф right? that's how I'll get a variation of Le. But all the formulae that I know about calculating D don't depend on Ф. Any help??

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