Self-Assembled Monolayer of Carboxylate On Calcite (101 ̅4) Surface: Molecular Dynamics Simulation Approach

Self-Assembled Monolayer of Carboxylate on Calcite Surface:

Molecular Dynamics Simulation Approach

Byeong Jae Chun, Seung Geol Lee, Giuseppe F. Brunello, Ji Il Choi and Seung Soon Jang

Computational NanoBio Technology Laboratory, School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive NW, Atlanta, GA 30332-0245

To study the wettability alteration of calcite surface in oil reservoir, we investigate the molecular adsorption of carboxylate on the calcite   surface using full-atomistic molecular dynamics (MD) simulation as well as density functional theory (DFT).  There are three types of carboxylate adsorbate used for this study; benzoate, dodecanoate (C12), and octadecanoate (C18). Using DFT calculation, the molecular details of carboxylates are determined including the quantum charge and optimal configuration. Once the fundamental information of the adsorption models is attained, MD simulations are performed to obtain the equilibrium structures and evaluate the thermodynamic stability of the chemisorbed carboxylate and their self-assembled monolayer (SAM) on calcite. Our force field contains the newly developed parameters for the off-diagonal van der Waals interactions of carboxylate functional group with calcite surface. The given original force fields are employed for van der Waals interactions between the same type of atoms or molecules. Based on the thermodynamic stability of each adsorption case achieved by our MD simulations, we determine the optimal packing density of carboxylate SAM on the calcite surface for each case and characterize the corresponding structures. In order to determine the effect of solvation on the packing and structures of SAM on calcite surface, we simulate the same systems in the presence of solvent phase such as water and octane. Once equilibrated structures of carboxylate SAM are obtained, the free energy barrier for detachment of carboxylate from the surface is evaluated by applying potential of mean force (PMF) in order to assess the kinetics of wettability alteration as well as thermodynamic stability.