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Molecular Structure of the Chloroform-Water and Dichloromethane-Water Interfaces†

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Molecular Structure of the Chloroform-Water and Dichloromethane-Water Interfaces† 8832 J. Phys. Chem. C 2007, 111, 8832-8842 Molecular Structure of the Chloroform-Water and Dichloromethane-Water Interfaces† Dennis K. Hore,‡ Dave S. Walker, Libby MacKinnon,§ and Geraldine L. Richmond* Department of Chemistry, UniVersity of Oregon, Eugene, Oregon 97403 ReceiVed: October 31, 2006; In Final Form: December 19, 2006 Using equilibrium molecular dynamics simulations, we investigate the structure of chloroform-water and dichloromethane-water interfaces. These systems are analyzed in terms of the orientation of water, chloroform, and dichloromethane molecules as a function of distance from the Gibbs surface. We also calculate order parameters for all molecules across the interface. The results show that the structures may be described in the context of a few distinct regions of the interface, where organic and water molecules generally arrange themselves either to maximize hydrogen-bonding interactions or to minimize the net dipole moment. In cases where molecular arrangements promote hydrogen-bonding interactions, they often do not provide complete dipole compensation. The orientation of organic and water molecules then creates a field across the interface. For the CHCl3-water interface, the field has a uniform direction with the positive end of the dipole pointing toward the bulk water phase. The interfacial field is more complicated in the case of CH2Cl2-water but is oriented in the same direction close to the bulk water side. 1. Introduction important contributor to surfactant structure. At biological The interface between water and chlorinated organic liquids interfaces, for example, the structure of many proteins is is of central importance to a broad category of scientific, stabilized by association with solvated ions. The historic opinion industrial, biological, and environmental disciplines. Chloroform has been that ions are repelled from the interface between water 32,33 and dichloromethane (DCM) in particular have received a lot and a hydrophobic medium. This view has been challenged of attention, primarily because of their widespread use as recently with detailed molecular dynamics studies that show - solvents but also because the small size of these molecules is an enhancement of polarizable anions at the air water inter- 35,36 amenable to computational study. Because chloroform is often face, and this provides some justification for the observed 37 considered a model hydrophobic polar liquid, there have been negative charge at many hydrophobic interfaces with water. many experimental and simulation studies of molecules adsorbed It has been demonstrated recently that the origin of this effect at the chloroform-water interface. These include ions of various may lie in a field created by oriented interfacial water 38 sizes,1,2 proteins,4,5 phospholipids,7 nanocrystals,8 polymer- molecules. This field would then attract hydroxide ions, either enzyme complexes,9,10 and polyelectrolytes.7 Given this wide from the autodissociation of water or from an additional source, range of applications, it is surprising that there have not been to the interface. The current study entails detailed investigations more studies of the neat chloroform-water interface, with the of the orientation of both water and organic molecules at the exception of early interfacial tension measurements11,12 and interface. We will therefore use our findings on the structure to studies of chloroform adsorbed to ice.13,14 The dichloromethane- additionally speculate on the origin, size, and polarity of any water interface has a similar spectrum of applications, such as orientation-induced field at the interface and discuss its conse- self-assembly of polymer surfactants,16-18 solvent dechlorina- quence on the surface charge. tion,20 ion partitioning,21 and microemulsions of these two We will begin with a brief overview of the details of our liquids.22,23 There is also biological interest in this liquid-liquid molecular dynamics simulations, followed by a description of interface with studies of adsorbed phosopholipids24 and the primary analyses performed with the resulting data. Two proteins.25-28 The neat dichloromethane-water interface has systems will then be discussed in turn, starting with the - - been studied by means of interfacial tension measurements30 chloroform water interface, followed by the dichloromethane and molecular dynamics simulation.31 In nearly all of the above water interface. A general comparison of these two systems then studies where structural information is sought, it is either of a follows. surfactant at the liquid-liquid interface or of the water molecules. This paper addresses the interfacial structure by 2. Simulation Details examining the mutual orientation of the water and organic Equilibrium molecular dynamics simulations were performed molecules as a function of depth through the interface. with the Amber 7 package39 (using the Amber 7 force field) The nature of the charge at a liquid interface is an important and an integration time of 1 fs. Initial configurations consisted consideration, both from a fundamental standpoint and as an of separate (40 Å)3 boxes, each containing 2135 POL339 water, † Part of the special issue “Kenneth B. Eisenthal Festschrift”. 