Hydrophobic Hydration andthe Effect of NaCl Salt in the Adsorptionof Hydrocarbons and Surfactants on Clathrate Hydrates

摘要

Adsorption of functional molecules on the surface of hydrates is key in the understanding of hydrate inhibitors. We investigate the adsorption of a hydrocarbon chain, nonionic and ionic surfactants, and ions at the hydrate–aqueous interface. Our results suggest a strong connection between the water ordering around solutes in bulk and the affinity for the hydrates surface. We distinguish two types of water ordering around solutes: (i) hydrophobic hydration where water molecules form a hydrogen bond network similar to clathrate hydrates, and (ii) ionic hydration where water molecules align according to the polarity of an ionic group. The nonionic surfactant and the hydrocarbon chain induce hydrophobic hydration and are favorably adsorbed on the hydrate surface. Adsorption of ions and the ionic headgroups on the hydrate surface is not favorable because ionic hydration and the hydrogen bond structure of hydrates are incompatible. The nonionic surfactant is adsorbed by the headgroup and tail while adsorption of the ionic surfactants is not favorable through the head. Waterordering is analyzed using the hydrogen bond and tetrahedral densityprofiles as a function of the distance to the chemical groups. Theadsorption of solutes is studied through the free energy profilesas a function of the distance to the hydrate surface. Salt lowersthe melting temperature of hydrates, disrupts hydrophobic hydration,reduces the solubility of solutes in the aqueous solution, and increasesthe propensity of solutes to be adsorbed on hydrate surfaces. Ourstudies are performed by the unbiased and steered molecular dynamicssimulations. The results are in line with experiments on the effectof salt and alkanes in hydrate antiagglomeration.