Molecular dynamics simulation investigation of hexanoic acid adsorption onto calcite (10 14) surface Mohammad Hadi Ghatee a, *, Mohammad Mehdi Koleini a , Shahab Ayatollahi b a Department of Chemistry, Shiraz University, Shiraz 71946, Iran b Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran A R T I C L E I N F O Article history: Received 17 July 2014 Received in revised form 26 November 2014 Accepted 28 November 2014 Available online 29 November 2014 Keywords: Calcite surface Hexanoic acid adsorption energy Molecular dynamics simulation Orientational bivariate map Self-diffusion A B S T R A C T In this paper we report the results of classical molecular dynamics (MD) simulation of hexanoic acid adsorption on calcite ð10 1 4Þ 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. ã 2014 Elsevier B.V. All rights reserved. 1. Introduction The structural and dynamical properties of liquids near solid surfaces have been the main focus of many theoretical and experimental studies. This interest is due to the important role of liquid–solid interface played in different practical and theoretical applications. In general, structural order of liquids near solid surface deviates considerably from their bulks. Knowledge of atomic level inorganic-solid-surfaces structure is of fundamental importance in investigation of processes such as growth, dissolution, and sorption of impurities. In particular, the surface activity of calcite (CaCO 3 ) is a key factor in a variety of industrial processes especially in crude oil extraction process. Since calcite is the most commonly occurring of the carbonate compound constituting the crude oil reservoir rocks, its most stable ð10 1 4Þ surface has been extensively studied by theoretical and experi- mental methods [1–7]. Carbonate rocks wettability alteration requires knowledge of adsorbate–adsorbent structural relation, which plays a key factor in industrial application such as crude oil in the secondary recovery stages. Reservoir wettabilites are strongly induced by adsorption of surfactants from crude oil on the pore walls of the reservoir. Little is known, however, about the surfactant-like behavior of the crude oil polar components toward the reservoir pore surface. Carboxylic acids are of very common polar component of crude oil of various reservoirs, acting as surface active agents. Their impacts are of considerable interest due to their very polar head group in contrast to existing nonpolar aromatic or aliphatic hydrocarbons. Due to the significant increase in computational power, the nature of such complex systems at the solid–liquid system can be exploited by simulation methods. Simulations are firm substituent to experiments because of possible modeling of the system of interest at molecular level, avoiding many assumptions, easy processing and understanding the results that are shown to be closely comparable with the experiment. * Corresponding author. Tel.: +98 71 613 7174. E-mail addresses: ghatee@susc.ac.ir (M.H. Ghatee), mhghatee2@gmail.com (M.M. Koleini). http://dx.doi.org/10.1016/j.fluid.2014.11.029 0378-3812/ ã 2014 Elsevier B.V. All rights reserved. Fluid Phase Equilibria 387 (2015) 24–31 Contents lists available at ScienceDirect Fluid Phase Equilibria journal homepage: www.else vie r.com/locat e/fluid