  Recognition that certain forces arising from the averaging of the multiple
impacts of a solute particle by the surrounding solvent particles undergoing
random thermal motion can be of an entropic nature has led to the incorporation
of these forces and their related entropies into theoretical protocols ranging
from molecular-dynamics simulations to the modeling of quarkonium suppression
in particle physics. Here we present a rigorous derivation of this Brownian
entropic force by means of the classical Gibbs canonical partition function and
in so doing provide a heuristic demonstration of its kinetic origin.
