Project Details Recommendation for the definition, preferred symbol for all transport properties

Project No.:
Start Date:
01 July 2014
End Date:


Propose definitions, preferred symbols and SI units for all of the transport properties and include them, together with a didactic description in an IUPAC Recommendation for potential incorporation in a new version of Chapter 2.15 of the Green Book. The work will involve the IATP (formerly the Sub-Committee on Transport Properties of Commission I.2 on Thermodynamics of IUPAC).


The IUPAC Green Book is the ultimate source of reference for the quantities, units and symbols used in physical chemistry. The growing interest in the transport properties of fluids in multi-component ionic systems, such as are often found in biological systems, exemplified by their treatment in Experimental Thermodynamics Volume IX, entitled Transport Properties of Fluids: Advances in Transport Properties (project 2013-003-3-100). During production of this volume it became clear that Chapter 2.15 of the Green Book requires expansion to include the definitions of a number of different diffusion coefficients as well as to remove ambiguity that has appeared in the publication of articles.

In a solution of n components there are (n – 1) independent interdiffusion fluxes and (n – 1)2 interdiffusion coefficients so that for a binary solution (for which n = 2) is described by a single interdiffusion coefficient D. The term interdiffusion refers to mass transport caused by gradients in chemical composition whereas intradiffusion refers to diffusion without mass transport in solutions of uniform chemical composition. For a binary mixture of water and hydrogen chloride there are intradiffusion coefficients for each of the ions in the solution and one interdiffusion coefficient for the binary molecular mixture. Each diffusion coefficient has a very different dependence on chemical composition.

A variety of definition and nomenclature has been adopted in the literature for these various diffusion coefficients and there is a significant lack of clarity and certainty about their usage as a result. This project sets out to provide definitions for the relevant coefficients as well as recommended notation for each diffusion coefficient. In particular, either clarification, definitions or recommended nomenclature, are required for all of the following:

  1. self-diffusion coefficients which are used for the intradiffusion coefficient of a pure substance;
  2. the interdiffusion coefficientsof a binary mixture, commonly called the mutual diffusion coefficients;
  3. the relationship of tracer diffusion to interdiffusion and intradiffusion;
  4. the use of term chemical interdiffusion, which is informative because it implies the mixing and mass transport of a different chemical component caused by gradients of chemical composition;
  5. commonly used diffusion coefficients that are neither interdiffusion coefficients nor intradiffusion coefficients that occur, for example, in dynamic light-scattering; and,
  6. the appropriate terms for the many diffusion coefficients that can be defined for the various ionic components of multicomponent electrolyte systems that include the relationships between diffusion coefficients, Onsager coefficients and the reciprocal relationships.

These definitions will ultimately make unnecessary the use of vague terminology such as translational diffusion, Fickian diffusion and Brownian diffusion.


Nov 2014 – Project announcement published in Chem. Int. Nov-Dec 2014, p. 23; DOI: 10.1515/ci-2014-0620

Dec 2020 update – A new project team has overtaken the responsibility for the project during the Boulder conference (20th Symposium on Thermophysical Properties) in 2018. Yet, the changes in the organization of the project became effective not before the beginning of 2019. In 2020, the Corona pandemic did not allow any travel activities for a project meeting and a revised plan targets completions towards the end of 2022.

Page last updated 14 Dec 2020