The pathway from the solution to the steps

The pathway of solute access to the kinks is an issue of unparalleled fundamental significance that also carries severe consequences for the morphologies, quality, and, utility of the grown crystals. Here we review experimental tests to distinguish between the two pathways of solute supply to the steps: directly from the solution or after first adsorbing on the terraces between steps and diffusing along the crystal surface towards the steps. Generalizing the evidence accumulated with ionic, organic, protein, and biomineral crystals to-date, we put forth a criterion to predict the pathway of solute supply to the kinks. The proposed criterion relies on the interactions of the solvent with solute and the crystal surface. Solvents that strongly associate to the crystal surfaces and solute molecules weaken solute adsorption and contribute to faster surface diffusion and abundant solute supply to the steps, which, in turn, makes the surface diffusion pathway faster than the one directly from the solution. Solvents that do not attenuate the solute-crystal surface attraction promote strong adsorption, which suppresses surface diffusion and encourages direct incorporation into the kinks. We propose that computational methods to predict a priori the pathway from the solution to the surface as a part of efforts to model crystal growth rates, morphologies, and quality need to faithfully account for solvent association to the crystal surfaces and to solute molecules.