Molecular association between water and dimethyl sulfoxide in solution: A molecular dynamics simulation study Ivana A. Borin and Munir S. Skaf Citation: The Journal of Chemical Physics 110, 6412 (1999); doi: 10.1063/1.478544 View online: http://dx.doi.org/10.1063/1.478544 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/110/13?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The influence of trehalose on hydrophobic interactions of small nonpolar solute: A molecular dynamics simulation study J. Chem. Phys. 139, 044508 (2013); 10.1063/1.4816521 The effect of pressure on the hydration structure around hydrophobic solute: A molecular dynamics simulation study J. Chem. Phys. 136, 114510 (2012); 10.1063/1.3694834 A molecular dynamics simulation study of the dimethyl sulfoxide liquid–vapor interface J. Chem. 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A molecular dynamics simulation study of the local structures and H-bond distribution for water– dimethyl sulfoxide ~DMSO! mixtures over the entire composition range is presented. Analysis of several site-site pair distribution functions reveals that two well-defined kinds of aggregates characterize the molecular association between water and DMSO in solution. One of them, already identified through recent neutron diffraction experiments and computer simulations, consists of two water molecules H-bonded to the oxygen atom of a DMSO molecule, such that the angle between the two H-bonds is nearly tetrahedral. The other complex features a central water molecule and two DMSO, making H-bonds to water hydrogens. According to the simulation data, these molecular aggregates coexist with each other in the mixture, but their proportions change with composition. 1DMSO-2water complexes predominate over 2DMSO-1water aggregates for water-rich mixtures ~water mole fractions .50%!, whereas the opposite is true for DMSO-rich mixtures. The present simulations also seem to indicate that an association between a pair of DMSO molecules through their oxygen atoms is made possible by the formation of the 2DMSO-1water complexes. Implications of the existence of these aggregates to the mobility and other dynamic properties of these mixtures are briefly discussed. © 1999 American Institute of Physics. @S0021-9606~99!50511-4# I. INTRODUCTION H-bond distribution,5~b!–7 and H-bond dynamics9 of DMSO– water mixtures. These studies have focused on water-rich Excess physicochemical properties of dimethyl sulfoxide mixtures. Vaisman and Berkowitz7 simulated diluted mix- ~DMSO!–water mixtures present strong deviations from ide- tures (xD50.005, 0.04, and 0.2!, focusing on the local struc- ality, with maximum deviation occurring around 30% to 1–4 ture of water upon addition of DMSO and on the H-bonding 40% mole fraction of DMSO. Examples of highly non- distribution between the two constituents. Their simulations ideal mixing behavior are found in measurements of dielec- show the existence of 1DMSO-2water H-bonded aggregates tric constants, NMR relaxation times, viscosity, density, and at the three compositions studied. Using different force heat of mixing, amongst others. Such strong deviations from fields, Luzar and Chandler6 performed MD simulations at the ideal mixing behavior have often been attributed to the two compositions (x 50.21 and 0.35!, focusing on the formation of 1DMSO-2H O molecular aggregates, where D 2 structural properties, H-bond distribution, and H-bond dy- two water molecules are supposedly tightly hydrogen ~H!- namics. Their simulations reveal several important aspects of bonded to the oxygen atom of a DMSO molecule. According these mixtures; three of them are worth mentioning here: to this interpretation, extrema of excess quantities would First, they show that the local tetrahedral order of water mol- then be likely found at DMSO mole fraction xD'0.33. Al- ecules is preserved even for the concentrated (xD50.35! so- though the existence of well-defined 1DMSO-2water com- 5 plexes in these mixtures has not been conclusively estab- lution, in agreement with previous neutron diffraction data. lished in the literature, neutron diffraction data suggest that Second, they show that, at the compositions studied, DMSO DMSO-water complexes do exist at that composition.5 The acts typically as a double H-bond acceptor, forming question as to whether such H-bonded aggregates are actu- 1DMSO-2water H-bonded aggregates of nearly tetrahedral ally long-lived complexes is more controversial since avail- geometry. Third, their analyses of the H-bond lifetimes indi- able experimental results do not provide sufficiently detailed cate that DMSO–water H-bonds are longer lived than information in this regard. water–water H-bonds in the mixture, which in turn are Another important and closely related aspect concerns longer lived than in neat water. In a more recent work, which the extent over which DMSO affects the structure of water in combined neutron scattering and computer simulations, 8 the mixture. In this respect, the recent neutron diffraction Soper and Luzar have found no evidence of hydrophobic experiments5 have shown that the local tetrahedral order of association of DMSO and that the enhanced correlations pre- water in these mixtures is preserved even at high (xD sented by water in these mixtures are due to the strong '0.35! concentrations of DMSO. This has been confirmed H-bonding of water to the oxygen atom of DMSO. by means of molecular dynamics ~MD! simulations as well.6 While great progress has been made toward obtaining an To our knowledge, several MD simulation studies have accurate molecular level understanding of DMSO–water been reported in the literature aimed at the structure,5~b!–8 mixtures on the water-rich side, mixtures rich in DMSO, 0021-9606/99/110(13)/6412/9/$15.006412 © 1999 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 143.106.1.143 On: Mon, 01 Sep 2014 13:17:36 J. Chem. Phys., Vol. 110, No. 13, 1 April 1999 I. A. Borin and M. S. Skaf 6413 where water plays the role of solute, have not been explored mixture’s thermodynamics and, to some extent, the intermo- from a microscopic point of view. We feel that further work lecular structure, are not strongly dependent on the DMSO is needed in order to characterize the molecular behavior of force field. Thus, one could, in principle, have used other these mixtures in a more complete way. In particular, very existing potentials for DMSO, or water for that matter, to little is known about way the mixture’s intermolecular struc- study these mixtures to the same level of reliability. Despite ture evolves with increasing concentration of DMSO, what that, we shall see below that the interactions between DMSO the local structures and H-bonding distributions for DMSO- and water in the present simulations may be somewhat over- rich mixtures are, and also, how does transport associated estimated compared to the real mixture. with each molecular species and other dynamical properties Both SPC/E and P2 are rigid, nonpolarizable intermo- behave as a function of composition? To answer these and lecular potentials whose site-site interactions are given by a other equally important questions, we have undertaken a se- sum of Lennard-Jones plus Coulomb terms, ries of MD simulations of DMSO–water mixtures spanning 12 6 sij sij qiqj the whole composition range. In this paper, we focus on the Vij~r!54«ij 2 1 , ~1! intermolecular structure given in terms of site-site pair dis- FS r D S r D G 4p«0r tribution functions, and the distribution of water–water and where qi is the partial charge on site i, «ij and sij are the water–DMSO H-bonds for mixtures of different composi- Lennard-Jones interaction parameters between sites i and j tions. We show in particular that, according to the simula- on distinct molecules, and r the separation between these tions, a distinct type of molecular aggregate, namely, one sites. The Lennard-Jones interaction parameters between consisting of two DMSO molecules H-bonded to the hydro- sites of different types are set by the usual Lorentz-Berthelot gen atoms of a central water molecule, is expected to be the combining rules, «ij5A«ii«jj and sij5(sii1sjj)/2. This predominant form of molecular association between water procedure has been used in other simulations of aqueous and DMSO in DMSO-rich mixtures. Some results are also mixtures, including DMSO–water mixtures, with apparent presented for the diffusion coefficients and the single-particle success. Site parameter and molecular geometries for the reorientational relaxation times. A more detailed study of the simulated models are found in Refs. 6 ~DMSO! and 10 ~wa- dynamics, dipole orientational correlations, and dielectric ter!. properties shall be reported separately.