A proportionality constant between the molar flux as the result of the molecular diffusion of one substance in a mixture and the driving force. The driving force is a concentration gradient across the mixture. The greater the value, the greater the diffusion of the substance into the other. The use of subscripts such as D_AB represents the diffusivity of substance A in substance B. The diffusivity of gases is dependent on temperature, pressure, and the nature of the gas components. The diffusivity of liquids, however, varies appreciably with concentration. For gases, the coefficients are either obtained experimentally or are based on formulae that are based on kinetic theory. For liquids, empirical correlations are generally used. The SI units are m² s⁻¹. Typical values for the diffusivity of gases is in the order of 10⁻⁵ m² s⁻¹ and for liquids in the order of 10⁻⁹ m² s⁻¹. See fick’s first law of diffusion; fick’s second law.