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Ilan Benjamin

Publications and source records attributed to Ilan Benjamin.

12 recordsLinked to original sources
PubMed2006

Theoretical studies of solute vibrational energy relaxation at liquid interfaces.

Recent advances in the theoretical understanding of solute vibrational energy relaxation at liquid interfaces and surfaces are described. Non-equilibrium molecular dynamics simulations of the relaxation of an initially excited solute molecule are combined with equilibrium force autocorrelation calculations to gain insight into the factors that influence the vibrational relaxation rate. Diatomic and triatomic nonpolar, polar, and ionic solute molecules adsorbed at the liquid/vapor interface of several liquids as well as at the water/CCl(4) liquid/liquid interface are considered. In general, the vibrational relaxation rate is significantly slower (a factor of 3 to 4) at the liquid/vapor and liquid/liquid interface than in the bulk due to the reduced density, which gives rise to a reduced contribution of the repulsive solvent-solute forces on the vibrational mode. The surface effects on the ionic solutes are much smaller (50% or less slower relaxation relative to the bulk). This is due to the fact that ionic solutes at the interface are able to keep part of their solvation shell to a degree that depends on their size. Thus, a significant portion of the repulsive forces is maintained. A high degree of correlation is found between the peak height of the solvent-solute radial distribution function and the vibrational relaxation rate. The relaxation rate at the liquid/liquid interface strongly depends on the location of the solute across the interface and correlates with the change in the density and polarity profile of the interface.

Journal Article↗
PubMed2006

Structure of the interface between two polar liquids: nitrobenzene and water.

Synchrotron X-ray reflectivity is used to study the electron density as a function of depth through the bulk nitrobenzene-water interface at four different temperatures. The measured interfacial width differs from the predictions of capillary wave theory with a progressively smaller deviation as the temperature is raised. Computer simulations suggest the presence of both molecular layering and dipole ordering parallel to the interface. Either layering or a bending rigidity, that can result from dipole ordering, can explain these measurements.

Computer Simulation↗
PubMed2006

Ion distributions near a liquid-liquid interface.

Mean field theories of ion distributions, such as the Gouy-Chapman theory that describes the distribution near a charged planar surface, ignore the molecular-scale structure in the liquid solution. The predictions of the Gouy-Chapman theory vary substantially from our x-ray reflectivity measurements of the interface between two electrolyte solutions. Molecular dynamics simulations, which include the liquid structure, were used to calculate the potential of mean force on a single ion. We used this potential of mean force in a generalized Poisson-Boltzmann equation to predict the full ion distributions. These distributions agree with our measurements without any adjustable parameters.

Journal Article↗
PubMed2005

Path-integral computations of tunneling processes.

The application of the path-integral methodology of Chandler and Wolynes [D. Chandler and P. G. Wolynes, J. Chem. Phys. 74, 4078 (1981)] to the calculation of one-electron-tunneling probabilities is revisited. We show that the evaluation of the kink free energy that is related to the tunneling splitting is associated with "polymer bead" distributions over a continuous distribution of scaled barriers, which makes both the calculation and its physical interpretation relatively difficult. In particular, we find that relative to other available techniques the method converges slowly and suffers from inaccuracies associated with the finite-temperature aspect of the calculation, and that past tentative identification of the bead distribution over the barrier with a physical picture of a "tunneling path" should be reassessed.

Journal Article↗
PubMed2005

Hydration shell exchange dynamics during ion transfer across the liquid/liquid interface.

We examine using molecular dynamics simulations the rate and mechanism of water molecules exchange around the Li(+) and Na(+) ions during ion transfer across the interface between water and nitrobenzene. As the ions are transferred from the water to the organic phase, they keep their first hydration shell and an incomplete second shell. The rate of water exchange between the first shell and the rest of the interfacial water molecule decreases during the transfer, which is consistent with an increase in the barrier along the ion-water potential of mean force. While in bulk water the exchange of water molecules around the Li(+) follows an associative (A) or associative interchange (I(a)) type mechanism, the fraction of exchange events of type A increases at the interface. In contrast, while in bulk water the exchange of water molecules around the six coordinated Na(+) hydrated species mainly follows a dissociative mechanism, the situation at the interface involves an equilibrium interchange between the four- and five-coordinated hydrated ion. Simulation of the reversed process, in which the hydrated Li(+) ion is transferred to the aqueous phase, shows the same general behavior as a function of location from the interface.

Cations, Monovalent↗
PubMed2005

Nucleophilic substitution reactions at liquid/liquid interfaces: molecular dynamics simulation of a model S(N)1 dissociation reaction at the water/carbon tetrachloride interface.

