44760-B6
Thermodynamics and Kinetic Studies of Hydrogen Isotope Binding on Selective Materials
Comparison of tritium and proton exchange: As a first step toward understanding the thermodynamics and kinetics of tritium exchange in a variety of materials, we considered the tritiated water auto-disssociation equilibria. Identifying the preferred form of the tritium (T+ or TO-) is key to choosing materials that can exchange it. To find the change in free energy of each of these reactions, we found the solvation free energy of the individual species using a quasi-chemical theory of solvation.[1-4] Lindiwe Ndebele ‘09 and Laura Fernandez ‘08 have found the most probable solvated structures for T+ and TO- using a quasi-chemical theory of solvation with harmonic and anharmonic frequency estimates. The most probable species using harmonic frequency estimates are the same as those found in previous quasi-chemical applications on H+ and HO-.[1-4]
Since differences between isotopes are crucial for the auto-dissociation of tritiated water, we have considered anharmonic frequencies. A second order pertubation approach yielded highly suspect frequencies for these highly anharmonic systems. We have also considered simple scaling relations. With scaled harmonic frequencies, we find a pKw of 17 rather than 14 for H2O and both HTO dissociations at room temperature. Since experimental studies suggest that raising the temperature from 20°C to 60°C changes the preferred HTO dissociation, we are considered the same equilibria at 60°C. At this elevated temperature, we found that the preferred number of waters solvating hydroxide shifted to smaller numbers. The pKw for water dissociation also decreased to 15 and 12 using the harmonic and anharmonic approximations. The direction of the shift agrees with experiment but the magnitude of the shift is too high. Further, dissociation of tritiated water into HO- and T+ species seems to be preferred at higher temperatures in agreement with experiment.
1. L. R. Pratt, R. A. LaViolette, M. A. Gomez, and M. E. Gentile*, “Quasi-chemical Theory for the Statistical Thermodynamics of the Hard Sphere Fluid.” J. Phys. Chem. B. 105, 11662 (2001).
2. P. Grabowski*, D. Riccardi*, M. A. Gomez, D. Asthagiri, and L. R. Pratt, “Quasi-chemical theory and the standard free energy of H+ (aq),” J. Phys. Chem. A. 106, 9145 (2002).
3. D. Asthagiri, L. R. Pratt, J. D. Kress, and M. A. Gomez, “The hydration state of HO-,” Chemical Physics Letters 380, 530 (2003).
4. D. Asthagiri, L. R. Pratt, J. D. Kress, and M. A. Gomez, “Hydration and mobility of HO- (aq),” Proc. Natl. Acad. Sci. USA 101, 7229 (2004).
