Mean Residence Time Of Molecules Research Articles

Nuclear Quantum Effects in Sodium Hydroxide Solutions from Neural Network Molecular Dynamics Simulations.

Nuclear quantum effects (NQEs) cause the nuclei of light elements like hydrogen to delocalize, affecting numerous properties of water and aqueous solutions, such as hydrogen-bonding and proton transfer barriers. Here, we address the prototypical case of aqueous NaOH solutions by investigating the effects of quantum nuclear fluctuations on radial distribution functions, hydrogen-bonding geometries, power spectra, proton transfer barriers, proton transfer rates, water self-exchange rates around the Na+ cations, and diffusion coefficients, for the full room-temperature solubility range. These properties were calculated from classical and ring-polymer molecular dynamics simulations employing a reactive high-dimensional neural network potential based on dispersion-corrected density functional theory reference calculations. We find that NQEs have a very small impact on the solvation structure around Na+, slightly strengthen the water-water and water-hydroxide hydrogen bonds, and lower the peak positions in the power spectra for the HOH bending and OH stretching modes by about 50 and 100 cm-1, respectively. Moreover, NQEs significantly lower the proton transfer barriers, thus increasing the proton transfer rates, resulting in an increase of the diffusion coefficient in particular of OH-, as well as a decrease of the mean residence time of molecules in the first hydration shell around Na+ at high NaOH concentrations.

Dynamical Study of a Micellar System

We investigated the dynamical behavior of equilibrium micellar aggregates. We found the characteristic times associated with the processes of exchange between the free surfactants and aggregates in dilute solutions. We employed Monte-Carlo simulations on a two-dimensional lattice, each amphiphile molecule occupying three sites, while all the remaining sites of the lattice are filled with a single water molecule per site. For a fixed concentration of the amphiphiles, we studied the dependence of mean residence time of molecules in aggregates as a function of temperature and of the size of the aggregates. We showed that the distribution curves for the residence time and that for the aggregation number are closely related. In particular, we could also characterize the transition to a micellar state by the behavior of the representative curves of the mean residence times as a function of temperature.

Mean residence time of molecules diffusing in a cell bounded by a semi-permeable membrane: Monte Carlo simulations and an expression relating membrane transition probability to permeability.

The rapid exchange of water across erythrocyte membranes is readily measured using an NMR method that entails doping a suspension of cells with a moderately high concentration of Mn(2-) and measuring the rate of transverse relaxation of the nuclear magnetisation. Analysis of the data yields an estimate of the rate constant for membrane transport, from which the membrane permeability can be determined. It is assumed in the analysis that the efflux rate of the water is solely a function of the rate of membrane permeation and that the time it takes for intracellular water molecules to diffuse to the membrane is relatively insignificant. The limits of this assumption were explored by using random-walk simulations of diffusion in cells modelled as parallel planes, spheres, and biconcave discs. The rate of membrane transport was specified in terms of a transition probability but it was not initially clear what the relationship should be between this parameter and the diffusional membrane permeability P(d). This relationship was derived and used to show that the mean residence time for a water molecule is determined by P(d) when the diffusion coefficient is above a certain threshold value; it is determined by the distance to the membrane below that value.

Calculation of the first stage of “ideal” emulsion polymerization

The kinetics of the micelle stage in “ideal” emulsion polymerization has been considered with the assumption that the rate of adsorption of free radicals by micelles and latex particles may be described by the Smoluchowski equation. The relationships found are very little different in form from those obtained previously with the assumption that the rate of adsorption of radicals is proportional to the particle surface area. The experimental data do not enable an unambigous selection to be made in favour of one of the assumptions. Data about the mean residence time of molecules of organic substances in a micelle give evidence in favour of the second assumption.