Molecular diffusion

monitoring diffusion processes | quality and accessability of catalyst carriers |

mobility of educts in chemical industry


EPR-active spin probes allow investigation of porous material on a molecular level. A closer look at the peak's shape, breadth and symmetry reveals fundamental information about the molecule's surrounding: in bulk solution, at an adsorption site (chemical or physical), in a capillary, etc.

Application Example: Molecular diffusion in functionalized porous hosts

Molecular movements through porous materials determines the speed of many industrial processes, like heterogenous catalysis or deionization of water. The process of diffusion can be distinguished on three scales: macro-scale (>1 µm), meso-scale (<1 µm, >10 nm) and molecular scale (<10 nm).

Winter et al. (2016) characterize the last one to be decisive for the diffusion process. Yet, there is little understanding of it.

EPR experiments are carried out to investigate molecular processes and mechanisms. Its advantages are high spatial resolution, fast data acquisition speed and there is no signal coming from porous hosts or solvents. Diffusion coefficients have been calculated from ESR measurements.

The results indicate a much stronger influence of the porous host's functional groups than pore-size. Different guest-surface interactions are given as a possible reason, but altered mobility within the solvent is also assumed.


  • Martin Wessig, Martin. Spitzbarth, Malte Drescher, Rainer Winter, Sebastian Polarz (2016): Multiple scale investigation of molecular diffusion inside functionalized porous hosts using a combination of magnetic resonance methods. Physical Chemistry, Chemical Physics 17, 15976 - 15988