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Adsorption Properties of Triethylene Glycol on a Hydrated {101̅4} Calcite Surface and Its Effect on Adsorbed Water
journal contribution
posted on 2015-08-11, 00:00 authored by Richard Olsen, Kim N. Leirvik, Bjørn Kvamme, Tatiana KuznetsovaMolecular dynamics
(MD) and Born–Oppenheimer MD (BOMD) simulations
were employed to investigate adsorption of aqueous triethylene glycol
(TEG) on a hydrated {101̅4} calcite surface at 298 K. We analyzed
the orientation of TEG adsorbed on calcite, as well as the impact
of TEG on the water density and adsorption free energy. The adsorption
energies of TEG, free energy profiles for TEG, details of hydrogen
bonding between water and adsorbed TEG, and dihedral angle distribution
of adsorbed TEG were estimated. We found that while the first layer
of water was mostly unaffected by the presence of adsorbed TEG, the
density of the second water layer was decreased by 71% at 75% surface
coverage of TEG. TEG primarily attached to the calcite surface via
two adjacent adsorption sites. Hydrogen bonds between water and adsorbed
TEG in the second layer almost exclusively involved the hydroxyl oxygen
of TEG. The adsorption energy of TEG on calcite in a vacuum environment
calculated by classical MD amounted to 217 kJ/mol, which agreed very
well with estimates found by using BOMD. Adsorption on hydrated calcite
yielded a drastically lower value of 33 kJ/mol, with the corresponding
adsorption free energy of 55.3 kJ/mol, giving an entropy increase
of 22.3 kJ/mol due to adsorption. We found that the presence of TEG
resulted in a decreased magnitude of the adsorption free energy of
water, thus decreasing the calcite wettability. This effect can have
a profound effect on oil and gas reservoir properties and must be
carefully considered when evaluating the risk of hydrate nucleation.
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Keywords
adsorption sitesAdsorption Propertiestriethylene glycolwater layergas reservoir propertiescalcite wettabilitydihedral angle distributionadsorption energyHydrogen bondshydrate nucleationAdsorbed WaterMolecular dynamicsenergy profilesMDTriethylene GlycolTEGhydroxyl oxygenentropy increaseBOMDadsorption energiesvacuum environment298 Kwater densitycalcite surface