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In Situ Monitoring of Diffusion of Guest Molecules in Porous Media Using Electron Paramagnetic Resonance Imaging

Published: September 2nd, 2016



1Department of Chemistry, Universität Konstanz

A protocol for the in situ monitoring of the diffusion of guest molecules in porous media using electron paramagnetic resonance (EPR) imaging is presented.

A method is demonstrated to monitor macroscopic translational diffusion using electron paramagnetic resonance (EPR) imaging. A host-guest system with nitroxide spin probe 3-(2-Iodoacetamido)-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (IPSL) as a guest inside the periodic mesoporous organosilica (PMO) aerogel UKON1-GEL as a host and ethanol as a solvent is used as an example to describe the protocol. Data is shown from a previous publication, where the protocol has been applied to both IPSL and Tris(8-carboxy-2,2,6,6-perdeutero-tetramethyl-benzo[1,2-d:4,5-d′]bis(1,3)dithiole) methyl (Trityl) as guest molecules and UKON1-GEL and SILICA-GEL as host systems.

A method is shown to prepare aerogel samples that cannot be synthesized directly in the sample tube for measurement due to a size change during synthesis. The aerogel is attached to sample tubes using heat shrink tubing and a pressure cooker to reach the necessary temperature without evaporating the solvent in the process. The method does not assume a clearly defined initial distribution of guest molecules at the start of the measurement. Instead, it requires a reservoir on top of the aerogel and experimentally determines the influx rate during data analysis.

The diffusion is monitored continually over a period of 20 hr by recording the 1d spin density profile within the sample. The spectrometer settings for the imaging experiment are described quantitatively. Data analysis software is provided to take the resonator sensitivity profile into account and to numerically solve the diffusion equation. The software determines the macroscopic translational diffusion coefficient by least square minimization of the difference between the experiment and the numerical solution of the diffusion equation.

Porous materials play a major role in practical applications such as catalysis and chromatography1. By adding surface groups and adjusting the pore size and surface properties, the materials can be tailored to the desired application2,3. The functionality of the porous material crucially depends on the diffusion properties of the guest molecules inside the pores. In porous materials, a distinction must be made between the microscopic translational diffusion constant D​micro, which describes diffusion on a molecular length scale on one hand and the macroscopic translational diffusion constant Dmacro....

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Caution: Please consult all relevant material safety data sheets (MSDS) before use. Ethanol is harmful if swallowed or inhaled and it is flammable.

1. Optimize the Continuous Wave (CW) EPR Parameters

  1. Prepare 40 µl of IPSL in ethanol (p.a.) at a concentration of 1 mM.
  2. Take a pipette controller and fill a capillary with the IPSL solution to a filling height of 2 cm. Pull the solution 1 cm further into the capillary so that there is an air gap below the solution. Seal t.......

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A photo and schematic of an aerogel within the shrinking tube is shown in Figures 2a and 2b. The 2d EPR image in Figure 2c clearly shows the upper edge of the aerogel. The intensity of ρ1d within the sample tube above the aerogel is lower although the concentration of the spin probe is at least as high as within the aerogel. However, the sample depth perpendicular to the picture plane is much smaller due to the sm.......

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The protocol allows monitoring of the diffusion of paramagnetic guest molecules. A 1d imaging approach has been chosen because it allows for a higher time resolution compared to 2d or 3d imaging. The 1d approach requires a constant cross sectional area of the sample because the intensity of the obtained 1d image depends not only on the concentration but also on the cross sectional area of the sample. The method also requires that the EPR spectra of the spin probes within the samples only change in intensity but not in sh.......

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The authors thank Prof. Peter Imming and Diana Müller for synthesis of the Trityl spin probe and Prof. Sebastian Polarz, Martin Wessig and Andreas Schachtschneider for the synthesis of the porous materials. Financial support by the DFG (DR 743/7-1) and within the SPP 1570 is gratefully acknowledged.


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Name Company Catalog Number Comments
X-Band spectrometer Bruker E580
Spectrometer software Bruker Xepr 2.6b.108
gradient coil system Bruker E540 GCX2
imaging resonator Bruker TMHS 1007
micro-classic pipette controller Brand 25900
microcapillary ringcaps 50 µl Hirschmann 9600150 inner diameter 0.5 mm
EPR sample tube 2 mm inner diameter Bruker ER 221TUB/2
EPR sample tube 4 mm inner diameter Bruker ER 221TUB/4
heat-shrink tubing DERAY-IB DSG-Canusa 2210048952 4.8 mm/2.4 mm, 2:1, 95 °C - 200 °C
heat gun Bosch PHG 600-3
PTFE  band VWR 332362S width 12 mm
test tube length 16 cm, diameter 1.5 cm
beaker 250 ml, height 9 cm, diameter 7 cm
capillary tube sealing Fisher Scientific 02-678
pressure cooker, 3l with trivet Beem Vital-X-Press V2, F1000675
magnetic stirrer with heating element
ethanol (p.a.)
ethanol (techn.)
syringe Hamilton 1705 0.05 ml, custom length: 20 cm,
Pasteur capillary pipette length 23 cm
data analysis software homemade Available for download at Requires Matlab.
UKON1-GEL kindly provided by Prof. Sebastian Polarz, Martin Wessig and Andreas Schachtschneider  See references 16, 18, 19 for the synthesis

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