2019-07 21/06/2019 ForsChem Research Reports 2019-07 (1 / 18) www.forschem.org On the Relationship between Molecular and Macroscopic Diffusion in Ideal Gases Hugo Hernandez ForsChem Research, 050030 Medellin, Colombia hugo.hernandez@forschem.org doi: 10.13140/RG.2.2.30954.98242 Abstract In this report, two different models of multicomponent diffusion in ideal gases are presented. The first model, introduced in a previous report, is derived from the probabilistic observation of individual molecules colliding with a specific molecular neighborhood. The second model is based on the macroscopic motion of a large number of molecules in one direction (molecular flux). Since both diffusion models are consistent with Fick’s laws of diffusion, it is proposed that the macroscopic diffusion coefficient is proportional to the molecular diffusion coefficient. It is also assumed that during the self-diffusion of a pure ideal gas, the symmetry of the collisions and the indistinguishability of individual molecules result in the equivalence between molecular and macroscopic diffusion coefficients. For multicomponent mixtures of ideal gases with different masses and sizes, the specific composition of the neighborhood influences both the molecular and macroscopic diffusion coefficients in different ways. It is also shown that each species has its own diffusion coefficients (macroscopic and molecular), which are a function of the composition of the mixture and the environmental conditions (e.g. pressure and temperature). Overall macroscopic diffusion coefficients of multicomponent systems can be obtained as a molar fraction weighted-average of the macroscopic diffusion coefficients for each species in the mixture. Significant differences with respect to conventional expressions used to estimate macroscopic diffusion coefficients of pure ideal gases and mixtures are found. However, some experimental results seem to be consistent with the proposed model of diffusion. Keywords Diffusion coefficients, Fick’s Laws, Ideal Gases, Molecular Flux, Multicomponent Systems, Self- diffusion