| AVS 55th International Symposium & Exhibition | |
| Nanometer-scale Science and Technology | Friday Sessions |
| Session NS+NC-FrM |
| Session: | Nanoscale Processes |
| Presenter: | M. Polak, Ben-Gurion University, Israel |
| Authors: | M. Polak, Ben-Gurion University, Israel L. Rubinovich, Ben-Gurion University, Israel |
| Correspondent: | Click to Email |
This study explores theoretically features unique to the chemical equilibrium in a nano-confined reaction mixture, which is closely related to newly developed routes for the synthesis of organic molecules and inorganic nanoclusters. Remarkable modifications in the equilibrium state of a closed small system compared to its (macroscopic) thermodynamic limit (TL) are predicted for several model reactions. Thus, canonical ensemble based statistical-mechanical formulation and computations show that a decrease in the overall number of molecules can lead to significant enhancement of the equilibrium extent of exothermic reactions. In particular, plots of lnK vs. 1/T exhibit below a certain temperature an increase in the slope, relative to the TL case, by a factor equal to the sum of the ingredient stoichiometric coefficients. In order to exemplify the smallness effect on K, the reaction 2AB(ad)=A2(ad)+B2(ad) on a surface is modeled for different overall number of molecules with emphasis on the roles of fluctuations and off-stoichiometric effects. The modeling is extended to the gaseous phase using the "harmonic oscillator-rigid rotor" approximation for the exothermic reaction 2NO(g)=N2(g)+O2(g), demonstrating the generality of the confinement-smallness effect. The enhancement is significant also in case of the addition reaction A+B=C, taking place in a closed tetrahedral "cage" as a model for a molecular capsule. The computations show that the temperature range of variations in the lnK vs. 1/T slope (from ΔHo to 2ΔHo) strongly depends on the reaction exothermity. These phenomena are relevant to reactions taking place under certain conditions in a confined nano-space, such as catalytic nanoreactors ranging from molecular capsules and micelles1 to zeolite cavities2 and carbon nanotubes.3 While in catalysis the system is typically open, in nanoreactors there can be situations, such as the occurrence of rapid pre-equilibrium or "product inhibition", in which the reaction mixture is confined effectively. Then, conclusions of the closed system present modeling should be valid and taken into account as an appreciable contribution to the reaction equilibrium yield.
1 T. S. Koblenz, J. Wassenaar, and J. N. H. Reek, Chem. Soc. Rev. 37, 247 (2008).
2 B. Smit, and T. L. M. Maesen, Nature 451, 671 (2008).
3 D.A. Britz et al., Chem. Commun. 37 (2005).