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Review date: July 24, 2003
PNNL-SA-27883

Basic Research at PNNL
Research areas: High-pressure nuclear magnetic resonance, Micelles and microemulsions in supercritical fluids, and Ion-water structure in hydrothermal water

Time-resolved FTIR instrument in operation

In situ laser photolysis in a ultra-high pressure NMR cell

High-Pressure Nuclear Magnetic Resonance
Since the mid 90's work at PNNL has focused on the investigation of supercritical fluid solutions using high-pressure nuclear magnetic resonance (NMR). There are numerous experimental techniques that have been used to investigate supercritical fluids. These range from FTIR, UV-Visible, fluorescence, ESR, and x-ray spectroscopies. NMR is a technique that has seen limited application to supercritical fluid solvents due to the specialized need for a high pressure, non-magnetic probe and its associated electronics. There have been different successful solutions to a functioning high pressure NMR probe ("A new apparatus for the convenient measurement of NMR spectra in high-pressure liquids", Yonker, C.R.; Zemanian, T.S.; Wallen, S.L.; Linehan, J.C.; Franz, J.A., J Magn Res A 1995, (113):102-107) and each of these probe designs has its own strengths and weaknesses. Overall, NMR is an information-rich spectroscopic technique that can describe the solvent environment about a solute molecule, determine self-diffusion coefficients, ascertain molecular structure, measure hydrogen bonding in solution, and describe molecular clustering as a function of density. NMR can provide important molecular level information about the density dependence of rotational and translational dynamics in supercritical fluid solutions. Similarly, high-pressure kinetics and chemical equilibria can be investigated by the use of NMR.

 

Cryomagnet for NMR spectrometer

Aggregation and association in alcohols have typically been used to study hydrogen bonding dynamics in solutions. Methanol can associate through hydrogen bonding, and details about the dynamics of this interaction in solution have been investigated for both liquid and supercritical conditions ("Density and temperature effects on the hydrogen bond structure of liquid methanol" Wallen, S.L.; Palmer, B.J.; Garrett, B.C.; Yonker, C.R., J Phys Chem 1996, (100):3959-3964 and "Pressure and temperature effects on the hydrogen-bond structures of liquid and supercritical fluid methanol". Bai, S.; Yonker, C.R., J Phys Chem A 1998, (102):8641-8647). The nuclear shielding constant is an absolute measure of the electronic distribution about the nucleus and its effect on the observed magnetic moment of that nuclei in the applied magnetic field, which is sensitive to a molecule's chemical structure and local solvation environment. For methanol, the CH3, and OH groups will each experience their own shielding environments. One assumes that changes in pressure or temperature affect the non-specific contributions to the nuclear shielding in a similar manner for all the different group's resonances. Thus, the difference between the shielding of the groups () can be related to the specific interactions in solution, which is due to hydrogen bonding of the OH group.

For methanol, the data was obtained over a wide range of pressure and temperature (50 to 500°C and 2 kbar). A dramatic change of in the vicinity of the methanol critical point (methanol Tc is 239.4°C) at low pressure was observed. This is related to the large changes in density and thus hydrogen bonding of solution in this region. As pressure is increased through this temperature region, hydrogen bonding increases which contributes to a change in shielding of the nucleus and thus to a change in . The slope for methanol increases with increasing temperature. This observation can be explained within the framework of the hydrogen bonding occurring in solution. Hydrogen bonding removes electron density from the vicinity of the ¹H nucleus contributing to the deshielding of the proton. Qualitatively, an increase in correlates with an increase in the deshielding of the OH proton relative to that of the CH3 group and thus an increase in hydrogen bonding in solution. This could be due to a change in both the extent and strength of the hydrogen bond network at high temperatures as one changes pressure as compared to a change in hydrogen bond strength alone at low temperatures with increasing pressure. The results demonstrate that increasing temperature at constant pressure tends to decrease the extent of hydrogen bonding in methanol, while increasing pressure at constant temperature increases hydrogen bonding in solution. One would anticipate that increasing temperature would more readily disrupt hydrogen bonds in solution. Increasing pressure at high temperatures should have a large effect on the solutions' hydrogen bond network, contributing to the larger slope seen at the higher temperatures. However, the NMR chemical shift data clearly indicates that significant hydrogen bond interactions exist for methanol at high temperatures and pressures and in the critical region.

