Surface chemistry effects on heterogeneous clathrate hydrate nucleation: A molecular dynamics study

We report results from a molecular dynamics study of clathrate hydrate nucleation near model hydrophilic and hydrophobic surfaces. –CH₃ and –OH terminated self-assembled monolayers (SAMs) are used as model surfaces. We study the nucleation of a soluble, structure II forming guest molecule with a coarse-grained model compatible with monatomic water. Despite the presence of SAMs, we show that nucleation occurs through a homogeneous mechanism in OHSAM and CH₃SAM systems. Formation of ice-like or intermediate water structure is not observed near either surface prior to nucleation. Nucleation occurs more quickly in OHSAM systems than CH₃SAM systems. However, the faster nucleation is driven by a partitioning of guest molecules which results in higher bulk guest concentration in OHSAM systems compared with CH₃SAM systems. Despite significant aggregation of guest molecules near CH₃SAM, no nucleation is observed near the surface. The formation of guest contact pairs, facilitated by the presence of CH₃SAM, may prevent nucleation in this region. Our results highlight the numerous routes by which surfaces can affect hydrate nucleation due to the multicomponent nature of the phenomena.