Enhanced Hydrate Nucleation Near the Limit of Stability Felipe Jim´ enez- ´ Angeles † and Abbas Firoozabadi *,†,‡ Reservoir Engineering Research Institute, Palo Alto, CA 94301, USA, and Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06510, USA E-mail: abbas.firoozabadi@yale.edu Phone: +1 (650)326-9172. Fax: +1 (650) 472-9285 Abstract Clathrate hydrates are crystalline structures composed of small guest molecules trapped into cages formed by hydrogen-bonded water molecules. In hydrate nucle- ation, 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 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 hy- drate 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 * To whom correspondence should be addressed † Reservoir Engineering Research Institute ‡ Yale University 1