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Micelles and microemulsion in carbon dioxide

Initial studies on the measurements of the micelle size and structure were conducted using light scattering on alkanes and CO2 micelles. At PNNL, the early realization of the importance of using angstrom-wavelength radiation (neutrons and x-rays) for characterization of micelle structures led to the first SANS and SAXS studies of these colloidal systems in fluids. In 1990, Fulton and other researchers at PNNL conducted the first SANS studies of microemulsions in compressible fluids and measured the short-range and highly attractive nature of these microemulsions droplets ("A Small-Angle Neutron Scattering Study of Intermicellar Interactions in Microemulsions of AOT, Water and Near-Critical Propane." Kaler, E.W.; Billman, J.F.; Fulton, J.L.; Smith, R.D., J. Phys. Chem. 1991, (95):458-462). These SANS studies used a cell designed at PNNL. In overcoming some of the limitations of SANS sensitivity and the limited availability of SANS facilities at the time, the method of SAXS for supercritical fluids and carbon dioxide was developed at PNNL including the development of a high-pressure SAXS cell. This work culminated in a collaborative study between Fulton and DeSimone in which the first reported measurements were made of the size and geometry of large (20 nm) surfactant and water aggregates using DeSimone polymeric surfactants ("Aggregation of Amphiphilic Molecules in Supercritical Carbon Dioxide: A Small Angle X-Ray Scattering Study." Fulton, J.L.; Pfund, D.M.; McClain, J.B.; Romack, T.J.; Maury, E.E.; Combes, J.R.; Samulski, E.T.; DeSimone, J.M.; Capel, M. Langmuir 1995, (11):4241-4249). This represented a large leap in our understanding of these systems since large aggregates of this size had not previously been known to exist in carbon dioxide.

Small angle X-ray-scattering (SAXS) equipment

In order to understand the nature of the high solubility of fluorinated compounds in CO₂, researchers at PNNL measured the molecular interactions between CO₂ and fluorinated compounds using Fourier Transform Infra-red spectroscopy ("Fourier Transform Infrared Spectroscopy of Molecular Interactions of Heptafluoro-1-butanol or 1-Butanol in Supercritical Carbon Dioxide and Supercritical Ethane." Yee, G.G.; Fulton, J.L.; Smith, R.D., J. Phys. Chem. 1992, (96):6172-6181). This study is widely cited in current efforts to design new CO₂ surfactant. Using the same technique, the inter-molecular hydrogen bonding of nonionic surfactants in CO₂ was reported ("Aggregation of Polyethylene Glycol Dodecyl Ethers in Supercritical Carbon Dioxide and Ethane." Yee, G.G.; Fulton, J.L.; Smith, R.D., Langmuir 1992, (8):377-384). The method of FTIR for studies of supercritical fluid microemulsion (including CO₂) were developed at PNNL, and techniques now used by others to probe water properties in CO₂ microemulsions were first demonstrate at PNNL ("Reverse Micelles and Microemulsions in Near-Critical and Supercritical Fluids." Smith, R.D.; Fulton, J.L.; Blitz, J.P.; Tingey, J.M. J. Phys. Chem. 1990, (94):781-787). Using the full Lifshitz theory, the van der Waals interaction between microemulsion droplets in SC fluids were calculated ("Interdroplet Attractive Forces in AOT Water-in-Oil Microemulsions Formed in Subcritical and Supercritical Solvents." Tingey, J.M.; Fulton, J.L.; Smith, R.D., J. Phys. Chem. 1990, (94):1997-2004) helping to define the way that colloid particles behave in fluids such as carbon dioxide.

In recent advance in surfactant technology for CO₂, a new system has been reported by Fulton at PNNL in which a conventional, inexpensive hydrocarbon surfactant (AOT) was used in combination with lesser amounts of a fluorinated cosurfactant to synthesize and stabilize large metallic (Ag) particles in CO₂ ("Synthesizing and Dispersing Silver Nanoparticles in a Water-in-Supercritical Carbon Dioxide Microemulsion." Ji, M.; Chen, X.; Wai, C.M.; Fulton, J.L., J. Am. Chem. Soc. 1999, (121): 2631-2632) ("Properties of an AOT Microemulsion formed in Supercritical Carbon Dioxide using a Fluorinated Co-Surfactant", Fulton, J.L.; Jackson, K. 215th ACS National Meeting in Dallas, Spring 1998). This approach decreases the reliance on the more expensive fluorinated compounds while still stabilizing macro-molecular species. More about Microemulsions.

