Hydrogen-bonding alcohol-water interactions in binary ethanol, 1-propanol, and 2-propanol + methane structure II clathrate hydrates

The small alcohols ethanol, 1-propanol, and 2-propanol are miscible in water, form strong hydrogen bonds with water molecules, and are usually known as inhibitors for clathrate hydrate formation. However, in the presence of methane or other help gases, clathrate hydrates of these substances have been synthesized. In this work, molecular dynamics simulations are used to characterize guest-host hydrogen bonding, microscopic structures, and guest dynamics of binary structure II clathrate hydrates of methane (small cages) with ethanol, 1-propanol, and 2-propanol in the temperature range of 100-250 K to gain insight into the stability of these materials. We observe that these alcohols form structures with dynamic long-lived (similar to 10 ps) guest-host hydrogen bonds in the hydrate phases while maintaining the general cage structure of the sII clathrate hydrate form. The hydroxyl groups of ethanol, 1-propanol, and 2-propanol act as both proton acceptors and proton donors and there is a considerable probability of simultaneous hydrogen bonding between O and H hydroxyl atoms with different cage water molecules. The presence of the nonpolar methane molecule and the hydrophobic moieties of the alcohols stabilize the hydrate phase, despite the strong and prevalent alcohol-water hydrogen bonding. The effect of the alcohol molecules on the structural properties of the hydrate and the effect of guest-host hydrogen bonding on the guest dynamics are studied. (C) 2010 American Institute of Physics. [doi:10.1063/1.3469776]