Numerous examples show that the dominant mechanism underlying host crystal/additive interactions is selective adsorption of the additive at the crystal/solution interface, with the main driving forces ranging from purely electrostatic to highly specific recognition of crystal faces by the additive.
Selective electrostatic interactions take place between growing crystals and flexible, highly charged small and macromolecules and/or surfactants because of differing ionic structures and charges of the crystal planes, some of them being shielded by hydrated layers. As in solution, surfactant molecules at high concentrations self-assemble into various superstructures (hemimicelles, bilayers) at the crystal/solution interface.
Recognition of crystal planes by rigid small molecules and macromolecules with partial β-sheet conformation (such as proteins or polyelectrolytes) is highly specific. It requires a dimensional fit between the distances of constituent ions protruding from the affected crystal plane(s) and the distances between functional groups that are part of the additive molecules. The consequences of selective additive/crystal interactions range from changes in crystal growth morphology to changes in the composition of the crystallizing phase. Examples showing the dual role of macromolecules as initiators and retarders of crystallization are discussed.