The ionic dissociation step of the nucleophilic substitution reaction t-BuCl --> t-Bu(+) + Cl(-) is studied at the water/carbon tetrachloride interface using molecular dynamics computer simulations. The empirical valence bond approach is used to couple two diabatic states, covalent and ionic, in the electronically adiabatic limit. The umbrella sampling technique is used to calculate the potential of mean force along the reaction coordinate (defined as the t-Bu to Cl distance) at several interface regions of varying distances from the Gibbs dividing surface. We find a significant increase of the ionic dissociation barrier height and of the reaction free energy at the interface relative to bulk water. This is shown to be due to the reduced polarity of the interface which causes a destabilization of the pure ionic state. However, deformation to the neat interface structure in the form of water protrusions into the organic phase may provide partial stabilization of the ionic species. The importance of these structural effects is examined by repeating the calculations with an artificially smooth interface. The destabilization of the ionic state at the interface also manifests itself with a rapid (picosecond time scale) recombination dynamics of the ions to form the parent molecule followed by a slow vibrational relaxation.

Journal Article↗
PubMed2005

Hydrogen bond dynamics at water/organic liquid interfaces.

Hydrogen bond dynamics at the neat interface between water and a series of organic liquids are studied with molecular dynamics computer simulation. The organic liquids are nonpolar (carbon tetrachloride), weakly polar (1,2-dichloroethane), and polar (nitrobenzene). The effect of surface polarity and surface roughness is examined. The dynamics are expressed in terms of the hydrogen bond population autocorrelation functions and are found to be nonexponential and strongly dependent on the nature of the organic phase. In particular, at all interfaces, the dynamics are slower at the interface than in the bulk and sensitive to the location of the water molecules along the interface normal.

Journal Article↗
PubMed2005

A molecular-dynamics study of a model S(N)1 dissociation reaction at the water liquid/vapor interface.

The thermodynamics and dynamics of a model S(N)1 reaction: t-BuCl --> t-Bu+ + Cl- is studied at the water liquid/vapor interface using molecular-dynamics computer simulations. The empirical valence bond approach is used to couple two diabatic states, covalent and ionic, in the electronically adiabatic limit. Umbrella sampling calculations are used to calculate the potential of mean force along the reaction coordinate (defined as the t-Bu to Cl distance) in bulk water and in several locations at the interface. We find a significant increase of the dissociation barrier height and of the reaction free energy at the interface relative to the bulk. This is shown to be due to the reduced polarity of the interface. Reactive flux correlation function calculations show significant deviation of the rate constant from the transition-state theory: The transmission coefficients range from 0.49 in the bulk to 0.05 above the Gibbs surface. The low transmission coefficient at the interface despite the lower friction is shown to be due to slow vibrational relaxation.

Journal Article↗
PubMed2005

Photo-induced ion transfer across the liquid/liquid interface.

Traditional studies of ion transfer across the interface between two immiscible liquids involve the establishment of a steady-state ion current across the interface. The data obtained from these studies are used to develop models of the interfacial kinetic. However, this approach, while straightforward to implement experimentally, is not sensitive to the microscopic structure of the interface region, which is only a few nanometers in size. We propose and examine the feasibility of a more direct approach to elucidating the ion dynamic by reporting the results of equilibrium and non-equilibrium molecular dynamics calculations in which an iodine ion is created at the interface between water and CCl4 by photodissociation of an adsorbed I2-. In this paper, we describe the model, examine the associated free energy curves and discuss the recombination of the parent molecule and its vibrational relaxation as a competing process for the ion transfer process.

Journal Article↗
PubMed2004

Photodissociation of ICN at the liquid/vapor interface of water.

The photodissociation of ICN adsorbed at the liquid/vapor interface of water is studied using classical molecular dynamics with nonadiabatic surface hopping. The cage escape, geminate recombination to form ICN and INC and the subsequent vibrational relaxation of these two molecules (on their ground electronic states) is compared with the same process in bulk water and with previous photodissociation studies at liquid interfaces. We find that the reduced surface density and weaker solvent-solute interactions give rise to reduced rate of nonadiabatic transitions and that the probability for cage escape at the interface is significantly enhanced due to the possibility that one or both of the photodissociation fragments desorb into the gas phase. The overall desorption probability varies from 75% to 92% for ICN initially located just below the Gibbs surface (50% bulk density) to ICN located just above the Gibbs surface, respectively. The corresponding geminate recombination probabilities are 18% and 9%, respectively. The vibrational relaxation rate of the recombined ICN is slower than in the bulk by a factor of 2.3.

Journal Article↗