The unique chemical-equilibrium in nano-systems: Confinement vs. quasi-confinement effects

Chemical-equilibrium involving a small number of molecules inside a confined nanospace can exhibit considerable deviations from the macroscopic thermodynamic limit due to reduced mixing entropy, as was predicted in several of our works using statistical-mechanics canonical partition-functions and the lattice-gas [1] as well as non-lattice [2] models. In particular, for exergonic addition and dimerization a considerable shift of the bimolecular reaction extent towards product formation is expected compared to only minor or even inverse effects in the case of endergonic reactions. This “nanoconfinement entropic effect on chemical-equilibrium” (NCECE) was verified by revised analysis [3] of reported measurements of DNA hybridization inside confined nano-fabricated chambers. In particular, the DNA dissociation constant and extent are consistently diminished under nanoconfinement, and it should be noted that this extra stabilization may have biological implications. Furthermore, a predicted enhanced Ir intra-core dimerization can affect catalytic properties of Pd-Ir nanoparticles such as those operating in several applications [2].

Using the grand-canonical ensemble, the modeling has been recently extended to the more common “quasi-confined nanosystems” exchanging molecules with a macroscopic environment. Even if the latter is not fully equilibrated, a limited number of molecules and a slow exchange rate (e.g., hindered by a narrow entrance) presumably facilitate fast local equilibration. As is exemplified by dimerization of alkali metal vapors (2Na = Na₂), distinctive entropy-related stabilization (or destabilization) of products is anticipated depending on the external reagent chemical-potentials and its internal sorption energies.

The unique chemical-equilibria under confinement or quasi-confinement are anticipated for a wide range of nanospaces (nanotubes, fullerenes, micelles, etc.), and thus can have implications for the growing nanotechnological utilization of chemical syntheses conducted within nanoreactors.

[1] M. Polak and L. Rubinovich, NanoLetters 8, 3543, (2008).

[2] L. Rubinovich, M. Polak, Topics in Catalysis 61, 1237 (2018).

[3] L. Rubinovich and M. Polak, NanoLetters 13, 2247 (2013).