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X-ray monochrometer and focusing mirrors at the National Synchrotron Light Source

Ion-Water Structure in Hydrothermal Water
Supercritical water has important potential applications in (1) oxidative destruction of hazardous waste (2) organic synthesis and oxidation reactions and (3) salt separation and solubility. Coordination structure and redox chemistry in supercritical water is also of high interest to the area of geochemistry and corrosion. Due to the difficulty of experimentally probing this extreme solvent environment there is a severe lack of fundamental structural information. In ongoing studies at PNNL, scientists are using XAFS (X-ray Absorption Fine Structure) to study the structure of supercritical water at the Advanced Photon Source at Argonne National Laboratory. In the first studies of this kind we have shown that XAFS is a powerful method to determine the local solvent environment around an ion in terms of the number of nearest solvent neighbors and the hydration distance. ("An XAFS Study of Strontium Ions and Krypton in Supercritical Water.", Pfund, D.M.; Darab, J.G.; Fulton, J.L.; Ma, Y., J. Phys. Chem. 1994, (98):13102-13107.)( "The ion pairing and hydration structure of Ni²⁺ in supercritical water at 425°C determined by x-ray absorption fine structure and molecular dynamics studies." Wallen, S.L.; Palmer, B.J.; Fulton, J.L., J. Chem. Phys. 1998, (108):4039-4046.)

High-pressure cell with the x-ray transmission and fluorescence detectors at the Advanced Photon Source, Argonne National Laboratory.

Another extremely important outcome of the research is that we have found an effective way to test and develop intermolecular potentials that are used in simulations ("Direct Modeling of XAFS Spectra from Molecular Dynamics Simulations." Palmer, B.J.; Pfund, D.M.; Fulton, J.L., J. Phys. Chem. 1996, (100):13393-13398.). These ion-water and ion-ion intermolecular potentials are used both for high-temperature and ambient water studies. The existing water models that have been developed for ambient conditions do not accurately predict structure in high-temperature water. From the XAFS experimental results we have implemented improvements to these models that have significantly boosted their performance by use of more realistic potentials.

First-shell structure about ions under hydrothermal conditions.

Many different ion-water systems have been studied with XAFS starting with an early landmark investigation of Sr²⁺ in supercritical water. ("An XAFS Study of Strontium Ions and Krypton in Supercritical Water.", Pfund, D.M.; Darab, J.G.; Fulton, J.L.; Ma, Y., J. Phys. Chem. 1994, (98):13102-13107.) We have found significant dehydration occurring under supercritical water conditions for mono- and di-valent cations (Sr²⁺ and Rb⁺) ("Rubidium ion hydration in ambient and supercritical water." Fulton, J.L.; Pfund, D.M.; Wallen, S.L.; Newville, M.; Stern, E.A.; Ma, Y., J. Chem. Phys. 1996, (105):2161-2166.) and for a monovalent anion (Br^-) ("Hydration of Bromide Ion in Supercritical Water: An X-ray Absorption Fine Structure and Molecular Dynamics Study." Wallen, S.L.; Palmer, B.J.; Pfund, D.M.; Fulton, J.L.; Newville, M.; Ma, Y.; Stern, E.A., J. Phys. Chem. A 1997, (101):9632-9640.) More recently we have explored NiBr2 hydration and ion pairing in high-temperature water. ("A Transition in the Ni²⁺ Complex structure from six- to four-coordinate upon formation of ion pair species in supercrtical water: an XAFS NIR and MD study." Hoffmann, M.M.; Darab, J.G.; Palmer, B.J.; Fulton, J.L., J. Phys. Chem. A 1999, (42):8471-8482) At room temperature, the octahedral Ni²⁺(H2O)6 species persists at all salt concentrations. This species is still prevalent at 325°C, but at higher temperatures it is replaced by four-coordinate structures. Above 425°C, at moderate pressures up to 700 bar, the stable structures are a family of four-coordinated species (NiBr(H2O)3Br, NiBr2(H2O)2, NiBr3(H2O)Na) where the degree of Br- adduction and replacement of H2O in the inner shell depends upon the overall Br- concentration. The most likely symmetry of these species is a distorted tetrahedron. Thus, we have completed the definitive structural characterization of several ionic species at high temperatures. Most recently we have investigated the structure about Cu¹⁺ above 300°C. We have found an unusual linear copper halide species that has now been characterized for the first time. Many other systems are currently under study using his powerful technique.

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