Showing papers by "Peter J. Rossky published in 2007"

Effect of surface polarity on water contact angle and interfacial hydration structure.

Abstract: We perform molecular dynamics simulations of water in the presence of hydrophobic/hydrophilic walls at T = 300 K and P = 0 GPa. For the hydrophilic walls, we use a hydroxylated silica model introduced in previous simulations [Lee, S. H.; Rossky, P. J. J. Chem. Phys. 1994, 100, 3334. Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G.; Phys. Rev. E 2006, 73, 041604.]. By rescaling the physical partial atomic charges by a parameter 0 ≤ k ≤ 1, we can continuously transform the hydrophilic walls (hydroxylated silica, k = 1) into hydrophobic apolar surfaces (k = 0). From a physical point of view, k is the normalized magnitude of a surface dipole moment, and thus it quantifies the polarity of the surface. We calculate the contact angle of water for 0 ≤ k ≤ 1. We find that, at least for the present homogeneous, atomically flat, and defect-free surface model, the magnitude of the surface dipole correlates with the contact angle in a one-to-one correspondence. In particular, we find that polar surfaces with 0 < ...

Hydration behavior under confinement by nanoscale surfaces with patterned hydrophobicity and hydrophilicity

Abstract: We perform molecular dynamics simulations of water confined between nanoscale surfaces (≈3.2 × 3.2 nm2) with various patterns of hydrophobicity and hydrophilicity at T = 300 K, −0.05 GPa ≤ P ≤ 0.2 GPa, and plate separations 0.5 nm ≤ d ≤ 1.6 nm. We find that the water surface density in the first hydration layer is considerably higher at a hydrophobic patch surrounded by hydrophilic borders than it is at a purely hydrophobic surface with the same area, highlighting the importance of heterogeneity on hydrophobicity at nanoscopic length scales. Increasing the pressure causes a progressive blurring of the difference between interfacial water densities manifest at hydrophilic and hydrophobic surfaces, with only minor differences remaining at 0.2 GPa. At P = −0.05 GPa and d = 0.6 nm, a single layer of hydrophilic sites along the border of the hydrophobic nanoscale plates is sufficient to prevent bulk cavitation, in contrast to the behavior observed in the absence of the hydrophilic sites. At small separation be...

How protein surfaces induce anomalous dynamics of hydration water.

Abstract: Water around biomolecules slows down with respect to pure water, and both rotation and translation exhibit anomalous time dependence in the hydration shell. The origin of such behavior remains elusive. We use molecular dynamics simulations of water dynamics around several designed protein models to establish the connection between the appearance of the anomalous dynamics and water−protein interactions. For the first time we quantify the separate effect of protein topological and energetic disorder on the hydration water dynamics. When a static protein structure is simulated, we show that both types of disorder contribute to slow down water diffusion, and that allowing for protein motion, increasing the spatial dimentionality of the interface, reduces the anomalous character of hydration water. The rotation of water is, instead, altered by the energetic disorder only; indeed, when electrostatic interactions between the protein and water are switched off, water reorients even faster than in the bulk. The dy...

Water-like solvation thermodynamics in a spherically symmetric solvent model with two characteristic lengths

Abstract: We examine by molecular dynamics simulation the solubility of small apolar solutes in a solvent whose particles interact via the Jagla potential, a spherically symmetric ramp potential with two characteristic lengths: an impenetrable hard core and a penetrable soft core. The Jagla fluid has been recently shown to possess water-like structural, dynamic, and thermodynamic anomalies. We find that the solubility exhibits a minimum with respect to temperature at fixed pressure and thereby show that the Jagla fluid also displays water-like solvation thermodynamics. We further find low-temperature swelling of a hard-sphere chain dissolved in the Jagla fluid and relate this phenomenon to cold unfolding of globular proteins. Our results are consistent with the possibility that the presence of two characteristic lengths in the Jagla potential is a key feature of water-like solvation thermodynamics. The penetrable core becomes increasingly important at low temperatures, which favors the formation of low-density, open structures in the Jagla solvent.

Excess electron relaxation dynamics at water/air interfaces

Abstract: We have performed mixed quantum-classical molecular dynamics simulations of the relaxation of a ground state excess electron at interfaces of different phases of water with air. The investigated systems included ambient water/air, supercooled water/air, Ih ice/air, and amorphous solid water/air interfaces. The present work explores the possible connections of the examined interfacial systems to finite size cluster anions and the three-dimensional infinite, fully hydrated electron. Localization site analyses indicate that in the absence of nuclear relaxation the electron localizes in a shallow potential trap on the interface in all examined systems in a diffuse, surface-bound (SB) state. With relaxation, the weakly bound electron undergoes an ultrafast localization and stabilization on the surface with the concomitant collapse of its radius. In the case of the ambient liquid interface the electron slowly (on the 10ps time scale) diffuses into the bulk to form an interior-bound state. In each other case, th...

Quantum density fluctuations in liquid neon from linearized path-integral calculations

Abstract: The Feynman-Kleinert linearized path-integral [J. A. Poulsen et al., J. Chem. Phys. 119, 12179 (2003)] representation of quantum correlation functions is applied to compute the spectrum of density fluctuations for liquid neon at $T=27.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, $p=1.4\phantom{\rule{0.3em}{0ex}}\text{bar}$, and $Q$ vector $1.55\phantom{\rule{0.3em}{0ex}}{\mathrm{Aa}}^{\ensuremath{-}1}$. The calculated spectrum as well as the kinetic energy of the liquid are in excellent agreement with the experiment of Cunsolo et al. [Phys. Rev. B 67, 024507 (2003)].

Nuclear quantum effects on the nonadiabatic decay mechanism of an excited hydrated electron

Abstract: We present a kinetic analysis of the nonadiabatic decay mechanism of an excited state hydrated electron to the ground state. The theoretical treatment is based on a quantized, gap dependent golden rule rate constant formula which describes the nonadiabatic transition rate between two quantum states. The rate formula is expressed in terms of quantum time correlation functions of the energy gap and of the nonadiabatic coupling. These gap dependent quantities are evaluated from three different sets of mixed quantum-classical molecular dynamics simulations of a hydrated electron equilibrated (a) in its ground state, (b) in its first excited state, and (c) on a hypothetical mixed potential energy surface which is the average of the ground and the first excited electronic states. The quantized, gap dependent rate results are applied in a phenomenological kinetic equation which provides the survival probability function of the excited state electron. Although the lifetime of the equilibrated excited state electr...