  The interfacial stiffness of structure II cyclopentane hydrate in
cyclopentane liquid has been measured by the Capillary Wave Fluctuation (CWF)
method for an equilibrated system with temperature of T=280K and pressure of
P~20bar, close to the upper (L$_w$-Hyd-Vap-L$_{HC}$) Quadruple-point, on the
boundary of the hydrate stable region.
  The hydrate-II/cyclopentane interfacial stiffness results fall broadly in the
range $\Gamma_{ho}$=(35-55)$\times 10^{-3}$J/m$^2$, with an average over all
simulations giving $\Gamma_{ho}$=45(5)$\times 10^{-3}$J/m$^2$. Earlier work of
the author has determined the cubic harmonic expansion of the natural
crystalline oscillations in structure II hydrate. Using this work, the
stiffness value translates directly into an orientation-averaged interface free
energy of $\gamma_{ho}$=45(5)$\times 10^{-3}$J/m$^2$. The latter is in
excellent agreement with the experimentally based estimates of
$\gamma_{ho}$=47(5)$\times 10^{-3}$J/m$^2$ from particle-particle adhesion
force measurements of Aman et. al. (Langmuir, 29(8):2676-2682, 2013) This
provides further evidence for the feasibility and robustness of the simulation
approach to measure interfacial free energies in molecular systems.
