A minimal colloid model of solution crystallization nucleates crystals classically.

A fundamental assumption of the classical theories of crystal nucleation is that the individual molecules from the "old" phase associate to an emerging nucleus individually and sequentially. Numerous recent studies of crystal nucleation in solution have revealed nonclassical pathways, whereby crystal nuclei are hosted and fed by amorphous clusters pre-formed in the solution. A sizable knowledge gap has persisted, however, in the definition of the molecular-level parameters that direct a solute towards classical or nonclassical nucleation. Here we construct a suspension of colloid particles of hydrodynamic diameter 1.1 μm and monitor their individual motions towards a quasi-two-dimensional crystal by scanning confocal microscopy. We combine electrostatic repulsion and polymer-induced attraction to obtain a simple isotropic pair interaction potential with a single attractive minimum of tunable depth between 1.2 and 2.7. We find that even the smallest aggregates that form in this system structure as hexagonal two-dimensional crystals and grow and maturate by the association and exchange of single particles from the solution, signature behaviors during classical nucleation. The particles in the suspension equilibrate with those in the clusters and the volume fractions of suspensions at equilibrium correspond to straightforward thermodynamic predictions based on depth of the interparticle attraction. These results demonstrate that classical nucleation is selected by particles interacting with a minimal potential and present a benchmark for future modifications of the molecular interactions that may induce nonclassical nucleation.