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

This paper resolve the salinity-dependent interactions of polar components of crude oil at calcite-brine interface in atomic resolution. Molecular dynamics simulations carried out on the present study showed that ordered water monolayers develop immediate to a calcite substrate in contact with a saline solution. Carboxylic compounds, herein represented by benzoic acid (BA), penetrate into those hydration layers and directly linking to the calcite surface. Through a mechanism termed screening effect, development of hydrogen bonding between –COOH functional groups of BA and carbonate groups is inhibited by formation of a positively-charged Na+ layer over CaCO3 surface. Contrary to the common perception, a sodium-depleted solution potentially intensifies surface adsorption of polar hydrocarbons onto carbonate substrates; thus, shifting wetting characteristic to hydrophobic condition. In the context of enhanced oil recovery, an ion-engineered waterflooding would be more effective than injecting a solely diluted saltwater.


Figure S1
Initial configuration of a typical simulation ensemble adopted in this study: 15 BA molecules and sufficient number of ions required to satisfy the desired concentration of the brine were randomly inserted into the middle of the ensemble, respectively. These species were inserted at locations at least 10 Å above the outermost layer of the substrate to avoid any coercive interaction of components with surface due to close proximity. Afterwards, the simulation ensemble was filled with 1350 water molecules. Note that the initial coordinates of BA molecules are identical in all simulations to avoid the effect of their initial position on the final adsorption structure onto calcite. Also, the number of NaCl pairs are different in brines as follows: 0 in DW, 13 in dSW, 26 in SW and 110 in HS. This snapshot is taken from the calcite/brine system with highest salinity (HS) at the beginning of simulation. For clarity, we reduced the size of ions (illustrated by purple and yellow spheres representing sodium and chloride, respectively) and water species (shown as blue dots).

S1. Evaluating finite size effect
Simulation system was designed large enough to preclude any potential artefact due to finite-size effect. For this purpose, a complementary simulation was performed for a larger calcite/SW (60,000 ppm) system with box dimensions 56.67 Å x 54.89 Å x 19.77 Å for a longer timespan, 50 ns. By doing so, we acquired number and charge density distribution profiles (Figures S2 and S3) identical to Figures 2 and 5. Also note the similarity of MSD diagram ( Figure S4) to that presented in Figure   11. This comparison confirms size-independency of our MD results.

S2. Molecular orientation
Here, we present the calculation procedure for obtaining the angle distribution map (Figure 6). First, an axis is defined for each BA molecule ( Figure S7) passing through the centroid of the BA phenyl ring and the carboxylic group. This axis is defined for each BA molecules as an vector, namely, Ԧ = ( , , , ).

As illustrated in
The preceding calculation was performed for each BA molecule at a series atomic trajectories produced every 2.5 ps. At each timeframe, a set of conceptual bins ( Figure S8) is assumed parallel to the calcite surface and the angle calculated for each molecules is        To this end, all atoms were allowed to relax until the magnitude of the residual force on all atoms reached 0.02 ev/Å and the total energy converged to 0.0001 eV. Integration over the Brillouin zone was found to fulfill convergence criteria with a (2 × 2 × 1) k-point grid sampling.
The VESTA and X-Crysden programs were used to draw the molecular structure and visualize the electronic results.