The vibrational OH-stretch spectra of large water clusters were measured by photofragment spectroscopy after the absorption of pulsed tunable infrared radiation in the frequency range from 3000 to 3800 cm -1 . The mean size of the clusters from 〈n〉 ) 20 to 1960 was measured by threshold photoionization of the water clusters doped with sodium atoms. The largest abundance of the fragments was that of water hexamers. The fragment intensities are measured for different excitation energies and different cluster temperatures as function of the cluster size. For the selected sizes 〈n〉 ) 48, 111, 631, and 1960 complete OH-stretch spectra have been measured. The comparison with calculations revealed that the method is mainly sensitive to the outer cluster surface which has for all sizes an amorphous structure dominated by 3-coordinated and to a lesser extent also by 4-coordinated molecules. The intensity of the hexamer fragments goes through a maximum at n ) 70 and drops to n ) 300 where it levels off with a different slope. This behavior is attributed to the number of available connected 3-coordinated water molecules and the influence of the emerging 4-coordinated molecules in these clusters.
The interface between the vapor and liquid phase of quadrupolar-dipolar fluids is the seat of an electric interfacial potential whose influence on ion solvation and distribution is not yet fully understood. To obtain further microscopic insight into water specificity we first present extensive classical molecular dynamics simulations of a series of model liquids with variable molecular quadrupole moments that interpolates between SPC/E water and a purely dipolar liquid. We then pinpoint the essential role played by the competing multipolar contributions to the vapor-liquid and the solute-liquid interface potentials in determining an important ion-specific direct electrostatic contribution to the ionic solvation free energy for SPC/E water-dominated by the quadrupolar and dipolar parts-beyond the dominant polarization one. Our results show that the influence of the vapor-liquid interfacial potential on ion solvation is strongly reduced due to the strong partial cancellation brought about by the competing solute-liquid interface potential.
Optimized structures and bonding energies have been calculated for ammonia clusters from n=3 to n=18 using a pairwise additive model potential. The trimer and tetramer are stable cyclic configurations. From the pentamer onward the structures are three dimensional with an increasing tendency to amorphous behavior. The exceptions are the heptamer with a Cs axis, the hexadecamer with a central atom, and the very stable and completely symmetric dodecamer with the D6h point group. Here each ammonia molecule is bound by two covalent and two hydrogen bonds. In general, the coordination number increases from 2.0 for the rings over 4.0 for n=12 to 4.2 for n=18. The structures agree where available with previously obtained results for a more elaborate potential.
Extensive molecular dynamics simulations of the flow of aqueous NaCl and NaI solutions through carbon nanotubes are presented, evidencing the dependence of diverse transport features on the solute specificity, the nanotube geometry, and the various atomic models employed, including polarizability. The simulated properties are in agreement with published results, indicating that ion translocation sets in only for nanotubes with chiralities higher than (7,7), and extend the explanation of the mechanisms governing ion transport to larger chiralities. The interpretation of the various dynamic quantities is developed in close connection with the structural details of the solution and the energy barriers the solute components have to overcome. Also, the role and relevance of water and ion polarizabilities are discussed in detail.
The present investigations continue previous research on transport in aqueous ionic solutions through carbon nanotubes. Specifically, the effects of the nanotube radius, solute concentration, and applied external electric fields on the transport properties are investigated in terms of mobilities, currents, and pairing times of the solute ions. The simulated transport features are corroborated with general theoretical results of nanofluidics (such as the linear log-log regime of the nanochannel conductance as function of the solute concentration and the current-voltage curve of the channel). Discontinuities in the partial ionic currents are explained on the basis of a recent theoretical model of quantized ionic conductance in nanopores, developed by Zwolak et al. Correlations between the structural and dynamic properties are established, linking causally the highly structured spatial density profiles, the ion pairing phenomenon and the ionic currents.
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