  Nanoclusters can form and grow by nanocluster-monomer (condensation) and
nanocluster-nanocluster (coagulation) collisions. During growth, product
nanoclusters have elevated thermal energies due to potential and thermal energy
exchange following a collision. Even though nanocluster collisional heating may
be significant and strongly-size dependent, no prior theory describes such
phenomenon. We derive a model to describe the excess thermal energy, the
kinetic energy increase of the product cluster, and latent heat, the heat
released to the background upon thermalization of the non-equilibrium cluster,
of collisional growth. Both quantities are composed of an enthalpic term,
related to potential energy minimum differences, and a size-dependent entropic
term, which hinges upon heat capacity and energy partitioning. Example
calculations using gold nanoclusters demonstrate that collisional heating can
be important and strongly size dependent, particularly for reactive collisions
involving nanoclusters composed of 14-20 atoms. Excessive latent heat release
may have considerable implications in cluster formation and growth.
