In many precipitation reactions, small primary particles (1-5 nm) are produced. The aggregation of these primary particles plays a significant role in determining final particle size distributions. Greater understanding of factors which control the strength of attraction between particles of this size would aid control of particle morphology as well as the range of particle sizes produced. In separate precipitation technologies, the object is often to produce high quality of crystals of large globular molecules (e.g., proteins). Solubility of these particles is controlled by pair interaction potentials. In both of these examples, aggregation and association of particles with 1-5 nm sizes are significant events. Unfortunately, little is known about how to control these processes. In this paper, methods are described of characterizing the interactions of particles in this nano size region. Working with silicotungstate (SiW12O40) and lysozyme, it is concluded that short range interactions of solvation of structural origin play a significant role in pair potentials and thus the particle’s state of aggregation. The implications of these observations are discussed in terms of the aggregation of primary particles produced in precipitation reactions and the solubility of globular macromolecules where it is concluded that short range forces can increase or decrease the extent of the precipitation.