The crystalline material used was sodium chlorate, as used by Kondepudi et al. (1990). Samples of L and D crystals are mixed with water in round-bottomed flasks and the system is stirred by a magnetic bar (of length 3–20mm) at 600 rpm. check details The system is maintained in a supersaturated state; small glass balls are added to continually crush the crystals.
The grinding is thus continuous, and crystals are maintained below a size of 200 μm. The chirality of the resulting crystals was determined by removing them from the flask, allowing them to grow and measuring their optical activity. The results show that, over time, the percentages of left- and right-handed crystals steadily change from about 50/50 to 100/0 or 0/100—a state which is described as complete chiral purity. With stirring only and no glass balls, the systems conserve their initial chiral excesses; with glass balls this website present and stirring, the chiral excess increases, and this occurs more rapidly if more balls are present or the speed of stirring is increased. More recently, Noorduin et al. (2008) have observed a similar effect with amino acids—a much more relevant molecule in the study of origins of life. This work has been reviewed by McBride and Tully (2008), who add to the speculation on the mechanisms responsible for the phenomenon. Noorduin et al. describe grinding as ‘dynamic dissolution/crystallization
Adavosertib datasheet processes that result in the conversion of one solid enantiomorph into the other’. They also note that ‘once a state of single chirality is achieved, the system is “locked” because primary nucleation to form and sustain new crystals from the opposite enantiomer is kinetically prohibited’. Both
these quotes include the crucial fact that the process evolves not towards an equilibrium solution (which would be racemic), but towards a different, dynamic steady-state solution. As noted by Plasson (personal communication, new 2008), this nonequilibrium state is maintained due to the constant input of energy into the system through the grinding process. McBride and Tully (2008) discuss the growth of one enantiomorph, and the dissolution of the other as a type of Ostwald ripening process; with the large surface area to volume ratio of smaller crystals giving a rapid dissolution rate, whilst larger crystals, have a lower surface area to volume ratio meaning that they dissolve more slowly. However appealing such an argument maybe, since surface area arguments can equally well be applied to the growth side of the process, it is not clear that this is either necessary or sufficient. Infact, the model analysed later in this paper will show that a critical cluster size is not necessary to explain homochiralisation through grinding. Our Aims We aim to describe the results of the crystal grinding phenomenon through a model which recycles mass through grinding, which causes crystals to fragment, rather than having explicit mass input and removal.