12247 IUPAC Gold Book - translational diffusion coefficient 10.1351/goldbook.12247 12247 current synonym: diffusion coefficient (in polymer science) 1 Proportionality constant relating the flux per unit area, \(J_{n}\), in unidirectional flow of amount, \(n\), of entities \(\ce{B}\) to their concentration gradient in the direction (\(x\)) of flow, \({\rm{d}}c_{\ce{B}}/{\rm{d}}x\), i.e., \(J_{n} = -D{\rm{d}} c_{\ce{B}}/{\rm{d}}x\). More generally, \(D\) is defined using vectorial flow as the proportionality constant, relating the flux per unit area, \(\boldsymbol{J_{n}}\) , of amount, \(n\), of entities \(\ce{B}\) to their concentration gradient, \(\boldsymbol{\nabla} c_{\ce{B}}\), i.e., \(J_{n} = -D \boldsymbol{\nabla} c_{\ce{B}}\). However, the more general definition is not needed in polymer science as experimental determinations of the translational diffusion coefficient all employ unidirectional flow. PAC, 2015, 87, 71. 'Definitions of terms relating to individual macromolecules, macromolecular assemblies, polymer solutions, and amorphous bulk polymers (IUPAC Recommendations 2014)' on page 98 (https://doi.org/10.1515/pac-2013-0201) https://goldbook.iupac.org/terms/view/12247/html https://goldbook.iupac.org/terms/view/12247/json https://goldbook.iupac.org/terms/view/12247/plain Citation: 'translational diffusion coefficient' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.12247 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using. 2026-06-13T16:05:00+00:00