Evaluation of Evaporation Fluxes for Pesticides and Low Volatile Hazardous Materials Based on Evaporation Kinetics of Net Liquids.

Evaporation is the phase transition process that plays a significant role in many spheres of life and science. Volatilization of hazardous materials, pesticides, petroleum spills, etc., impacts the environment and biosphere. Predicting evaporation fluxes under specific environmental conditions is challenging from theoretical and empirical points of view. A new practical method for estimating fluxes is proposed based on our experimental results and previously published data. It is demonstrated that some parameters in theoretical equations for near-equilibrium evaporation can be estimated from experiments, and these formulas can be exploited to predict steady-state evaporation fluxes in the air in a range of 8 orders of magnitude based on a single experiment carried out for nontoxic volatile compounds.

Spontaneous Dipole Reorientation in Confined Water and Its Effect on Wetting/Dewetting of Hydrophobic Nanopores.

The properties of nanoconfined fluids are important for a broad range of natural and engineering systems. In particular, wetting/dewetting of hydrophobic nanoporous materials is crucial due to their broad applicability for molecular separation and liquid purification; energy storage, conversion, recuperation, and dissipation; for catalysis, chromatography, and so on. In this work, a rapid, orchestrated, and spontaneous dipole reorientation was observed in hydrophobic nanotubes of various pore sizes d (7.9-16.5 Å) via simulations. This phenomenon leads to the fragmentation of water clusters in the narrow nanopores (d = 7.9, 10 Å) and strongly affects dewetting through cluster repulsion. The cavitation in these pores has an electrostatic origin. The dependence of hydrogen-bonded network properties on the tube aperture is obtained and is used to explain wetting (intrusion)-dewetting (extrusion) hysteresis. Computer simulations and experimental data demonstrate that d equals ca. 12.5 Å is a threshold between a nonhysteretic (spring) behavior, where intrusion-extrusion is reversible, and a hysteretic one (shock absorber), where hysteresis is prominent. This work suggests that water clustering and the electrostatic nature of cavitation are important factors that can be effectively exploited for controlling the wetting/dewetting of nanoporous materials.

Effect of the Topology on Wetting and Drying of Hydrophobic Porous Materials.

Establishing molecular mechanisms of wetting and drying of hydrophobic porous materials is a general problem for science and technology within the subcategories of the theory of liquids, chromatography, nanofluidics, energy storage, recuperation, and dissipation. In this article, we demonstrate a new way to tackle this problem by exploring the effect of the topology of pure silica nanoparticles, nanotubes, and zeolites. Using molecular dynamics simulations, we show how secondary porosity promotes the intrusion of water into micropores and affects the hydrophobicity of materials. It is demonstrated herein that for nano-objects, the hydrophobicity can be controlled by changing the ratio of open to closed nanometer-sized lateral pores. This effect can be exploited to produce new materials for practical applications when the hydrophobicity needs to be regulated without significantly changing the chemistry or structure of the materials. Based on these simulations and theoretical considerations, for pure silica zeolites, we examined and then classified the experimental database of intrusion pressures, thus leading to the prediction of any zeolite's intrusion pressure. We show a correlation between the intrusion pressure and the ratio of the accessible pore surface area to total pore volume. The correlation is valid for some zeolites and mesoporous materials. It can facilitate choosing prospective candidates for further investigation and possible exploitation, especially for energy storage, recuperation, and dissipation.

Subnanometer Topological Tuning of the Liquid Intrusion/Extrusion Characteristics of Hydrophobic Micropores.

Intrusion (wetting)/extrusion (drying) of liquids in/from lyophobic nanoporous systems is key in many fields, including chromatography, nanofluidics, biology, and energy materials. Here we demonstrate that secondary topological features decorating main channels of porous systems dramatically affect the intrusion/extrusion cycle. These secondary features, allowing an unexpected bridging with liquid in the surrounding domains, stabilize the water stream intruding a micropore. This reduces the intrusion/extrusion barrier and the corresponding pressures without altering other properties of the system. Tuning the intrusion/extrusion pressures via subnanometric topological features represents a yet unexplored strategy for designing hydrophobic micropores. Though energy is not the only field of application, here we show that the proposed tuning approach may bring 20-75 MPa of intrusion/extrusion pressure increase, expanding the applicability of hydrophobic microporous materials.

Molecular dynamics investigation of the transient regime in the freezing of salt clusters.

Molecular dynamics (MD) investigations of the freezing of supercooled liquids can identify nuclei far smaller than can be detected in laboratory experiments, to date, and consequently can provide information about nucleation so far inaccessible to experiment. In a recent MD study of the freezing of clusters of SeF6, a new method of recording nucleation events was introduced. It involved the observation of times of first appearance of nuclei of the size of n. An advantage of the new approach is that it provides information about the size of the critical nucleus. For nuclei smaller than the critical size, it also avoids the overshoots of nucleation rates that precluded the application of the Miloshev-Wu method in the subcritical region. Kinetic information in the transient regime can be characterized by three parameters, the time lag, the reduced moment, and the steady state nucleation rate. To get some idea of how general the new approach is, a very different system was investigated, that of clusters of NaCl. Two different fitting functions were used to analyze the results. The first one adopted the log-normal probability distribution of Wu. The second function was a modification of Shneidman's analytical solution appropriate for large nuclei. The second function gave a rather good account of MD data for all nuclear sizes and temperatures and gave more stable results in the subcritical region. Several inferences of the sizes of critical nuclei were made. Applying the criterion for n* based on the Zeldovich solution of the Becker-Döring equations, we estimated the critical nucleus sizes to be 14, 18, and 24 ions for quench temperatures of 640, 690, and 740 K, respectively. Even though the interionic interactions initiating nucleation in salt are very different from the van der Waals interactions in clusters of SeF6, the characteristic aspects of the transient regimes of the two systems were quite similar.