Molecular Dynamics Simulation of Secondary Sorption Behavior of Montmorillonite Modified by Single Chain Quaternary Ammonium Cations Qian Zhao † and Susan E. Burns* School of Civil and Environmental Engineering 790 Atlantic Drive, N.W. Georgia Institute of Technology, Atlanta, Georgia 30332-0355, United States ABSTRACT: Organoclays synthesized from single chain quaternary ammonium cations (QAC) ((CH 3 ) 3 NR + ) exhibit different mechanisms for the sorption of nonpolar organic compounds as the length of the carbon chain is increased. The interaction between a nonpolar sorbate and an organoclay intercalated with small QACs has been demonstrated to be surface adsorption, while partitioning is the dominant mechanism in clays intercalated with long chain surfactants. This study presents the results of a molecular dynamics (MD) simulation performed to examine the sorption mechanisms of benzene in the interlayer of three organoclays with chain lengths ranging from 1 to 16 carbons: tetramethylammonium (TMA) clay; decyltrimethylammonium (DTMA) clay; and hexadecyltrimethylammonium (HDTMA) clay. The basis of the overall simulation was a combined force field of ClayFF and CVFF. In the simulations, organic cations were intercalated and benzene molecules were introduced to the interlayer, followed by whole system NPT and NVT time integration. Trajectories of all the species were recorded after the system reached equilibrium and subsequently analyzed. Simulation results confirmed that the arrangement of the surfactants controlled the sorption mechanism of organoclays. Benzene molecules were observed to interact directly with the clay surface in the presence of TMA cations, but tended to interact with the aliphatic chain of the HDTMA cation in the interlayer. The simulation provided insight into the nature of the adsorption/partitioning mechanisms in organoclays, and explained experimental observations of decreased versus increased uptake capacities as a function of increasing total organic carbon (TOC) for TMA clay and HDTMA clay, respectively. The transition of sorption mechanisms was also quantified with simulation of DTMA clay, with a chain length between that of TMA and HDTMA. Furthermore, this study suggested that at the molecular level, the controlling factor for the ultimate sorption capacity is available surface sites in the case of TMA clay, and density of aliphatic chains within the interlayer space for HDTMA clay. ■ INTRODUCTION Clay mineral surfaces, such as montmorillonite, have relatively low sorption affinity for nonpolar organic compounds due to the hydrophilic/organophobic interlayer volume that results from the permanent structural surface charge of the mineral. 1,2 In montmorillonite, the surface charge results primarily from isomorphic substitution within the mineral crystal, and typically yields an excess of negative charge. The resulting surface charge is counteracted by cations that are present in the interlayer of the clay mineral. Most commonly, this excess negative charge has two primary results: interlayer attraction of inorganic metallic cations (e.g., Na + , Ca 2+ ) that hydrate strongly, and the development of highly ordered water layers 3,4 within the interlayer space. The limited uptake capacity of natural montmorillonite for nonpolar organic compounds can be improved by exchanging their naturally occurring inorganic cations with organic cations, resulting in organic-clay complexes, known as organoclays. Increasing the total organic content in the interlayer of the clay mineral changes the interlayer space from organophobic to organophillic, resulting in an interlayer space that may exhibit much larger uptake capacity for organic compounds. 5-8 The enhanced sorption capacity of organoclays for organic compounds results in their potential applications in waste containment, landfill liners, and slurry walls. 9-11 Most commonly, organoclays are created by exchanging quaternary ammonium cations (QACs) onto the clay surface. QACs are composed of an ammonium headgroup, with four exchangeable positions that can be occupied by organic functional groups of varying size and structure. The process of intercalating quaternary ammonium organic cations onto the montmorillonite surface by replacing their naturally occurring inorganic cations will herein be referred to as primary adsorption of organic cations. By varying the molecular structure and quantity of the organic cations that are loaded Received: June 21, 2011 Revised: February 17, 2012 Accepted: February 27, 2012 Published: February 27, 2012 Article pubs.acs.org/est © 2012 American Chemical Society 3999 dx.doi.org/10.1021/es202115v | Environ. Sci. Technol. 2012, 46, 3999-4007