ReactionCoordinate of Incipient Methane ClathrateHydrate Nucleation.

Nucleation from solution is a ubiquitousphenomenon with relevanceto myriad scientific disciplines, including pharmaceuticals, biomineralization,and disease. One prominent example is the nucleation of clathratehydrates, multicomponent crystalline inclusion compounds relevantto the energy industry where they block pipelines and also constitutea potential vast energy resource. Despite their importance, the molecularmechanism of incipient hydrate formation remains unknown. Herein,we employ advanced molecular simulation tools (pBhistogram, equilibrium path sampling) to provide a statistical-mechanicalbasis for extracting physical insight into the molecular steps bywhich clathrates form. Through testing the Mutually Coordinated Guest(MCG) order parameter, we demonstrate that both guest (methane) andhost (water) structuring are crucial to accurately describe the nucleationof hydrates and determine a critical nucleus size of MCG-1 = 16 at255 K and 500 bar. Equipped with a validated (and novel) reactioncoordinate, subsequent equilibrium path sampling simulations yieldthe free energy barrier and nucleation rate. The resulting quantitativenucleation process is described by the MCG clustering mechanism. Thisconstitutes a significant advance in the field of hydrates research,as the fitness of a molecular descriptor has never been statisticallyverified. More broadly, this work has significance to a wide rangeof multicomponent nucleation contexts wherein the formation mechanismdepends on contributions from both solute and solvent. [ABSTRACT FROM AUTHOR]