  The hydration of ions in nanoscale hydrated clusters is ubiquitous and
essential in many physical and chemical processes. Here we show that the
hydrolysis reaction is strongly affected by relative humidity. The hydrolysis
of CO32- with n = 1-8 water molecules is investigated by ab initio method. For
n = 1-5 water molecules, all the reactants follow a stepwise pathway to the
transition state. For n = 6-8 water molecules, all the reactants undergo a
direct proton transfer to the transition state with overall lower activation
free energy. The activation free energy of the reaction is dramatically reduced
from 10.4 to 2.4 kcal/mol as the number of water molecules increases from 1 to
6. Meanwhile, the degree of the hydrolysis of CO32- is significantly increased
compared to the bulk water solution scenario. The incomplete hydration shells
facilitate the hydrolysis of CO32- with few water molecules (especially for n =
6) to be not only thermodynamically favorable but also kinetically favorable.
This discovery provides valuable insights for designing efficient sorbents
including that for CO2 air capture.
