Enhanced hydrate nucleation near the limit of stability

Clathrate hydrates are crystalline structures composed of small guest molecules trapped into cages formed by hydrogen-bonded water molecules. In hydrate nucleation, water and the guest molecules may stay in a metastable fluid mixture for a long period. Metastability is broken if the concentration of the guest is above a certain limit. Here we study propane hydrates by means of molecular dynamics simulations. First we simulate three-phase equilibrium of water, propane, and propane hydrates; the simulated melting temperature and solubility of propane in water are agreement with experimental measurements. In the main part we simulate hydrate nucleation in water-propane supersaturated solutions. At moderate temperatures we show that hydrate nucleation can be very fast in a very narrow range of composition, namely, close to the limit of stability. Propane density fluctuations near the fluid-fluid demixing are coupled with crystallization, producing enhanced nucleation rates. This is the first report of propane-hydrate nucleation by molecular dynamics simulations. We observe motifs of the crystalline structure II in line with experiments and new hydrate cages not reported in the literature. Our study relates nucleation to the fluid-fluid spinodal decomposition and demonstration that the enhanced nucleation phenomenon is more general than short-range attractive interactions as suggested in nucleation of proteins.