Propensity of formate, acetate, benzoate, and phenolate for the aqueous solution/vapor interface: Surface tension measurements and molecular dynamics simulations,

The properties of aliph. and arom. carboxylates and phenolate-mimicking functional groups of humic acid are discussed with regard to their behavior in aq. solns. close to the surface. Both surface tension measurements and MD simulations confirm that Na formate behaves in accord with the classical theory of surfaces of electrolytes, whereas NaOAc and, much more pronounced, NaOBz and Na phenolate show a more hydrotropic behavior with surface active anions. Further to the surface tension data, the MD results suggest that these hydrotropes are highly oriented at the soln.-vapor interface

Propensity of citric, maleic, oxalic and succnic acids for the acquous solution- vapour interface : Surface tension measurment and molecular dynamics simulations

Behaviour of oxalic, citric, succinic, and maleic acids at the air/water interface is characterized and quantified by surface tension measurements in a broad concentration range and molecular dynamics simulations in slab geometry employing a polarizable force field. The relative order of surface propensities of these atmospherically relevant acids is established in this study with results being also in a very good agreement with previous measurements

Electronic structure, chemical bonding features, and electron charge density of the double-cubane single crystal [Sb7S8Br2](AlCl4)3

The present calculations were performed using all-electron full potential linearized augmented plane wave method based on the density functional theory. We have optimized the structure of the double-cubane single crystal [Sb7S8Br2](AlCl4)3, starting with the x-ray diffraction data Zhang et al., [J. Am. Chem. Soc. 131, 9896 (2009)] , by minimization of the forces (1 mRy/au) acting on the atoms, keeping the lattice parameters fixed at the experimental values. Our calculations show that [Sb7S8Br2](AlCl4)3 possesses a wide indirect energy band gap of about 1.6 eV (2.03 eV) using local density approximation (Engel–Vosko generalized gradient approximation) exchange correlation potentials. To describe the bonding properties we have evaluated the electronic charge space density contour in four planes-namely (001), (110), (100), and (010) which show that this compound possesses a considerable anisotropy. The contour plot shows partial ionic and strong covalent bonding between S–Sb, Al–Cl, S–Br, S–S, Cl–Cl, and Sb–Br atoms

X-ray reflectometry studies on the effect of water on the surface structure of [C(4)mpyr][NTf2] ionic liquid.

The effect of water on the surface structure of 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonylimide [C(4)mpyr][NTf2] ionic liquid was investigated using X-ray reflectometry. The measured reflectivity data suggests a significant amount of water is adsorbed at the surface, with the first layer from the gas (nitrogen)-liquid phase boundary mainly occupied by a mixture of cations and water. Beyond the cation + water layer, the scattering length density increases towards the bulk value, indicating a decreasing amount of water and cations, and/or an increasing amount of anions. The orientation of the butyl chain of cation at the phase boundary and the population of water at the surface were described based on results from an independent molecular dynamics (MD) simulation. We show that the presence of water in the ionic liquid has a non-monotonic effect on the overall thickness of the surface. At low water content, the addition of water does not change the surface thickness since water is mainly present in the bulk. As the water content increases, the surface swells before eventually shrinking down close to the solubility limit of water. The non-monotonic surface thickness is used to explain the anomalous trend of surface tension in ionic liquid-water mixtures reported in the literature. © 2009, Royal Society of Chemistr

Ultrasonic velocities, densities, viscosities, electrical conductivities, Raman spectra, and molecular dynamics simulations of aqueous solutions of Mg(OAc)2 and Mg(NO3)2: Hofmeister effects and ion pair formation

The ultrasonic velocities, densities, viscosities, and electrical conductivities of aqueous solutions of magnesium nitrate and magnesium acetate have been measured from dilute to saturation concentrations at 0 < or = t/degrees C < or = 50. The temperature derivative of the isentropic compressibility, kappa(s), became zero at 2.28 and 2.90 mol kg(-1) for Mg(OAc)2 and Mg(NO3)2 solutions, respectively, at 25 degrees C. The total hydration numbers of the dissolved ions were estimated to be, respectively, 24.3 and 19.2 at these concentrations. Differences in kappa(s) for various M2+ salts, using the present and literature data, correlated with reported M2+-OH2 bond lengths and to a lesser extent with cationic charge densities (ionic radii). The influence of anions on kappa(s) appears to follow the Hofmeister series and also correlates approximately with the anionic charge density. Substantial differences between Mg(OAc)2(aq) and Mg(NO3)2(aq) occur with respect to their structural relaxation times (derived from compressibility and viscosity data) and their electrical conductivities. These differences were attributed to a much greater ion association in Mg(OAc)2 solutions. Raman spectra recorded at 28 degrees C confirmed the presence of various types of contact ion pairs including mono- and bidentate complexes in Mg(OAc)2(aq). In Mg(NO3)2(aq), only noncontact ion pairs appear to be formed even at high concentrations. The experimental results are supported by molecular dynamics simulations, which also reveal the much stronger tendency of OAc- compared to NO3- to associate with Mg2+ in aqueous solutions. The simulations also allow an evaluation of the ion-ion and ion-water radial distribution functions and cumulative sums and provide a molecular picture of ion hydration in Mg(OAc)2(aq) and Mg(NO3)2(aq) at varying concentrations