An off-lattice Wang-Landau study of the coil-globule and melting transitions of a flexible homopolymer

The Wang-Landau Monte Carlo approach is applied to the coil-globule and melting transitions of off-lattice flexible homopolymers. The solid-liquid melting point and coil-globule transition temperatures are identified by their respective peaks in the heat capacity as a function of temperature. The melting and theta points are well separated, indicating that the coil-globule transition occurs separately from melting even in the thermodynamic limit. We also observe a feature in the heat capacity between the coil-globule and melting transitions which we attribute to a transformation from a low-density liquid globule to a high-density liquid globule.This work was supported by an Australian ARC Discovery Grant

Derjaguin’s Water II: a surface hydration phenomenon

B.V. Derjaguin’s promotion of anomalous water II (polywater) in the early 1970s was an embarrassing point in the career of an illustrious chemist, and quickly repudiated by Derjaguin himself. Water II does not exist as a bulk liquid. And yet a theoretical model of the hydration of ions developed by I. Klugman, consistent with electrolyte properties such as equivalent conductivity, diffusion coefficient, and viscosity, found a density of water molecules in the hydration shell of ions to be 1.4 g/cm3, close to the density of water II reported by Derjaguin and Churaev. Given Derjaguin and Churaev’s use of adsorption in fine capillaries, Klugman postulates that their anomalous experiments can be understood as measuring the hydration layer of adsorbed water rather than bulk water. Derjaguin’s last publication in 1994 on violation of Archimedes’ Law during adsorption may be intended to hint at this conclusion. Perhaps Derjaguin’s involvement with water II can, in the end, be celebrated not as a study of bulk liquid but as a study of adsorption phenomena and hydration

The effects of saline water on the recovery of lead and zinc sulfide during froth flotation

In this study, we investigated the effects of water salinity on the flotation performance of pure lead and zinc sulfide mineral samples as well as a Pb/Zn complex sulfide ore by means of micro-flotation and batch flotation experiments. Our results showed higher PbS and ZnS recoveries in more concentrated NaCl salt solutions. The results for the experiments using seawater demonstrated that in the presence of additional ions, such as Ca2+ and Mg2+, the recovery of PbS and ZnS was significantly reduced. As part of this investigation, we developed and implemented a surface complexation model for ZnS based on the presence of two differently charged surface sites. Zeta potential measurements of ZnS particles were used to optimise the parameters of our model. It was found that the surface potentials calculated using this model were in good agreement with the experimental zeta potentials, validating the model for predicting the zeta potential behaviour of ZnS particles over a broad range of pH and NaCl concentrations. Additionally, total interaction free energies were determined as a function of separation distance, representing particle–particle and particle-bubble interactions of our study in different NaCl concentrations. The theoretical analyses showed that asymmetric Pb/Zn particle–particle interactions were repulsive at lower NaCl concentrations, before becoming purely attractive at higher NaCl concentrations. For the case of the symmetric particle–particle interactions, attraction controlled all interactions, regardless of NaCl concentration. The calculated PbS-bubble interactions were repulsive in lower NaCl concentrations but became increasingly attractive in higher NaCl concentrations. Strong repulsions controlled all ZnS-bubble interactions, and these interactions remained repulsive with increasing NaCl concentration. The theoretical projections presented in this study were in good agreement with the measured saline water flotation phenomena

