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44487-B4
Investigating the Molecular Interactions Between Solute and Cosolvent Molecules in Supercritical CO2
Project Activities
The ACS-PRF grant provided me with salaries for two students for two summers as well as my salary for three summers. The equipment funds were used to acquire a Thermo Scientific 380 FT-IR spectrometer. This spectrometer in combination with a Raman spectrometer that I purchased with a grant from the National Science Foundation, has been a tremendous asset to my research program. The proposed research requires that a very large number of spectra be collected for the solute-solvent/cosolvent systems under investigation and we have made tremendous progress toward that goal primarily due to the inherent efficiency of the FT-Raman system as well as being able to perform two separate experiments at once using both spectrometers.
In previous work, I demonstrated that a polar, aprotic cosolvent in supercritical CO2 will solvate a zwitterion such that the cybotactic region gives rise to a dramatic absorbance enhancement of the vibrational modes of the cosolvent molecule. The Thermo Scientific 380 spectrometer was installed in May of 2006 and the Raman spectrometer was installed in October 2006. During the summer of 2006, my group worked on optimizing the experimental conditions in order to collect meaningful spectra. By the end of the ten week period, we had worked through several experimental difficulties to include background contamination in the carbon dioxide source and several leaks in the high pressure system. The initial data that we collected allowed us to reach preliminary conclusions regarding two zwitterions, rhodamine B and merocyanine, in the acetonitrile/sc CO2 cosolvent. During summer 2007, my group conducted a series of experiments to investigate how the individual modes of vibration are affected differently depending upon the concentration and temperature. So far, we have collected a large set of Raman spectra of several concentrations of both neat acetonitrile/sc CO2 and rhodamine B in acetonitrile/sc CO2 while varying the temperature over a range from 313 K to 333 K. We also collected one set of spectra on merocyanine in acetonitrile/scCO2. The following table shows the experiments performed to date on the Raman spectrometer at 313 K, 323 K, and 333 K.
Table 1: Mixtures for which Raman spectra have been collected at the specified temperatures.
Mixture
10% acetonitrile
3% acetonitrile
1% acetonitrile
0.5% acetonitrile
3% acetonitrile/10-4 M rhodamine B
3% acetonitrile/10-3 M rhodamine B
0.5% acetonitrile/10-3 M rhodamine B
3% acetonitrile/10-3 M merocyanine
Each entry in Table 1 consists of 10 separate spectra of 32 scans each at 1 cm-1 resolution. In this way, we can calculate the standard error such that the peak shifts that we observed will be statistically meaningful.
In addition to the Raman experiment, we developed a new piece of equipment to collect infrared spectra on the solute-solvent/cosolvent systems. While infrared spectroscopy is an extremely sensitive technique for probing subtle changes in the local solvent environment, high-density fluids such as scCO2, are essentially opaque over the regions of the spectrum where infrared active modes absorb energy; thereby, limiting the technique as a probe of solute-solvent interactions. The cell that we constructed is an adaptation of a standard part for the Thermo Scientific SmartOrbit ATR module. We tested the cell by collecting a series of infrared spectra on scCO2 at various temperatures and pressures and found that we can work at the same temperatures and pressures that we used for the Raman experiments.
Project Findings
The spectra that we collected reveal several trends that are indicative of the mechanical constraint on the acetonitrile in supercritical CO2. An increase in temperature causes the stretching modes to blue shift, whereas; an increase in the acetonitrile concentration causes the same modes to red shift. An interesting effect that we did not anticipate is that the bending modes appear to be completely unaffected by either a change in temperature or concentration. Furthermore, the individual stretching modes of both acetonitrile and CO2 are shifted by a different amount, which may lend some insight into spatial orientation of the molecules in the cybotactic region. At these concentrations of acetonitrile in scCO2, we do not observe any difference in the spectra with the inclusion of rhodamine B. This is in direct contrast with the effect of rhodamine B on the cosolvent that we reported in the literature for low concentrations (~0.001%) of acetonitrile. Apparently, acetonitrile at a concentration of 0.5% in scCO2 behaves more like it would in the bulk phase than it does at much lower concentrations. Curiously, merocyanine (a much smaller zwitterion with a larger charge separation) shows the opposite behavior, at least with the one sample that we investigated. We are currently pursuing this promising lead.
