Some of my earliest work in the Chandler group concerned the role of solvent fluctuations in the aggregation of two nanoscale hydrophobic solutes. At the interface of a nanoscale hydrophobic solute, water forms an interface which is akin to the interface found between its coexisting liquid and vapor phases. Fluctuations in this solute-solvent interface play an important role in the aggregation of such particles. In other words, the dimerization is poorly described in the absence of a solvent-based reaction coordinate.

We find that the precursor to the aggregation of two solute particles is the formation of a vapor tunnel that bridges the two solutes, which is typically followed by a rapid collapse of the solutes involved. This tunnel forms through the mutual fluctuation of the interface surrounding each particle. The free energy cost associated with the formation of a vapor tunnel grows rapidly with the distance between the particles. Thus typical reactive trajectories pass through a region of phase space for which the formation of the vapor tunnel is essentially activation-less. For further reading refer to the article below.

1. "The Role of Solvent Fluctuations in Hydrophobic Assembly," A.P. Willard and D. Chandler. J. Phys. Chem. B, 112, 6187-6192 (2008).