London-van Der Waals Research Articles

Resin bound gold nanocomposites assisted SE/ATR-FTIR spectroscopy for detection of pymetrozine insecticide in vegetable samples

The goal of this work was to assess the competence of organic hydrophobic resin bound gold nanocomposites (OH/R-AuNCs) for detection of pymetrozine insecticide from vegetable samples employing surface-enhanced/attenuated total reflectance-Fourier transform infrared (SE/ATR-FTIR) spectroscopy. The adsorption isotherm models, including the Langmuir, Freundlich and Temkin, are tested to reveal the interactive behaviour between the OH/R-AuNCs and pesticide. The adsorption occurs principally by London–Van der Waals dispersion interactions and hydrogen bonding interactions between the surface of OH/R-AuNCs materials and the hydrophobic part of pesticide molecule. The characteristic absorption band obtained at 3019.94 cm−1 was utilized for the quantitative analysis of pymetrozine insecticide in vegetable samples. The method was found to be accurate and precise, with mean recovery values in the range of 94.5–110 %, correlation coefficient of 0.992 %, and detection limit of 2.65 μg mL−1. The adsorption efficiency of the designed OH/R-AuNCs significantly influences the SE/ATR-FTIR response of the pymetrozine around 90 %. The optimized and validated method was applied to determine the residual concentrations of the pymetrozine that had been applied to vegetable samples.

Influence of surfactant on clay swelling inhibition of weathered crust elution-deposited rare earth ores

Clay minerals are the main components of the weathered crust elution-deposited rare earth ores (WREOs), and their swelling properties have a significant effect on the safe and green in-situ leaching of WREOs. Few researches have been reported on the inhibition of clay swelling in WREOs and our teammates used combination of ammonium salts to suppress the clay swelling in previous work. Surfactants combined with (NH4)2SO4 were utilized as inhibitor in this study to reduce the amount of ammonium salts and thus decrease the ammonium pollution. Effects of different surfactants on inhibition of clay swelling were investigated, and the swelling inhibition mechanism was further discussed by DLVO theory. CTAB exhibited best performance among five surfactants. The swelling efficiency of kaolinite, halloysite and illite in DI water is 2.52%, 1.65% and 1.32% respectively, and these values can be reduced to 0.45%, 0.65% and 0.82% respectively in presence of single (NH4)2SO4. As (NH4)2SO4 was compounded with 0.04% CTAB, the swelling efficiency of kaolinite, halloysite and illite was further reduced to 0.25%, 0.45% and 0.31% respectively. The XRD and SEM analysis indicated that no change of the morphology and crystal structure of minerals after soaking with surfactants were presented. It was further clarified by DLVO theory that the London-van der Waals energy between clay particles increased, while both of the repulsion electrostatic energy and the total potential energy decreased in the presence of CTAB, and this was beneficial to the clay swelling inhibition.

Interaction of Nanoparticles in Electrolyte Solutions.

The interaction between nanoparticles includes several components; however, the most frequently used are electrostatic, caused by overlapping double electrical layers, and London-van der Waals interactions, caused by quantum and thermodynamic fluctuations of electromagnetic fields. Only these two kinds of interaction are considered below. The electrostatic interaction is calculated based on the linearized Poisson-Boltzmann equation for particles with constant electrical potential of the surfaces (constant ζ potentials). An exact solution of the problem is obtained for both identical particles and particles of different sizes. For the London-van der Waals interaction, the screening of static fluctuations and the retardation of electromagnetic fields for the dispersive part of the interaction are taken into account. The total interaction energy for two particles was calculated for a range of possible nanoparticle sizes from 1 to 103 nm and electrolyte concentration from 10-2 to 10-6 mol/L. The predominance of the London-van der Waals force over the shielded electrostatic repulsion force was found at high electrolyte concentrations in the range from 10-2 to 10-3 mol/L at large interparticle distances.

Transport and removal of stormwater vehicle-related mobile organic contaminants in geomedia-amended sand columns

Green infrastructure drainage systems are innovative treatment units that capture and treat stormwater. Unfortunately, highly polar contaminants remain challenging to remove in conventional biofilters. To overcome treatment limitations, we assessed the transport and removal of stormwater vehicle-related organic contaminants with persistent, mobile, and toxic (in short: PMTs) properties, such as 1H-benzotriazole, NN'-diphenylguanidine, and hexamethoxymethylmelamine (PMT precursor), using batch experiments and continuous-flow sand columns amended with pyrogenic carbonaceous materials, like granulated activated carbon (GAC) or wheat-straw derived biochar. Our results indicated that all investigated contaminants were subjected to nonequilibrium interactions in sand-only and geomedia-amended columns, with kinetic effects upon transport. Experimental breakthrough curves could be well described by a one-site kinetic transport model assuming saturation of sorption sites, which we inferred could occur due to dissolved organic matter fouling. Furthermore, from both batch and column experiments, we found that GAC could remove contaminants significantly better than biochar with higher sorption capacity and faster sorption kinetics. Hexamethoxymethylmelamine, with the lowest organic carbon-water partition coefficient (KOC) and largest molecular volume among target chemicals, exhibited the lowest affinity in both carbonaceous adsorbents based on estimated sorption parameters. Results suggest that sorption of investigated PMTs was likely driven by steric and hydrophobic effects, and coulombic and other weak intermolecular forces (e.g., London–van der Waals, H-bonding). Results from extrapolating our data to a 1-m depth geomedia-amended sand filter suggested that GAC and biochar could enhance the removal of organic contaminants in biofilters and last for more than one decade. Overall, our work is the first to study treatment alternatives for NN'-diphenylguanidine and hexamethoxymethylmelamine, and contributes to better PMT contaminant removal strategies in environmental applications.

