Showing papers by "Peter J. Rossky published in 2009"

Phase transitions induced by nanoconfinement in liquid water.

Abstract: We present results from molecular dynamics simulations of water confined by two parallel atomically detailed hydrophobic walls. Simulations are performed at T = 300 K and wall-wall separation d = 0.6-1.6 nm. At 0.7 < or = d < or = 0.9 nm, a first order transition occurs between a bilayer liquid (BL) and a trilayer heterogeneous fluid (THF) as water density increases. The THF is characterized by a liquid (central) layer and two crystal-like layers next to the walls. The BL-THF transition involves freezing of the two surface layers in contact with the walls. At d = 0.6 nm, the THF transforms into a bilayer ice (BI) upon decompression. Both the BL-THF and BI-THF transitions are induced by the surface regular atomic-scale structure.

Enhanced surface hydrophobicity by coupling of surface polarity and topography

Abstract: We use atomistic computer simulation to explore the relationship between mesoscopic (liquid drop contact angle) and microscopic (surface atomic polarity) characteristics for water in contact with a model solid surface based on the structure of silica. We vary both the magnitude and direction of the solid surface polarity at the atomic scale and characterize the response of an aqueous interface in terms of the solvent molecular organization and contact angle. We show that when the topography and polarity of the surface act in concert with the asymmetric charge distribution of water, the hydrophobicity varies substantially and, further, can be maximal for a surface with significant polarity. The results suggest that patterning of a surface on several length scales, from atomic to μm lengths, can make important independent contributions to macroscopic hydrophobicity.

Interior- and surface-bound excess electron states in large water cluster anions.

Abstract: We present the results of mixed quantum/classical simulations on relaxed thermal nanoscale water cluster anions, (H(2)O)(n)(-), with n=200, 500, 1000, and 8000. By using initial equilibration with constraints, we investigate stable/metastable negatively charged water clusters with both surface-bound and interior-bound excess electron states. Characterization of these states is performed in terms of geometrical parameters, energetics, and optical absorption spectroscopy of the clusters. The calculations provide data characterizing these states in the gap between previously published calculations and experiments on smaller clusters and the limiting cases of either an excess electron in bulk water or an excess electron at an infinite water/air interface. The present results are in general agreement with previous simulations and provide a consistent picture of the evolution of the physical properties of water cluster anions with size over the entire size range, including results for vertical detachment energies and absorption spectra that would signify their presence. In particular, the difference in size dependence between surface-bound and interior-bound state absorption spectra is dramatic, while for detachment energies the dependence is qualitatively the same.

Nonadiabatic Mixed Quantum−Classical Dynamic Simulation of π-Stacked Oligophenylenevinylenes

Abstract: We present results from the first nonadiabatic (NA), nonequilibrium mixed quantum-classical molecular dynamics simulations of pi-stacked oligophenylvinylene (OPV) chains with a quantum electronic Hamiltonian (Pariser-Parr-Pople with excited states given by configuration interaction) that goes beyond the tight-binding approximation. The chains pack approximately 3.6 A apart in the ground state at 300 K, and we discuss how thermal motions, chiefly a relative sliding motion along the oligomer backbone, affect the electronic structure. We assign the electronic absorption spectrum primarily to the S(0) --> S(2) transition as transitions from the ground state to S(1) and S(3) are particularly weak. After photoexcitation, the system rapidly decays via NA transitions to S(1) in under 150 fs. On S(1), the system relaxes as a bound exciton, localized on one chain that may hop between chains with a characteristic time between 300 and 800 fs. We find that the system does not make a rapid transition to the ground state because both the NA and radiative couplings between S(1) and S(0) are weak.

Nuclear quantum effects in electronically adiabatic quantum time correlation functions: application to the absorption spectrum of a hydrated electron.

Abstract: A general formalism for introducing nuclear quantum effects in the expression of the quantum time correlation function of an operator in a multilevel electronic system is presented in the adiabatic limit. The final formula includes the nuclear quantum time correlation functions of the operator matrix elements, of the energy gap, and their cross terms. These quantities can be inferred and evaluated from their classical analogs obtained by mixed quantum-classical molecular dynamics simulations. The formalism is applied to the absorption spectrum of a hydrated electron, expressed in terms of the time correlation function of the dipole operator in the ground electronic state. We find that both static and dynamic nuclear quantum effects distinctly influence the shape of the absorption spectrum, especially its high energy tail related to transitions to delocalized electron states. Their inclusion does improve significantly the agreement between theory and experiment for both the low and high frequency edges of ...

Response to “Comment on ‘An electron-water pseudopotential for condensed phase simulation’ ” [J. Chem. Phys.131, 037101 (2009)]

Abstract: We are grateful for the detailed re-examination of Larsen et al. of the electron-water pseudopotential that we proposed more than 20 years ago. The potential was among the first of a number of potentials that were used by various groups in studies of the structure, dynamics, and spectroscopy of solvated electrons. While our potential was based on a more detailed analysis than had been carried out in previous work, it nevertheless included many assumptions and strong approximations. Effectively, it was an ad hoc model potential—irrespective of its construction from basic physical arguments. It has been validated, at least on a qualitative level, by comparison to experiment in a number of contexts, most importantly excited state dynamics. When devising the potential, we considered four fundamental contributions: i static Coulomb interactions between the excess electron and the dipolar water molecules, ii electrostatic polarization effects, iii Pauli repulsion reflecting the orthogonality constraint between the excess electronic wave function and the solvent wave functions, and iv exchange interactions. The last were omitted from the final potential, but the other three terms were deemed significant enough to be included. As Larsen et al. showed, an error was apparently made in the calculation of the parameters that are associated with one of the three terms that are represented in the potential. This means that our model potential does not, in fact, follow from the procedure as originally described. Effectively, a correct calculation following the route originally proposed shifts the balance in favor of the repulsive orthogonality term, correspondingly de-emphasizing the Coulomb interaction with the polar water molecules. However, what Larsen et al. observed when simulations are carried out with the “corrected” potential is remarkable. Despite the considerable change in water distribution at a short distance from the electron, once again a cavity-like state for the hydrated electron is found—just as has been seen in numerous other studies of hydrated electrons that employed alternative potential functions. There are some differences, in that the relatively smaller contribution of the polar interaction term leads to a solvation structure that is considerably less akin to that of an anion than was observed in the original simulations. One would then conclude that not only anions but also small nonpolar solutes can provide a meaningful reference point for the description of the hydrated electron. The calculated absorption spectrum is found to be redshifted, albeit without any real change in the lineshape or the underlying origin of that lineshape. While the spatial details of the hydration structure of hydrated electrons remain unsettled, we believe that the comparison of Larsen et al. of the two potential functions only confirms the general robustness of our understanding that has emerged over the past two decades: There is a cavity with an electronic ground state that can be s-like, and the main band of the absorption spectrum can be explained by transitions to three excited states that can be p-like and that are only approximately degenerate. Simulations of hydrated electrons with sophisticated many-electron methods indicate that the precise description of the hydrated electron may be more involved, but the simple cavity model nevertheless continues to serve as a most useful reference point.