Nanodiamond-treated flax: improving properties of natural fibers

Synthetic fibers are used extensively as reinforcement in composite materials, but many of them face environmental concerns such as high energy consumption during production and complicated decommissioning. Natural fibers have been considered as an attractive solution for making composites more sustainable. However, they are generally not as strong as synthetic fibers. It is therefore of interest to investigate ways to improve the properties of natural fibers without compromising environmental issues. Here, we present a study of the moisture absorption and mechanical properties of flax that has been exposed to hydrogenated nanodiamonds through an ultrasonic dispersion treatment. Nanodiamonds are known to be non-toxic, unlike many other carbon-based nanomaterials. We show that nanodiamond-treated flax fabric has a lower moisture content (~ −18%), lower moisture absorption rate and better abrasion resistance (~ +30%). Single yarns, extracted from the fabric, show higher tensile strength (~ +24%) compared to untreated flax. Furthermore, we present a theoretical model for the nanodiamond fiber interaction, based on the Derjgauin–Landau–Verwey–Overbeek (DLVO) theory of colloid interactions. The simulations indicate that the mechanical properties improve due to an enhancement of the electrolytic force, dispersion force and hydrogen bonding of nanodiamond-treated fibers, which strengthens the cohesion between the fibers. We also apply the model to nanodiamond-treated cotton. The lower zeta potential of cotton increases the electrolytic force. Comparing the results to experimental data of nanodiamond-treated flax and nanodiamond-treated cotton suggests that the fiber’s zeta potential is critical for the improvements of their mechanical properties. Graphical abstract: [Figure not available: see fulltext.

Effective Polarisability Models

Theories for the effective polarisability of a small particle in a medium are presented using different levels of approximation: we consider the virtual cavity, real cavity and the hard-sphere models as well as a continuous interpolation of the latter two. We present the respective hard-sphere and cavity radii as obtained from density-functional simulations as well as the resulting effective polarisabilities at discrete Matsubara frequencies. This enables us to account for macroscopic media in van der Waals interactions between molecules in water and their Casimir-Polder interaction with an interface

Ultrathin Metallic Coatings Can Induce Quantum Levitation between Nanosurfaces

There is an attractive Casimir-Lifshitz force between two silica surfaces in a liquid (bromobenze or toluene). We demonstrate that adding an ultrathin (5-50{\AA}) metallic nanocoating to one of the surfaces results in repulsive Casimir-Lifshitz forces above a critical separation. The onset of such quantum levitation comes at decreasing separations as the film thickness decreases. Remarkably the effect of retardation can turn attraction into repulsion. From that we explain how an ultrathin metallic coating may prevent nanoelectromechanical systems from crashing together.Comment: 4 pages, 5 figure

Role of zero point energy in promoting ice formation in a spherical drop of water

We demonstrate that the Lifshitz interaction energy (excluding the self-energies of the inner and outer spherical regions) for three concentric spherical dielectric media can be evaluated easily using the immense computation power in recent processors relative to those of a few decades ago. As a prototype, we compute the Lifshitz interaction energy for a spherical shell of water immersed in water vapor of infinite extent while enclosing a spherical ball of ice inside the shell, such that two concentric spherical interfaces are formed: one between solid ice and liquid water and the other between liquid water and gaseous vapor. We evaluate the Lifshitz interaction energy for the above configuration at the triple point of water when the solid, liquid, and gaseous states of water coexist, and, thus, extend the analysis of Elbaum and Schick in Phys. Rev. Lett. 66 (1991) 1713 to spherical configurations. We find that, when the Lifshitz energy contributes dominantly to the total energy of this system, which is often the case when electrostatic interactions are absent, a drop of water surrounded by vapor of infinite extent is not stable at the triple point. This instability, that is a manifestation of the quantum fluctuations in the medium, will promote formation of ice in water, which will then grow in size indefinitely. This is a consequence of the finding here that the Lifshitz energy is minimized for large (micrometer size) radius of the ice ball and small (nanometer size) thickness of the water shell surrounding the ice. These results might be relevant to the formation of hail in thunderclouds. These results are tentative in that the self-energies are omitted; surface tension and nucleation energy are not considered.Comment: 15 pages, 9 figures, 1 table, conclusions toned dow

Sign of the Casimir-Polder interaction between atoms and oil-water interfaces: Subtle dependence on dielectric properties

We demonstrate that Casimir-Polder energies between noble gas atoms (dissolved in water) and oil-water interfaces are highly surface specific. Both repulsion (e.g. hexane) and attraction (e.g. glycerine and cyclodecane) is found with different oils. For several intermediate oils (e.g. hexadecane, decane, and cyclohexane) both attraction and repulsion can be found in the same system. Near these oil-water interfaces the interaction is repulsive in the non-retarded limit and turns attractive at larger distances as retardation becomes important. These highly surface specific interactions may have a role to play in biological systems where the surface may be more or less accessible to dissolved atoms.Comment: 5 pages, 6 figure