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 as well as to several of my colleagues at the
I began the summer 2008 research with a new group of three undergraduate students. The first two weeks were devoted to training the students on the equipment and explaining the overall scope of the project. Given our success with observing fundamental molecular interactions in solute/cosolvent systems, I decided to investigate the interaction between CO2 and H2O. I chose this current project in response to the intense interest, within the scientific community, in understanding the fundamental molecular interactions between CO2 and H2O. Further knowledge of this system may lead to the development of new technology to capture and sequester CO2. Our ability to observe changes in the vibrational spectra of a molecule is a very sensitive technique for probing the local environment. However, CO2 and H2O are essentially opaque in the infrared region of the electromagnetic spectrum. To solve this experimental problem, we designed a high-pressure flow cell for infrared spectroscopy of gases, liquids, and supercritical fluids, specifically those that have an extremely large absorption cross-section. The design is specific to the Thermo Scientific Smart Orbit attenuated total reflectance (ATR) module that is part of the Thermo 380 FT-IR spectrometer. This cell will withstand pressures up to 35 MPa (5130 psi); thereby, allowing us to duplicate the pressures and temperatures of the deep ocean, where it has been proposed to sequester CO2. The manuscript reporting this design has been submitted to the marketing department at Thermo Scientific for inclusion in their technical communication.
We used the high-pressure cell to collect a series of infrared spectra in addition to the Raman spectra that were collected simultaneously. The Table 2 lists the conditions at which spectra were collected.
Table 2: Mixtures of CO2 and H2O for which Raman and infrared spectra have been collected at the specified temperatures and 1500 p.s.i. pressure.
Concentration of CO2 in H2O
| 279 K
| 285 K
| 295 K
| 305 K
| 315 K
| 335 K
|
90%
| x
| x | x | x
| x | x |
80%
| x
| x | x | x | x | x |
70%
| x
| x | x | x | x | x |
60%
| x
| x | x | x | x | x |
50%
| x
| x | x | x | x | x |
40%
| x
| x | x | x | x | x |
30%
| x
| x | x | x | x | x |
20%
| x
| x | x | x
| x | x |
10%
| x
| x | x | x | x | x |
Each entry in Table 2 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. Furthermore, for the 90% and 10% mixtures, spectra were collected at pressures from 1000 to 3000 p.s.i. in increments of 100 p.s.i. at all six temperatures listed.
Findings
We were able to reproduce the Raman spectra obtained by Sum, et al. (J. Phys. Chem. B 1997, 101, 7371-7377) of CO2 incorporated into clathrate hydrates and then use the identical conditions to collect infrared absorption spectra of these structures. We found that the enclathrated CO2 exhibited rotational structure similar to gas phase CO2. Calculation of the rotational constant suggests that the CO2 freely rotates inside the clathrate. We predicted this phenomenon based on a suggestion by Consani and Pimentel in their paper on enclathrated C2D2 (J. Phys. Chem., 1987, 91, 289-293). Furthermore, we found that the infrared absorption spectra revealed that CO2 exists in several different environments that are highly dependent upon the concentration of CO2 in H2O, the temperature, and the mixing time.
