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We have performed intensive computer simulations of the irreversible annihilation reaction: A + A → C + C and of the trapping reaction: A + B → C + B for a variety of three-dimensional fluids composed of identical spherical particles. We have found a significant difference in the asymptotic behavior of the rate coefficients for these reactions. Both the rate coefficients converge to the same value with time t going to infinity but the convergence rate is different: the O(t-1/2) term for the annihilation reaction is higher than the corresponding term for the trapping reaction. The simulation results suggest that ratio of the terms is a universal quantity with the value equal to 2 or slightly above. A model for the annihilation reaction based on the superposition approximation predicts the difference in the O(t-1/2) terms, but overestimates the value for the annihilation reaction by about 30%. We have also performed simulations for the dimerization process: A + A → E, where E stands for a dimer. The dimerization decreases the reaction rate due to the decrease in the diffusion constant for A. The effect is successfully predicted by a simple model. [ABSTRACT FROM AUTHOR]

We performed molecular dynamics simulations of liquid film evaporation into vacuum for two cases: free evaporation without external supply of energy and evaporation at constant average liquid temperature. In both cases we found that the pressure inside a liquid film was constant, while temperature decreased and density increased as a function of distance from the middle of the film. The momentum flux in the vapor far from the liquid was equal to the liquid pressure in the evaporating film. Moreover the pseudopressure (stagnation pressure) was found to be constant in the evaporating vapor and equal to the liquid pressure. The momentum flux and its relation to the pressure determined the number of evaporating molecules per unit time and as a consequence the mass evaporation flux. We found a simple formula for the evaporation flux, which much better describes simulation results than the commonly used Hertz–Knudsen relation. [ABSTRACT FROM AUTHOR]

The influence of concentrations of reagents on the rate of reaction: A+B→C+B for low density equimolar mixtures of spherically symmetric ions immersed in the Brownian medium has been investigated by performing large scale molecular dynamics simulations. The Coulomb potential of ion-ion interactions is truncated at the cutoff distance large enough to make the kinetics of the reaction independent of its value. The simulations have been performed at conditions close to that for quenching reactions for fluophores. One of the simulation results is that the excess in the rate coefficient Δk is always positive and converges to a constant value which is two to three orders in magnitude higher than that for the soft spheres immersed in the Brownian medium [Litniewski, J. Chem. Phys. 124, 114502 (2006)]. Δk is approximately proportional to c however, if the concentration is high, positive deviations [O(c2)] are noticeable. The simulation results are compared with simple model that bases on the superposition approximation. The model predicts most of the properties of Δk. The predicted values are about 30%–40% lower than that from the simulations. [ABSTRACT FROM AUTHOR]

The influence of the interactions between reagents on the excess in the rate coefficient, Δk, for the instantaneous reaction A+B→C+B have been investigated by performing large scale molecular dynamics simulations for simple soft spheres. The simulation method has enabled us to determine the contributions to Δk coming from A-B as well as B-B interactions. The simulations have shown that positive values of Δk that appear both for the liquid and for the Brownian system [M. Litniewski, J. Chem. Phys. 123, 124506 (2005); 124, 114501 (2006)] result from B-B interactions. If B-B interactions were absent, Δk was always negative. The influence of B-B interactions was about three times higher for the Brownian system than for the liquid. A qualitative explanation for the effect has been proposed basing on a simple model and analyzing the influence of B-B interactions on fluctuations in concentrations of reagents. The influence of A-B interactions was completely negligible except for the liquid at short times, for which the cancellation of A-B interaction noticeably decreased Δk. [ABSTRACT FROM AUTHOR]

Molecular dynamics investigations on the influence of the concentration of B (quencher) on the rate coefficient, k(t), for the reaction A+B→C+B are continued [M. Litniewski, J. Chem. Phys. 123, 124506 (2005); 124, 114501 (2006)]. The problem is investigated by analyzing the excess in the two-particle probability density function and in its radial moments. The simulations have been performed for the deterministic systems as gas and liquid as well as for the Brownian system. The influence of moderate changes of the reaction radius resulting in changes of the activation energy has been also considered. The most important result is that the excess in k(t) may be not only a direct consequence of fluctuations in concentrations. For the gas, the excess in the mean radial velocity of A towards B dominated over the excess in the value of the probability density function. As a result, the excess in k(t) was negative in spite of the excess in the relative spatial correlations between A and B was positive. The excess in the mean radial velocity was completely unimportant for dense liquids and the Brownian system. [ABSTRACT FROM AUTHOR]

In this paper new results of the simulations [M. Litniewski, J. Chem. Phys. 123, 124506 (2005)] on the influence of the quencher concentration on the reaction A+B→C+B for the identical soft sphere system are presented. The problem is generalized by considering also the case when the spheres are immersed in the Brownian medium. A significant difference between simple deterministic systems and the Brownian ones is found: the excess in the rate coefficient for the Brownian system is constant and positive, except for very short times. The reaction has been simulated for a very long time, but any tendency to decrease the excess has not been noted. It is also shown that the relative excess in the surviving probability is a universal quadratic function of the quencher concentration for the range of time much longer than the result from the previous simulations. A very strong correlation between the excess in the relative value of spatial correlations between the reagents and the excess in the rate coefficient is shown. It is also shown that the A-A and A-C interactions have some influence on the excess values. A simple model for this effect is presented. [ABSTRACT FROM AUTHOR]

