The Impact of Salinity on the Interfacial Structuring of an Aromatic Acid at the Calcite/Brine Interface: An Atomistic View on Low Salinity Effect

Abstract

This study aims to elucidate the impact of salinity on the interactions governing the adsorption of polar aromatic oil compounds onto calcite. To this end, molecular dynamics simulations were employed to assess adsorption of a model polar organic molecule (deprotonated benzoic acid, benzoate) on the calcite surface in NaCl brines of different concentration levels, namely, deionized water (DW), low-salinity water (LS, 5000 ppm), and sea water (SW; 45,000 ppm). Calcite was found to be completely covered by several wellordered water layers. The top hydration layer is very compact and prevents direct adsorption of benzoates onto the substrate. Instead, Na + ions form a distinct positively charged layer by adhering on the calcite substrate through inner-sphere complexion mode. Cl − ions mostly lodge on top of the adsorbed sodium cations, forming a negatively charged layer. The distribution of ions at the calcite/brine interface thus exhibits the features of an electrical double layer, composed of a Stern-like positive layer followed by a negative one. The positive charged layer attracts benzoates toward the surface. As such, the sodium ions attached onto the calcite can act as adsorption sites to connect benzoates to the surface. By increasing the salinity, more Na + ions adsorb onto the calcite surface, and the density of benzoate molecules at the interface is enhanced as a result of more Na + bridging ions. The monotonic salinity-dependent adsorption of benzoate molecules on calcite offers a mechanism driving additional oil recovery upon injection of diluted brine into subsurface carbonate reservoirs.

How do ions contribute to brine-hydrophobic hydrocarbon Interfaces? An in silico study

“…For instance, Koleini et al reported a charge density of a calcite-brine system at a similar salinity, which was three orders of magnitude greater than the value presented for LS NaCl in Fig. S8 [46]. According to the magnified pictures taken at the marked interfaces, one will find out a biased distribution of cations and anions in CaCl 2 and NaCl solutions adjacent to the oil phases.…”

Atomistic insight into salinity dependent preferential binding of polar aromatics to calcite/brine interface: implications to low salinity waterflooding

“…Despite vast progress in the empirical lab-scale research, the atomic-level interactions behind the salinity-response of calcite minerals is still not well understood. Koleini et al 32 recently showed that salinity-dependence of calcite wettability is essentially driven by adsorption/desorption process. They observed stronger surface attachment of polar hydrocarbons, represented there by benzoate, upon increasing salinities.…”

A Deep Look into the Dynamics of Saltwater Imbibition in a Calcite Nanochannel: Temperature Impacts Capillarity Regimes

“…It suggests the sodium and chloride ions constantly appear in the interfacial region of the advancing meniscus, possibly due to the favorable interactions with the calcite surface. It comes with the observation made by Koleini et al that Na + ions tend to bind onto the protruding oxygen atoms on the basal plane of a calcite slab 52,53 . As a result, a positively charged layer by adsorbed sodium cations is formed on the calcite surface, which in turn draws the chloride anions towards the surface 52,54 .…”

“…Even, ions localization at the calcite interface is connected with axial travelling of water molecules. We know from former investigations that sodium cations tend to adhere directly on the calcite surface through pairing with protruding oxygen atoms of basal carbonate groups; thus, forming a positivelycharged layer over the calcite surface 53,69 . It is mirrored into the sharp peak of Na + distribution profile, shown in Figure 8.…”