Viedma Ripening in the Eyes of Cristobal Viedma

A simple experiment with glass beats could have profound implications for the origin of life

 

"I teach; I touch the future."

Cristobal Viedma

 
This article was only possible with the help of  Prof. Dr. Cristobal Viedma from the Complutense University of Madrid, Spain.

 

Chirality is widely present in nature and has fascinated the mankind to date. All amino acids in our body, expect glycine, are chiral. What is even more remarkable is that naturally occurring amino acids share the same chirality. Nevertheless, is hard to understand how homochirality arose in the first place before life began. Energetically, the left and right handed isomers are identical and should form with the same probability. The unlikelihood of breaking symmetry by chance has led to different theories; one more exotic then the other one. Fortunately, in 2004 Prof. Dr. Cristobal Viedma has added a new twist, by establishing the Viedma ripening as a reliable solid-state method for the deracemization of racemic mixtures of crystalline compounds into single enantiomers, simply by continuously grinding a suspension. His discovery has made a significant contribution to the accelerated development of new pharmaceuticals.

Viedma showed the remarkable and dramatic emergence of single chirality crystals of sodium chlorate (NaClO3) and sodium bromate (NaBrO3) in saturated suspensions of the two enantiomorphous crystals. Experiments were conducted with a racemic mixture; so the system was equilibrated with an equal amount of right- and left-handed crystals in saturated aqueous solution. No new crystals nucleate under such conditions. It is demonstrated that abrasive grinding of crystals by stirring in the presence of glass beads promotes dynamic dissolution/crystallization processes that lead to a solid state of single chirality. One of the chiral populations of crystals disappears totally in an irreversible autocatalytic process that nurtures the other one. The system evolves from a similar amount of crystals of both hands to a single enantiomorphic population of crystals. The conversion takes place randomly to the left or right hand with equal probability. The system moves from an equilibrium stage between left- and right-handed crystals to a single chiral population of crystals promoted by grinding.

The first explanation of this striking observation was that attrition by glass beads produced a great number of smaller crystals, whose increase in the mixture causes in turn a slight supersaturation of NaClO3 in solution—although not enough to support primary nucleation. This is possible because, according to the Gibbs–Thomson effect, small particles have a higher solubility than large ones, and therefore small crystals dissolve more readily than large crystals. The latter is a direct consequence of the surface to volume ratio of the crystals as the system minimizes its total surface free energy. In a saturated solution in contact with crystals of different sizes, the phenomenon also leads to Ostwald ripening. Large crystals grow at the cost of smaller ones. Thus, grinding enhances Ostwald ripening in the system and permanently generates particles of different size, therefore increasing the dissolution-growth process.

Thanks to Prof. Dr. Cristobal Viedma and the Viedma ripening method, many novel applications have been developed in the last years which enable the use of Viedma ripening on a large industrial scale as well as in synthetic organic chemistry. Viedma ripening is a more robust and reliable method as crystals of the unwanted enantiomer, which often prevent total spontaneous resolution from reaching an enantiopure end state, are transformed into the desired enantiomer. Although seeding is used in total spontaneous resolution to obtain the best results in terms of chiral purity of the final product, Viedma ripening leads to complete deracemisation without seeding, even when starting from a racemic mixture of crystals.