Articles written in Journal of Chemical Sciences

    • The pervasive solvent-separated sodium chloride ion pair in water-DMSO mixtures

      Ashok K Das B L Tembe

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      Sodium chloride exists as a contact ion pair (CIP) as well as a solvent-separated ion pair (SSIP) in its solutions in water and in dimethyl sulphoxide (DMSO). In a mixture of these two solvents, the CIP is not formed in the two mixture compositions with χdmso=0.35 and 0.21 and the ions stay as the SSIP near an interionic distance of 5.0 Å. This has been shown by constructing the ion-ion potentials of mean force and by following the ion-pair trajectories initiated at various initial ion-pair separations in the two solvent mixtures.

    • Salting-out of methane in the aqueous solutions of urea and sarcosine


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      Hydrophobic association and solvation of methane molecules in aqueous solutions of urea and sarcosine(sa) have been studied using MD simulations. The potentials of mean force (PMFs) between methanemolecules in water, water-sa, water-urea and water-urea-sa mixtures show an enhancement of methane associationon the addition of these osmolytes. These observations are well supported by calculation of equilibriumconstants. Calculation of thermodynamic parameters shows that the association of methane is stabilized byentropy and favored by enthalpy. The hydrophobic solvation of methane is stabilized by enthalpy and destabilizedby entropy. The calculated solvation free energies of methane in these mixtures show that methane isless soluble in the mixtures of urea and sarcosine than in water. The solubility is the least in the water-urea-samixture. Analysis of distributions of solvent and co-solvent around methane suggests that the local densities ofboth urea and sarcosine are diminished around the methane in the mixtures of these osmolytes. The selectivereduction of both urea and sarcosine from methane surface suggests that both urea and sarcosine push methanemolecules towards water and increase the interaction between methane molecules i.e., salting-out of methane.

    • Na+ Cl- ion pair association in water-DMSO mixtures: Effect of ion pair model potentials


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      Potentials of Mean Force (PMF) for the Na+ Cl- ion pair in water–dimethyl sulfoxide (DMSO)mixtures for three DMSO mole fractions have been computed using constrained Molecular Dynamics (MD)simulations and confirmed by dynamical trajectories and residence times of the ion pair at various inter-ionicseparations. The three ion-ion direct potentials used are 12-6-1, exp-6-1 and exp-8-6-1. The physical picturethat emerges is that there is a strong contact ion pair (CIP) and strong to moderate solvent separated ion pair(SSIP) in these solutions. Analysis of local ion clusters shows that ions are dominantly solvated by watermolecules. The 12-6-1 potential model predicts running coordination numbers closest to experimental data.

    • Thermodynamics of association of water soluble fullerene derivatives [C ₆₀ (OH)n, n = 0, 2, 4, 8 and 12] in aqueous media


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      The thermodynamics of association of fullerene [C₆₀] and water-soluble fullerene derivatives, i.e., fullerols [C ₆₀ (OH)n, n = 0, 2, 4, 8 and 12] in aqueous solutions have been studied using molecular dynamics simulations. The potentials of mean force (PMFs) bring out the tendency of aggregation of these nanostructuresin water. The extent of hydroxylation seems to have a minor effect on the depth of the contact minima (the first minimum in the PMFs). The positions of the subsequent minima and maxima in the PMFs change with the size of the solute molecules. Higher stability of the contact state of highly hydroxylated fullerols is due to the van der Waals interactions whereas intermolecular solute-solvent hydrogen bonding nearly flattens the PMFs beyond the 2nd minima for higher fullerols. The solvent contributions to the PMFs for all the solute particles studied here are positive. Entropic and enthalpic contributions to the association of solute molecules are calculated in the isothermal-isobaric (NPT) ensemble. We find that the contact pair formation is governed by entropy with the enthalpic contributions being highly unfavorable, whereas the solvent assisted and solvent separated configurations show entropy-enthalpy compensation.

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