Diffusion accelerates and enhances chirality selection

The diffusion effect on chirality selection in a two-dimensional reaction-diffusion model is studied by Monte Carlo simulations. The model consists of achiral reactants A which turn into either of the chiral products R or S in a solvent of chemically inactive vacancies V . The reaction contains a nonlinear autocatalysis as well as a recycling process, and the chiral symmetry breaking is monitored by an enantiomeric excess (ee) phi. Without dilution a strong nonlinear autocatalysis ensures chiral symmetry breaking. By diluting a diffusionless system, the ee phi decreases and a racemic state is recovered below a critical concentration c(c) . When the diffusion is allowed, the steady value of phi increases and c(c) decreases. As for the relation between the ee phi and the concentration c , a formula interpolating between the diffusionless (D=0) and the homogeneous (D=infinity) limits is proposed by incorporating the diffusional enhancement of the concentrations of chiral species. Diffusion also accelerates the development of the chiral order, and the time required to establish the order in a system of a size L(2) is inversely proportional to the diffusion constant D as L(2)/D .