Theoretical methods demonstrate that crystalline hydrates can form from single-phase systems consisting of both vapor water with gaseous hydrate former and liquid water with dissolved hydrate former; two phase system consisted of both liquid water with gaseous hydrate former and with liquid hydrate former on the surface. At these conditions, the pressure required for hydrate formation will be equal to or greater than that required when a separate gas or liquid phase of hydrate former is present. It is possible to form hydrates from a two phase system consisted of liquid water and gaseous hydrate former when the mole fraction of the dissolved hydrate former on the water surface is less than that which would exist in the presence of a gas phase at the three phase vapor liquid hydrate (VLH) equilibrium pressure. The pressure requirement for the formation of hydrate increases as the mole fraction of hydrate former decreases under these conditions. Here, we have presented a model that uses intermolecular potential parameters obtained from interaction energies and extends Langmuir based equations to single guest clathrates. It is shown that the temperature dependence of Langmuir constants contains all the information needed to determine spherically averaged intermolecular potentials
In 1811 Sir Humphrey Davy [1] was the first to document the existence of a gas hydrate (Chlorine hydrate) or clathrate, a class of compounds in which water forms a continuous and known crystal structure with small cavities or cages. Gas hydrates are crystalline solids formed from mixtures of water and low molecular weight compounds, referred to as guests that typically are gases at ambient conditions [2]. These cavities encapsulate guests, such as methane, needed to stabilize the water lattice. Since Davy's initial discovery, more than 130 hydrophobic compounds have been found as guests in water clathrates [2].
Generally, hydrates are formed in the laboratory from twophase systems by contacting a hydrate former or formers in the gas or liquid phase with liquid water and increasing the pressure or decreasing the temperature until crystalline hydrate forms. At sufficiently high pressure and low temperature, gas hydrates can form at conditions above the normal freezing point of water, e.g., methane hydrate is known to be stable up to 330.5 K and 1.5 GPa, above which ice VI is more stable [3].
However, the formation of hydrate from a single-phase aqueous system using only the hydrate former dissolved in the aqueous phase demonstrated [4] and more recently by Buffett and Zatsepina [5]. In addition, other works demonstrate the equilibrium between a gaseous phase and hydrates indicate that hydrates could form from a single gaseous phase containing sufficient gaseous water [6,7]. Gas hydrates with hydrophobic formers, commonly found in nature and industry, are known to form three distinct crystal structures.
Structure I (sI) has a body-centered cubic structure with a lattice parameter of 12.03? [8] when for example ethylene oxide is the guest.
