Free energy of separation of structure II clathrate hydrate in water and a light oil.

The adhesion forces and free energies of separation of structure II clathrate hydrates in vacuum and submerged in water and a model oil are investigated by molecular dynamics simulation. The water molecules are modeled by the TIP4P/ice model and the alkanes by the OPLS_AA force field. The results are compared with theory and earlier work. It is observed that the adhesive forces between the simulated surfaces have an effective range of no more than 1.5-2 nm. The hydrate-hydrate interaction force is attractive in vacuum and oil, larger in vacuum. In water the interaction force is very slightly repulsive on average and much weaker than in the two other systems with a larger uncertainty. In all cases the interaction is largely entropically driven. The separation energies in vacuum and oil (octane) are stronger than predicted by theory, with free energies of approximately 4 and 0.7 aJ, respectively, likely due to lack of polarization effects. The hydrate-hydrate interaction in water is too weak for quantitative comparisons to be made.