Dynamics of ionic and hydrophobic solutes in water-methanol mixtures of varying composition

Dynamical Anomaly of Aqueous Amphiphilic Solutions: Connection to Solution H-Bond Fluctuation Dynamics?

We have investigated the possible connection between "dynamical anomaly" observed in time-resolved fluorescence measurements of reactive and nonreactive solute-centered relaxation dynamics in aqueous binary mixtures of different amphiphiles and the solution intra- and interspecies H-bond fluctuation dynamics. Earlier studies have connected the anomalous thermodynamic properties of binary mixtures at very low amphiphile concentrations to the structural distortion of water. This is termed as "structural anomaly." Interestingly, the abrupt changes in the composition-dependent average rates of solute relaxation dynamics occur at amphiphile mole fractions approximately twice as large as those where structural anomalies appear. We have investigated this anomalous solution dynamical aspect by considering (water + tertiary butanol) as a model system and performed molecular dynamics simulations at several tertiary butanol (TBA) concentrations covering the extremely dilute to the moderately concentrated regimes. The "dynamical anomaly" has been followed via monitoring the composition dependence of the intra- and interspecies H-bond fluctuations and reorientational relaxations of TBA and water molecules. Solution structural aspects have been followed via examining the tetrahedral order parameter, radial and spatial distribution functions, numbers of H bonds per water and TBA molecules, and the respective populations participating in H-bond formation. Our simulations reveal abrupt changes in the H-bond fluctuations and reorientational dynamics and tetrahedral order parameter at amphiphile concentrations differing approximately by a factor of 2 and corroborates well with the steady-state and the time-resolved spectroscopic measurements. This work therefore explains, following a uniform and cogent manner, both the experimentally observed structural and dynamical anomalies in microscopic terms.

Rigid Cations Induce Enhancement of Microheterogeneity and Exhibit Anomalous Ion Diffusion in Water-Ethanol Mixtures

Because of the amphiphilic nature of ethanol in the aqueous solution, ions cause an interesting microheterogeneity where the water molecules and the hydroxy groups of ethanol preferentially solvate the ions, while the ethyl groups tend to occupy the intervening space. Using computer simulations, we study the dynamics of rigid monovalent cations (Li⁺, Na⁺, K⁺, and Cs⁺) in aqueous ethanol solutions with chloride as the counterion. We vary both the size of the ions and the composition of the mixture to explore size- and composition-dependent ion diffusion. The relative stability of enhanced microheterogeneous configurations makes ion diffusion slower than what would be surmised by using the bulk properties of the mixture, using the Stokes-Einstein relation. We study the structure through partial radial distribution functions and the stability through coordination number fluctuations. The ion diffusion coefficient exhibits sharp re-entrant behavior when plotted against viscosity varied by composition. Our studies reveal multiple anomalous features of ion motion in this mixture. We formulate a mode-coupling theory (MCT) that takes into account the interaction between different dynamical components; MCT can incorporate the effects of heterogeneous dynamics and nonlinearity in composition dependence that arise from the feedback between mutually dependent ion-solvent dynamics.

Experiments on tracer diffusion in aqueous and non-aqueous solvent combinations

Forced Rayleigh scattering is used to study the tracer diffusion of an azobenzene in binary combinations of polar solvents, including water. In the absence of water, the tracer diffusion coefficient D in the mixture lies between the diffusion coefficients within the pure solvents, on a curve that is reasonably close to the prediction of free-volume theory. If water is present, on the other hand, the diffusion coefficient displays a minimum that is less than the smaller of the two pure-solvent values. We attempt to understand the different behavior in water by concentrating on the fairly hydrophobic nature of the solute, leading to a first solvent shell that is hydrophobic on the inside and hydrophilic on the outside. We also believe that clusters of amphiphiles explain the observation that, in aqueous combinations, D is nearly constant above a certain amphiphile mole fraction.

SIZE DEPENDENCE OF SOLVATION STRUCTURE AND DYNAMICS OF IONS IN LIQUID N-METHYLACETAMIDE: A MOLECULAR DYNAMICS SIMULATION STUDY

The solvation structure and dynamics of alkali metal (Li+, Na+, K+, Rb+, Cs+) and halide (F-, Cl-, Br-, I-) ions in liquid N -methylacetamide (NMA) are calculated at two different temperatures T = 313 K and 453 K, by using classical molecular dynamics simulations. We have also considered [Formula: see text] and some larger cations such as I +, Me 4 N +, and Et4N+ in this study to investigate the size dependence solvation structure and dynamics of ions in liquid NMA. With the increase of ion size, the self-diffusion coefficients of cations are found to increase and the maximum is observed at Me4N+ , whereas for halide ions the increase of diffusion coefficient with ion size continues up to I- and no maximum is observed. However, the relative increase of the diffusion coefficients of larger ion compared to those of Li+ and F+ are found to be significantly higher at low temperature. Results are very good in agreement with experimental observation.

Nonideality in diffusion of ionic and hydrophobic solutes and pair dynamics in water-acetone mixtures of varying composition

We have performed a series of molecular dynamics simulations of water-acetone mixtures containing either an ionic solute or a neutral hydrophobic solute to study the extent of nonideality in the dynamics of these solutes with variation of composition of the mixtures. The diffusion coefficients of the charged solutes, both cationic and anionic, are found to change nonmonotonically with the composition of the mixtures showing strong nonideality of their dynamics. Also, the extent of nonideality in the diffusion of these charged solutes is found to be similar to the nonideality that is observed for the diffusion and orientational relaxation of water and acetone molecules in these mixtures which show a somewhat similar changes in the solvation characteristics of charged and dipolar solutes with changes of composition of water-acetone mixtures. The diffusion of the hydrophobic solute, however, shows a monotonic increase with increase of acetone concentration showing its different solvation characteristics as compared to the charged and dipolar solutes. The links between the nonideality in diffusion and solvation structures are further confirmed through calculations of the relevant solute-solvent and solvent-solvent radial distribution functions for both ionic and hydrophobic solutes. We have also calculated various pair dynamical properties such as the relaxation of water-water and acetone-water hydrogen bonds and residence dynamics of water molecules in water and acetone hydration shells. The lifetimes of both water-water and acetone-water hydrogen bonds and also the residence times of water molecules are found to increase steadily with increase in acetone concentration. No maximum or minimum was found in the composition dependence of these pair dynamical quantities. The lifetimes of water-water hydrogen bonds are always found to be longer than that of acetone-water hydrogen bonds in these mixtures. The residence times of water molecules are also found to follow a similar trend.