Prediction and understanding of the thermodynamic properties and kinetics of phase transitions in molecular systems depends on tuning intermolecular interactions such that the desired structures are assembled. These interactions can depend on the solvent temperature and composition and are difficult to determine in an a priori manner. This is especially true for large and complex molecules and nanoparticles with functionalized surfaces. Here, we demonstrate the use of the pair contribution of the long-time self-diffusivity determined by pulsed-field gradient spin-echo nuclear magnetic resonance as a probe of these interactions. Materials with high solubilities have scaled long-time self-diffusivity, D-2, values that are close to hard sphere values and decrease as the solubility decreases. We find a remarkable correlation between solubility and D2 for a wide range of hydrogen-bonding solutes that crystallize upon quenching solutions from high temperature. This generalized phase behavior can be understood in terms of the solutes' interacting with attractive forces that have an extent that is only a small fraction of their diameters.