Interfacial Structure, Thermodynamics, and Electrostatics of Aqueous Methanol Solutions via Molecular Dynamics Simulations Using Charge Equilibration Models

Abstract

We present results from molecular dynamics simulations of methanol-water solutions using charge equilibration force fields to explicitly account for non-additive electronic interaction contributions to the potential energy. We study solutions across the concentration range from 0.1 to 0.9 methanol mole fraction. At dilute concentrations, methanol density is enhanced at the liquid-vapor interface, consistent with previous molecular dynamics and experimental studies. Interfacial thickness exhibits a monotonic increase with increasing methanol mole fraction, while surface tensions display monotonic decrease with methanol concentration, in qualitative agreement with experimental data and previous molecular dynamics predictions using polarizable force fields. In terms of interfacial structure, in keeping with predictions of traditional force fields, there is a unique preferential orientation of methanol molecules at the interface. Moreover, there is a free energetic preference for methanol molecules at the interface as evidenced by potential of mean force calculations. The pmf calculations suggest an interfacial state with 0.8 kcal/mole stability relative to the bulk, again, in qualitative agreement with previous simulation and experimental studies. Interfacial potentials based on double integration of total charge density range from −610 mV to −330 mV over the dilute to concentrated regimes, respectively. The preponderance of methanol at the interface at all mole fractions gives rise to a dominant methanol contribution to the total interfacial potential. Interestingly, there is a transition of the water surface potential contribution from negative to positive upon the transition from methanol mole fraction of 0.1 to 0.2. The dipole and quadrupole contributions to the water component of the total interfacial potential are effectively of equal magnitude and opposite sign, thus canceling one another. We compute the in-plane component of the dielectric permittivity along the interface normal. We observe a non-monotonic behavior of the methanol in-plane dielectric permittivity that tracks the methanol density profiles at low methanol mole fractions. At higher methanol mole fractions, the total in-plane permittivity is dominated by methanol, and displays a monotonic decrease from bulk to vapor. We finally probe the nature of hydration of water in the bulk versus interfacial regions for methanol mole fractions of 0.1 and 0.2. In the bulk, methanol perturbs water structure so as to give rise to water hydrogen bond excesses. Moreover, we observe negative hydrogen bond excess in the vicinity of the alkyl group, as reported by Zhong et al for bulk ethanol-water solutions using charge equilibration force fields, and positive excess in regions hydrogen bonding to nearest-neighbor methanol molecules. Within the interfacial region, water and methanol density reduction lead to concomitant water hydrogen bond deficiencies (negative hydrogen-bond excess).

Role of spatial ionic distribution on the energetics of hydrophobic assembly and properties of the water/hydrophobe interface

“…The longer z -dimension accommodates a vapor space above the bulk liquid; thus our system contains a liquid-vapor interface that is included in order to probe the relative propensities of different anions and cations between the free liquid-vapor interface and the liquid-hydrophobe interface. This simulation scheme is a standard protocol 36,51,52 . The hydrophobic surfaces consisted of two parallel plates, each comprised of 31 atoms arranged in a triangular lattice with bond length of 3.2 Å (Figure 1); both plates were solvated with TIP4P-FQ water molecules 53 .…”

An overview of current issues in the uptake of atmospheric trace gases by aerosols and clouds

“…Simulations of molecules interacting with the vapour/liquid interface of water have been performed for a variety of molecular species, (Pohorille and Benjamin, 1991;Matsumoto et al, 1993;Pohorille and Benjamin, 1993;Matsumoto, 1996;Sokhan and Tildesley, 1996;Tarek et al, 1996a, b;Taylor et al, 1997;Wilson and Pohorille, 1997;Benjamin, 1999;Taylor and Garrett, 1999;Dang and Feller, 2000;Shin and Abbott, 2001;Roeselova et al, 2003;Morita, 2003;Dang and Garrett, 2004;Paul and Chandra, 2004;Roeselova et al, 2004;Vacha et al, 2004;Vieceli et al, 2005;Canneaux et al, 2006;Minofar et al, 2007;Partay et al, 2007;Carignano et al, 2008;Mahiuddin et al, 2008;Morita and Garrett, 2008;Miller et al, 2009;Patel et al, 2009;Sun et al, 2009) including hydrophilic species such as ethanol, acids including oxalic and citric, amphiphilic molecules such as peptides, hydrophobic species such as oxygen and nitrogen, and radical species such as OH and HO 2 . The condensation of water on aqueous droplets is a special case of molecular uptake, but for molecular simulations, which treat all molecules as distinguishable, the methods used are the same as for other molecular species.…”

“…1 Similarly, the adsorption free energy can be obtained from the free energy profiles and related to the surface excess. Comparison of computed (Pohorille and Benjamin, 1991;Pohorille and Wilson, 1993;Taylor et al, 1997;Wilson and Pohorille, 1997;Benjamin, 1999;Taylor and Garrett, 1999;Dang and Feller, 2000;Shin and Abbott, 2001;Dang and Garrett, 2004;Paul and Chandra, 2004;Roeselova et al, 2004;Vacha et al, 2004;Vieceli et al, 2005;Canneaux et al, 2006;Partay et al, 2007;Carignano et al, 2008;Morita and Garrett, 2008;Patel et al, 2009;Sun et al, 2009) and experimental (Ben-Naim and Marcus, 1984;Castro et al, 1991;Hanson et al, 1992;Dabkowski et al, 1996;Karpovich and Ray, 1998;Allen et al, 1999;Sander, 1999;Autrey et al, 2004) solvation and adsorption free energies provide a test of the accuracy of the molecular interactions used in simulations of uptake. The comparisons show that the interaction potentials are capable of reproducing energetic quantities to within several kJ/mol.…”

Molecular dynamics simulations of nonpolarizable inorganic salt solution interfaces: NaCl, NaBr, and NaI in transferable intermolecular potential 4-point with charge dependent polarizability (TIP4P-QDP) water

“…83 To explore this idea, we compute the longitudinal ͑z-dependent͒ profile of the in-plane dielectric permittivity of water using the approach of Stern and Feller. [98][99][100] The water contribution to the in-plane dielectric constant is shown in Fig. 4 for each salt solution and the pure solvent.…”