Publication

Abstract : Gas hydrates (GH) are crystalline compounds composed of small hydrophobic molecules encapsulated within cavities formed by hydrogen bonded water molecules. These compounds are an important source of fuel and are of application in gas storage and transportation. Processes such as nucleation, crystal growth, stability and decomposition of GH are controlled by the introduction of molecules referred to as additives. A challenging aspect in the study of GH is to explain at a molecular level the influence of additives on these processes. An intriguing case of additives is that of amphiphilic molecules which influence hydrate processes through their complex effects on the organization of water as well as on gas solubility. In this study, we apply molecular dynamics simulations to examine the effect of CH3OH, the simplest amphiphile and the most commonly used additive, on the organization of H2O molecules in the hydration shell of CO2, a hydrate forming gas. The interface between methanol–water liquid mixture and CO2 is examined by applying the Identification of Truly Interfacial Molecules (ITIM) technique, thereby minimizing the errors due to molecular level roughness at the interface. The organizational order of hydration shell water (HSW) is examined by evaluating the F4 order parameter. The results show that the presence of CH3OH enhances the availability of aqueous CO2 near the interface which makes the arrangement of HSW in this region more hydrate-like, resulting in an increase in value of F4 on approaching the interface from the bulk. It is also observed that CH3OH molecules tend to orient their hydrophobic CH3 group towards aqueous CO2 thereby inducing hydrate-like ordering of water in the hydration shell. However, on approaching the liquid surface, hydrate-like organization of HSW is opposed by CH3OH molecules at the surface which, owing to their –OH group oriented towards the liquid phase, significantly influences water orientation near the interface and decreases the value of F4. The net outcome of these opposing effects of CH3OH on hydrate-like ordering of HSW is the observation of a maximum in the value of F4 order parameter at a distance beneath the surface of the liquid and a progressive decrease from this maximum on moving further closer to the surface. The observed features of the value of F4 order parameter profile are explained based on the analysis of the intrinsic structure of the liquid–gas interface. The results indicate that, the presence of CH3OH leads to a region beneath the liquid surface where the extent of hydrate-like organization of HSW becomes maximum. However, CH3OH molecules significantly limits the availability of water molecules around aqueous gas, justifying the action of methanol as an inhibitor for hydrate formation when present at high concentrations. The results indicate that the effect of an amphiphile on GH formation is determined by its interaction with dissolved gas as well as by the influence of surface adsorbed amphiphile on water orientation and gas availability near the interface.