Comment on "Technical note: An assessment of the relative contribution of the Soret effect to open water evaporation" by Roderick and Shakespeare (2025)

Abstract. This comment addresses the definition of Fick’s 1^st law employed in the paper “Technical note: An assessment of the relative contribution of the Soret effect to open water evaporation" by Roderick and Shakespeare (2025), and defended by the authors during the on-line discussion phase of their manuscript’s peer review process. Based on precedence in chemical engineering literature, the authors argue "the complete equivalence of mass- and molar-based frameworks for describing diffusion". On the contrary, here a very simple example shows that the authors’ preferred molar-based framework neglects the key role of inertia in momentum conservation, violates Newton’s laws of motion, and leads to different conclusions with regard to isotopic discrimination. It therefore ought not be considered equivalent to the inertial framework that is consistent with the laws of physics.