The effect of L-valine on the growth of the Lalanine (011) surface from solution crystallization is studied by combining single-crystal growth experiments and molecular simulation. For the first time, an unusual promotion effect of Lvaline on L-alanine crystal growth, after the initial inhibition effect, is found when the impurity concentration is higher. Through molecular simulation, it is revealed that this unusual promotion effect is due to the close interaction between Lalanine and L-valine, which repels H 2 O around solute molecules and therefore eliminates the negative effect of H 2 O on surface diffusion of L-alanine.

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The effects of L-valine on the lateral growth of ( 011) and ( 120) surfaces of L-alanine are studied by combining molecular simulation and experimental studies. The growth of the (120) surface is significantly inhibited by L-valine, while the growth of the (011) surface is less affected. An intrinsic impurity effectiveness parameter is proposed by taking into consideration the selectivity of impurities to adsorb on both step and terrace, which explains the different inhibition effects of an impurity on different surfaces.