475 chloroform, or 601 dichloromethane molecules. These were * Corresponding author. E-mail: [email protected]. Phone: 541- subject to energy minimization and were then equilibrated for 346-4635. Fax: 541-346-5859. 200 ps. Temperature was controlled by weak coupling to a heat ‡ Present address: Department of Chemistry, University of Victoria, bath at 300 K; molecular geometries were constrained using Victoria, British Columbia, V8W 3V6, Canada. § Department of Chemistry, California Polytechnic State University, San the SHAKE algorithm; long-range interactions were cut off at Luis Obispo, CA 93407. 8 Å using the Particle Mesh Ewald technique. The partial atomic 10.1021/jp067176t CCC: $37.00 © 2007 American Chemical Society Published on Web 02/22/2007 Chloroform-Water and Dichloromethane-Water J. Phys. Chem. C, Vol. 111, No. 25, 2007 8833 Figure 1. Organic and water boxes, showing the configuration of the system for molecular dynamics simulations. Values of n depend on the density of the organic liquid. z ) 0 corresponds to the lower Gibbs surface. For all analyses, molecules above the system center are treated as if they originated in a corresponding location in the lower half of the box. The dotted lines depict 1-Å-thick slabs that were used to obtain the statistics for the analyses. charges that were used to construct the organic models were used from the Amber 7 library for chloroform (same as Dang’s polarizable model40) and from the literature for dichlo- romethane.41 Water-organic boxes were then assembled as illustrated in Figure 1. Such configurations allowed for easier control of the center of mass while instituting periodic boundary conditions in all three directions. The energy of the entire system was then minimized and allowed to reach equilibrium by following the dynamics for 2 ns. Snapshots of all of the coordinates were then recorded every 50 fs for 10 ns, thereby Figure 2. In the laboratory frame, the Gibbs dividing surface is in the collecting 200 000 configurations. It was necessary to collect xy plane, so z is the normal to this plane. (a) In the frame of the water such a large amount of data (nearly 1 Tb of files on disk) molecule, c is defined to be along the C2 symmetry axis, pointing away from the hydrogens. The molecule lies in the ac plane. The Euler angle, because the sought information on the molecular orientation θ, is defined as the angle between the z and c unit vectors. The twist would be obtained through the generation and analysis of joint angle, ψ, parametrizes the rotation about the molecular c axis. (b) For tilt-twist angle histograms. It has been recognized that con- chloroform, the C-H vector defines the molecular c axis. (c) The structing such plots with useful resolution typically requires dichloromethane axes are defined in an analogous manner to those of about an order of magnitude more data than would be used for the water molecule, with H-C-H defining the ac plane, and the c - - one-dimensional analyses.42,43 All calculations and analyses were axis bisecting the H C H angle. performed on our 16-processor AthlonMP/Opteron cluster, running Fedora Core 3 linux, and using the LAM message- 3.1. Quantitative Description of Molecular Orientation. passing libraries. There are two useful metrics for describing the orientation of molecules at a liquid interface. One is the extent to which the 3. Results collection of molecules is ordered with respect to a designated direction in the laboratory frame; the other is the actual We now examine all of the data produced by the molecular orientation of the molecules, specified by Euler angles (θ, φ, dynamics simulations to arrive at a structural description of the ψ) that relate the molecule-fixed to laboratory frames. The first interface between water and the two chlorinated organic liquids. description is achieved by specifying three biaxial order First, we will describe the two categories of quantities that we parameters.44-46 One is for the tilt angle, θ, defined as the angle have calculated, namely, order parameters and orientation between the molecular c axis and the interface normal, z. (The histograms. Next, we will outline the results of each system in relevant molecular frame axes for each molecule are defined in turn, beginning with the chloroform-water interface. For each Figure 2.) system, we will first show the order parameters for water and This order parameter is the organic species to gain a qualitative understanding of interfacial features. This will be followed with an analysis of 1 S ) 〈3 cos2 θ - 1〉. (1) the orientation histograms to arrive at a detailed picture of the 1 2 water and organic structure. This data will then be summarized with structural diagrams to illustrate the orientation of both For chloroform, we will assume that there is very weak (if any) species throughout the interface.
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