The paper presents the results of large-scale molecular dynamics simulations of the irreversible bimolecular reaction A+B→C+B for the simple liquid composed of mechanically identical soft spheres. The systems with the total number of molecules corresponding to 107–109 are considered. The influence of the concentration of a quencher (B) on the surviving probability of A and the reaction rate is analyzed for a wide range of the concentrations and for two significantly different reduced densities. It is shown that the quencher concentration dependence effect (QCDE) is, in fact, a composition of two QCDE effects: the short-time QCDE that increases the reaction rate and the long-time QCDE that decreases it. The paper also analyzes the influence of the concentration on the steady-state rate constant, kSS, obtained by integrating the surviving probability. The excess in kSS due to finite quencher concentration changes the sign from negative to positive while going from low to high concentrations. Generally, the excess is extremely weak. It attains a 1% level only if the concentration is very high. [ABSTRACT FROM AUTHOR]

The Smoluchowski approach to description of fluorescence quenching is tested by comparing the theory with computer simulations for the case of spherical molecules. The distance dependent sink terms describing the electron transfer mechanism and the Forster model for the energy transfer are considered. It is shown that the agreement between the rate coefficient from the model and from simulations depends on the strength of the solute-solvent interactions as well as on the speed of reaction itself. Comparing results of simulations for different quencher concentrations we estimate the strength of quencher concentration dependence effect and the range of times the effect may be significant. In the long time limit the increase in quencher concentration decreased the rate coefficient. [ABSTRACT FROM AUTHOR]

We derive an approximate expression for the time-dependent reaction rate coefficient, k(t), of the Smoluchowski equation for the step function nonradiative lifetime (SFNL) model in the case of structureless liquid (i.e., if there are no spatial correlations between molecules of reactants). The SFNL model assumes that a reaction occurs with equal probability for reactants at distances between r[sub 0] and r[sub 1]. The accuracy of the obtained analytical formula for k(t) is absolutely sufficient for practical applications like the interpretation of experiments on fluorescence quenching. A molecular dynamics has shown that the SFNL model much better describes the simulation results than the Smoluchowski–Collins–Kimball model if the distance between r[sub 1] and r[sub 0] cannot be neglected. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulations for a liquid of identical soft spheres we analyze the relative diffusion constant DΣn(r) and the self diffusion constant Dn where r is the interparticle distance and n = 2, 3 denotes the dimensionality. We demonstrate that for the periodic boundary conditions, Dn is a function of the system size and the relation: DΣn(r = L/2) ≅ 2Dn(L), where L is the length of the cubic box edge, holds both for n = 2 and 3. For n = 2 both DΣ2(r) and D2(L) increase logarithmically with its argument. However, it was found that the diffusive process for large two dimensional systems is very sensitive to perturbations. The sensitivity increases with L and even a very low perturbation limits the increase of D2(L→∞). Nevertheless, due to the functional form of DΣ2(r) the standard assumption for the Smoluchowski type models of reaction kinetics at three dimensions:DΣn(r) ≈ 2Dn leads to giant errors if applied for n = 2. [ABSTRACT FROM AUTHOR]

The influence of concentrations of reagents on the rate of reaction: A + B → C + B has been investigated by performing large scale computer simulations on systems of spherically symmetric molecules. The problem has been analyzed both for deterministic systems (gas, liquid) and for the stochastic ones where the particles are immersed in the Brownian medium. Significant deviations from the law of mass action have been found. The deviations result not only from fluctuations in concentrations of reagents. For the Brownian systems the simulations have given a positive value of the excess in the rate coefficient even for very long times, which contradicts with general expectations. The reason for the positive excess values is repulsion between B particles (the so called excluded volume effect). The effect is strongly influenced by the shape of the probability density function for B-B pairs. As a result, for the liquids the effect is weaker: for short times the excess in rate coefficient is positive but for long times it becomes negative. [ABSTRACT FROM AUTHOR]

In this paper we consider a chemical process, for which the reagent of interest decays shortly after it is created. For such a system the standard theory of nonequilibrium spatial correlations in the density fluctuations, which leads to Ornstein-Zernike type of expression, does not work. We compare results of molecular simulations with another theoretical description based on the assumption that the lifetime of the reagent is short enough to treat its motion as a ballistic rather than diffusive one. [ABSTRACT FROM AUTHOR]

Adsorption of nanoparticles on a spherical colloidal particle is studied by molecular dynamics simulations. We consider a generic model for a mixture of nanoparticles with energetically favored self-assembly into alternating layers of the two components. When both components are attracted to the colloidal particle, the adsorbed nanoparticles self-assemble either into alternating parallel tori and clusters at the two poles of the colloidal particle, or into alternating spirals wrapped around the spherical surface. The long-lived metastable states obtained in simulations follow from the spherical shape of the adsorbing surface and the requirement that the neighboring chains of the nanoparticles are composed of different components. A geometrical construction leading to all such patterns is presented. When the second component particles are repelled from the colloidal particle and the attraction of the first component is strong, the attracted particles form a monolayer at the surface of the colloidal particle that screens the repulsion of the second component. The subsequent adsorbed alternating spherical layers of the two components form together a thick shell. This structure leads to the adsorption that is larger than in the case of the same attraction of the two components to the colloidal particle.