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

The overall goal of this procedure is to monitor the diffusion of guest molecules in porus hosts. This method can help answer key questions in physical chemistry, such as unravelling the diffusion of different time and lengths simultaneously. The main advantage of this technique is that it captures the diffusion process in its entirety, and the extra initial boundary conditions are determined experimentally.

To begin this procedure fill a 10 centimeters diameter petri dish with ethanol up to a height of five millimeters. Place an aero gel into the petri dish, and cut off a cylindrical piece of five millimeters to one centimeter in length. Following this, cut a piece of heat shrink tubing that is about one centimeter longer than the cylindrical piece of aero gel.

Using a glass tubing cuter, break a two millimeter inner diameter glass sample tube into two four centimeter long pieces. Insert one of the sample tube pieces five millimeters deep into one end of the heat shrink tubing. Then use the heat gun to carefully heat the end of the shrink tubing without shrinking the rest of the tubing.

Next submerge the sample tube and the head shrink tubing in the petri dish containing the aero gel. Carefully put push the cylindrical piece of aero gel into the open end of the heat shrink tubing. If you try to hold the aero gel with your fingers or if you apply pressure while it is wet between the shrink tubing and the side of the petri dish, the aero gel will break.

Fill a test tube with ethanol, up to a height of seven centimeters. Insert the second four centimeter long piece of sample tube into the open end of the heat shrink tubing. Transfer the sample from the petri dish into a test tube, making sure the open end of the heat shrink tubing is oriented to the top.

After placing a pressure cooker on a magnetic stirrer. Add a stir bar and a trivet. Fill the pressure cooker with at least 500 milliliters of ethanol.

Place the beaker containing the sample on the trivet inside the pressure cooker. Cook and stir the sample at a pressure setting of one bar above ambient pressure. Allow the sample and pressure cooker to cool down as soon as the pressure is released and the pressure valve releases ethanol vapor.

At this point, place a finger on top of the sample to keep the ethanol from flowing out. After removing the sample from the test tube, use a syringe to remove some ethanol from the bottom of the lower sample tube, and seal that end with tube sealing compound. Using a Pasteur capillary pipet, remove all the ethanol from the sample tube except for 3 millimeters above the aero gel.

Next, inject 20 microliters of spin label solution in ethanol on top of the aero gel. Making sure to not create an air bubble. Mark the correct time as the start of the diffusion process.

Place the sample in a four millimeter inner diameter sample tube. Use ptfe tape to center the sample. Using a felt tipped pen, mark the outer sample tube at a position of 68 millimeters above the upper edge of the aero gel.

For the diffusion experiment, place the sample in a resonator so that the marking aligns with the top of the ptfe holder of the resonator. Finally, tune the spectrometer for critical coupling as is described in the operating manual of the spectrometer. Start the experiment and follow the text protocol to process the data.

The two dimensional epr image clearly shows the uper edge of the aero gel within the shrinking tube. The intensity of the one dimensional spin density distribution within the sample tube above the aero gel is lower. Although the concentration of the spin probe is at least as high as within the aero gel.

Diffusion heat maps of Trityl, and IPSL and Yukon one gel are shown here. The color code is proportional to the concentration. Each vertical slice can be seen as a snapshot of the concentration profile at a single point in time.

The numerical solutions for the diffusion equations that mapped the experimental data for Trityl and IPSL are displayed here. The heat maps show qualitatively that the macroscopic translational diffusion of Trityl is significantly slower than the macroscopic translational diffusion of IPSL. The macroscopic translational diffusion coefficients for Trityl and IPSL in Yukon one gel and silica gel are shown here.

The quantitative analysis shows slower diffusion for the larger Trityl molecule compared to IPSL. Adding glycerol to the solvent increases the viscosity and shows a further decrease of the diffusion coefficient for Trityl. Once mastered, the sample preparation can be done in 45 minutes if it is performed properly, following this procedure the data from the spectrometer can be analyzed quantitatively to determine the macroscopic diffusion coefficient.

Also a two dimensional EPR imaging experiment can be performed in order to check for cracks in the aero gel. After development, this technique paved the way for researchers in the fields of catalysis and chromatography to explore the behavior of gas molecules in porous media. After watching this video, you should have a good understanding of how to prepare an aero gel sample prior to monitoring diffusion in an EPR imaging experiment.

The aero gel is placed inside heat shrink tubing to prevent diffusion around the sides. Don't forget that working with a heat gun and hot ethanol can be extremely hazardous and precautions such as safety glasses, appropriate gloves, and working in a fume hood should always be taken when performing this procedure.