Molecular dynamics simulation investigation of hexanoic acid adsorption onto calcite (101¯4)surface

• Adsorption of hexanoic acid on calcite (101−4) surface was studied by MD simulation.
• Adsorption interaction via H and O atoms has found as a lock and key relation with calcite surface.
• Bivariate orientational maps indicate O atom wobbles between two adsorption sites.
• Head group and alkyl-chain atoms diffusion coefficients are the same order of magnitudes.
• This adsorption dynamics of hexanoic acid on calcite surface is of considerable interest for industrial applications.

In this paper we report the results of classical molecular dynamics (MD) simulation of hexanoic acid adsorption on calcite (101−4) surface plane using Pavese and AMBER force fields for calcite and hexanoic acid, respectively. Pair correlation function, strictly suggests a well-structured adsorption. Density profile indicates the adsorption occurs through double-bonded O atom of the acid head group by direct interaction with Ca atom at calcite surface. Adsorption orientation of H and double-bonded O atoms was found to be as lock and key with respect to calcite surface Ca and O atoms, facilitating an effective adsorption. Adsorption time evolution indicates that O atom adsorption is accompanied by wobbling between two Ca sites. Hence, in spite of H-bonding by acid group, the surface atoms matrix enforces adsorption with some dynamics. Hexanoic acid molecules adopt a well-defined adsorption layer immediate to the surface patterned by the calcite surface structure. The adsorption energy was estimated to be almost 187 times that of octane, a relevant nonpolar fraction of crude oil. These are useful information on the microscopic behavior and adsorption mechanism of crude oil polar fraction in calcite reservoir. The density profile over simulation time steps is proposed as a practical tool to study the dynamics of adsorbed layer in the same way as the most surface spectroscopic method like surface scattering spectroscopy do but in dynamic mode sub-picosecond scale. The intrinsic orientation of double-bonded O and H atoms of the head-group are presented by bivariate maps which helps establishing the key factors of calcite surface activity.

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