Study of Molecular Behavior in a Water Nanocluster: Size and Temperature Effect

Temperature dependence of the contact angle of water: A review of research progress, theoretical understanding, and implications for boiling heat transfer

Contact angle, a quantitative measure of macroscopic surface wettability, plays an important role in understanding liquid-vapor heterogeneous phase change phenomena, e.g., boiling heat transfer. The contact angles of water at elevated temperatures are of particular interest for understanding of wettability-regulated boiling heat transfer in steam-based power generation. From a more theoretical perspective, the temperature dependence of contact angle of water is also essential to estimation of several key surface thermodynamic properties, such as the solid surface tension, the surface entropy, and the heats of immersion and adsorption. Here, a comprehensive review of historical efforts in measuring the contact angles of water over a wide temperature range on a variety of solids, not limited to metallic surfaces, is presented. As suggested by the literature data, the temperature dependence of contact angle of water may be classified into three regimes: (a) low temperatures below the saturation point (i.e., 100 °C at atmospheric pressure), (b) medium temperatures up to ~170 °C, and (c) high temperatures up to 300 °C at pressurized conditions. A slightly-decreasing or nearly-invariant trend of the contact angles of water on both non-metallic and metallic surfaces was reported for the low-temperature regime. In contrast, a steeper linear decline in water contact angle was demonstrated at temperatures above 100 °C. The few experimental data available on several metallic surfaces showed that the contact angle of water either again becomes nearly temperature-independent or further decreases with temperature above 210 °C. A theoretical understanding of the temperature dependence is given based on surface thermodynamic analysis, although the exact molecular mechanisms underlying these experimental observations remain unclear. Consequently, the theoretical model for predicting the variation of the contact angle of water with temperature is not well-developed. As the critical point of water (374 °C and 22.1 MPa) is approached, the surface tension, and hence the contact angle, should become vanishingly small. However, this theoretical expectation has not yet been verified due to the lack of experimental data at such high temperatures/pressures. Finally, future research directions are identified, including a systematic exploration of the contact angle at near-critical temperatures, the effects of surface oxidation, corrosion, and deposition on contact angle during operation of boilers and reactors, and the particular effect of irradiation on contact angle in nuclear reactor applications.

High-dilution effects revisited. 1. Physicochemical aspects

Several lines of evidence suggest that homeopathic high dilutions (HDs) can effectively have a pharmacological action, and so cannot be considered merely placebos. However, until now there has been no unified explanation for these observations within the dominant paradigm of the dose-response effect. Here the possible scenarios for the physicochemical nature of HDs are reviewed. A number of theoretical and experimental approaches, including quantum physics, conductometric and spectroscopic measurements, thermoluminescence, and model simulations investigated the peculiar features of diluted/succussed solutions. The heterogeneous composition of water could be affected by interactive phenomena such as coherence, epitaxy and formation of colloidal nanobubbles containing gaseous inclusions of oxygen, nitrogen, carbon dioxide, silica and, possibly, the original material of the remedy. It is likely that the molecules of active substance act as nucleation centres, amplifying the formation of supramolecular structures and imparting order to the solvent. Three major models for how this happens are currently being investigated: the water clusters or clathrates, the coherent domains postulated by quantum electrodynamics, and the formation of nanoparticles from the original solute plus solvent components. Other theoretical approaches based on quantum entanglement and on fractal-type self-organization of water clusters are more speculative and hypothetical. The problem of the physicochemical nature of HDs is still far from to be clarified but current evidence strongly supports the notion that the structuring of water and its solutes at the nanoscale can play a key role.

Size and chain length effects on structural behaviors of biphenylcyclohexane-based liquid crystal nanoclusters by a coarse-grained model

Size and chain length effects on structural behaviors of liquid crystal nanoclusters were examined by a coarse-grained model and the configurational-bias Monte Carlo (CBMC) simulation. The nanoclusters investigated in this study are composed of the biphenylcyclohexane-based BCH5H liquid crystal molecule and its derivatives. Results of the study show that the average energy decreases (i.e., more negative) as the cluster size (i.e., the number of molecules) increases. With the increasing cluster size, the equilibrium conformation of the nanocluster changes gradually from a pipe-like structure (for the smaller systems) to a ball-like cluster (for the larger systems). The order parameter of the system reduces with the transition of the equilibrium conformation. Regarding the chain length effect, the pipe-like equilibrium conformation (for the smaller systems) was observed more close to a pipe as the length of the tail alkyl chain of the derivatives extended. However, due to the flexibility of the tail alkyl chain, the pipe conformation of the system deflects slightly about its cyclohexyl group as the tail extends further.

Dynamical property of water droplets of different sizes adsorbed onto a poly(methyl methacrylate) surface

A molecular dynamics approach has been employed to study the dynamical behavior of a water droplet adsorbed on a poly(methyl methacrylate) (PMMA) surface. Several sizes of water droplets are considered in order to understand the size influence of the droplet on the dynamical properties of water molecules on the PMMA substrate. The local density profile of water molecules in the droplet upon impact with the PMMA surface is calculated, and the result shows an increase in water penetration with a decrease in the size of the droplet. By examining the velocity field, the regular motion of the water droplet is found both during the equilibrium process and after the droplet reaches the equilibrium state. The dynamical behavior of water molecule is studied by the velocity autocorrelation function (VACF) in different regions for different sizes of water droplets. The result shows that VACFs in different regions are significantly influenced for the droplet with 500 water molecules than for that with 2000 water molecules. Calculations in different regions are made for the vibrational spectrum of the oxygen atom, as well as for hydrogen bond dynamics, the lifetime, and the relaxation time of the hydrogen bond. The changes in the hydrogen bond dynamics are consistent with the change in the distribution of the hydrogen bond angle. We conclude that the dynamical properties of the water molecule are significantly affected by the region relative to the surface but only weakly influenced by the size of the droplet.

Water-chloride and water-bromide hydrogen-bonded networks: influence of the nature of the halide ions on the stability of the supramolecular assemblies

Two compounds, namely, [TTPH(2)](Cl)(2) x 4 H(2)O (1) and [TTPH(2)](Br)(2) x 4 H(2)O (2), (TTP = 4'-p-tolyl-2,2':6',2''-terpyridine) were synthesized from purely aqueous media and characterized by physical techniques. In the solid-state structures of these compounds, interesting supramolecular assemblies are observed. In 1, an unusual staircase-like architecture of the tape of edge-shared planar water hexamer is of importance, where the chloride ions are at the two edges of the tape. In 2, the polymeric nature of the water-bromide assembly is of interest, where discrete open-cube water octamers are doubly bridged by bromide ions. Semiempirical and DFT calculations confirm that the nature of the anion indeed affects the topology of the water-halide assemblies. We conclude that the protonated [TTPH(2)](2+) species can act as appropriate receptors for halide ions, which in turn act as a matrix for the formation of polymeric 1D water-halide assemblies.

Simulations of RNA base pairs in a nanodroplet reveal solvation-dependent stability

We show that RNA base pairs have variable stability depending on their degree of solvation. This finding has far-reaching biological implications for nucleic acid structure in a partially solvated cellular environment such as inside RNA-protein complexes. Molecular dynamics simulations of partially solvated Watson-Crick RNA base pairs show that whereas water serves to destabilize a base pair by competing for and disrupting base-base hydrogen bonds, when sufficient water molecules are present, fewer hydrogen bonds are available to disrupt the base pairs and the destabilization effect is reduced. The result is that base pairs exist at a stability minimum when solvated in between 20 and 100 water molecules, the upper limit of which corresponds to the approximate number of water molecules contained in the first hydration shell.