A new approach for studying nucleation phenomena using molecular simulations:: Application to CO2 hydrate clathrates

We use an order-parameter formulation, in conjunction with non-Boltzmann sampling to study the nucleation of clathrate hydrates from water-CO2 mixtures, using computer simulations. A set of order parameters are defined: Phi(i)(gg) (i=1,2,...,n and gg for guest-guest), which characterize the spatial and orientational order of the CO2 molecules, and Phi(i)(hh) (hh for host-host), which govern the ordering of the water molecules. These are bond-orientational order parameters based on the average geometrical distribution of nearest-neighbor bonds. The free-energy hypersurface as a function of the order parameters is calculated using the Landau-Ginzburg approach. The critical cluster size that leads to the nucleation of the clathrate phase is determined accurately by analyzing the free energy surface. We find that the nucleation proceeds via "the local structuring mechanism," i.e., a thermal fluctuation causing the local ordering of CO2 molecules leads to the nucleation of the clathrate, and not by the current conceptual picture, called "the labile cluster hypothesis." The local ordering of the guest molecules induces ordering of the host molecules at the nearest- and next-to-nearest-neighbor shells, which are captured by a three-body host-host order parameter, zeta(hh); these thermodynamic fluctuations lead to the formation of the critical nucleus. Our results are significant in understanding the proposed sequestration of CO2 by direct ocean injection in order to mitigate the greenhouse effect. (C) 2002 American Institute of Physics.