Van der Waals–London interaction of atoms with pseudorelativistic kinetic energy

We consider a multiatomic system where the nuclei are assumed to be point charges at fixed positions. Particles interact via Coulomb potential and electrons have pseudo-relativistic kinetic energy. We prove the van der Waals-London law, which states that the interaction energy between neutral atoms decays as the sixth power of the distance $|D|$ between the atoms. We rigorously compute all the terms in the binding energy up to the order $|D|^{-9}$ with error term of order $\mathcal{O}(|D|^{-10})$ . As intermediate steps we prove exponential decay of eigenfunctions of multiparticle Schr\"odinger operators with permutation symmetry imposed by the Pauli principle and new estimates of the localization error.

Pour point depressant efficacy as a function of paraffin chain-length

Pour point depressant (PPD) activity mechanisms include solubilization, morphology alteration, and steric/entropic repulsion. A wealth of complex intermolecular interactions governs PPD efficacies in real crude compositions. The current study utilizes single-component model waxes (n-C24H50, n-C28H58, n-C32H66 or n-C36H74) dissolved in dodecane to reveal chain-length dependent PPD efficacies. Wax appearance temperature depression, effected by PPD, diminishes as paraffin chain-length increases. Stronger London van der Waals attractions between longer wax components effectively diminish PPD solubilization activity. Controlled-temperature centrifugation reveals partitioning of PPD polymers and paraffin waxes between solid and liquid phases. PPD polymers typically contain a polydisperse molecular weight distribution. The highest molecular weight PPD polymers partition preferentially to solid phases, including PPD aggregates and PPD-modified wax crystals. The lowest molecular weight PPD polymers remain preferentially soluble in the liquid phase, binding only weakly to precipitated wax. The presence of precipitated wax counteracts the solubilization activity of PPD polymer. At high precipitated wax fractions, PPD shows no wax solubilization activity. As the precipitated wax fraction decreases, the wax solubilization activity of the PPD progressively increases. The wax solubilization activity of the PPD attains a maximum at the WAT. Finally, the results are consistent with a single optimal polymer molecular weight for PPD activity occurring at a single temperature. PPD polymers larger than the optimal MW undergo a coil-to-globule transition prior to wax crystallization, deactivating the polymer. Polymers smaller than the optimal molecular weight show weaker binding to wax crystals, consistent with a smaller Gibbs free energy of binding, and partition preferentially to the liquid phase. Modern PPD formulations should utilize polymers that are tailored according to molecular weight. Optimally tailored PPD polymers should be less polydisperse in nature than current PPD formulations.

London Dispersion Corrections to Density Functional Theory for Transition Metals Based on Fitting to Experimental Temperature-Programmed Desorption of Benzene Monolayers.

Standard implementations of generalized gradient approximation (GGA)-based density functional theory (DFT) describe well strongly bound molecules and solids but fail to describe long-range London dispersion or van der Waals (vdW) attraction interactions that are important in molecular crystals and two-dimensional solids. To provide accurate values for the vdW distance and energies for the metals Cu, Ag, Au, Ni, Pd, and Pt, we determined empirical vdW corrections to Perdew, Burke, and Ernzerhof (PBE) DFT by fitting the experimental adsorption enthalpies measured by temperature-programmed desorption (TPD) from benzene monolayers by Campbell and co-workers ( J. Phys. Chem. C 2016, 120, 25161-25172). Benzene physisorbed to these metals without chemical reaction; therefore, we consider the bonding to be vdW. We use the low gradient form for the vdW corrections, EvdW-LG = -C6LG/[R6 + RvdwLG6] with just two parameters per atom (C6LG and RvdwLG). This LG form leads to negligible changes in bond distances and angles, so adjusting the parameters should not sacrifice accuracy for the bonding interactions. We demonstrate that the parameters fitted to benzene also describe well the physisorption enthalpies for other hydrocarbons (naphthalene, cyclohexane, methane, ethane, and propane) on Pt. We also report low gradient vdW correction parameters for the noble gases that fit the equilibrium lattice parameter and heat of vaporization of the crystals.

Ultrafiltration of up-flow biological activated carbon effluent: Extracellular polymer biofouling mechanism and mitigation using pre-ozonation with H2O2 backwashing

Biofouling is a key problem in membrane filtration, and extracellular polymer substances (EPS) play a key role in biofouling. Biofouling contributes to membrane fouling during ultrafiltration of up-flow biological activated carbon filter (UBACF) effluent. EPS are released when pollutants get attached with membrane surface and when pollutants are in solution phase from cell lysis and by cell secretions. In our study of EPS + humic acid (HA) prepared as the effluent pollutants for ultrafiltration, we found that EPS increased the interfacial forces between the pollutants and the membrane, resulting in membrane fouling. In the early stages of filtration, the main contribution of EPS to membrane fouling was to bond with organic colloids, which led to an increase in the pollutant particle size and zeta potential. This increased the short-range Lewis acid-base (AB) forces from −4.89 nN to −12.59 nN and accelerated the formation of a cake layer. In the late stage of filtration, the EPS increased both the AB and London–van der Waals (LW) forces, thus accelerating membrane fouling. In order to mitigate biofouling, we developed a method of pretreating the effluent with 0.4 mg/L ozone prior to ultrafiltration and backwashing with 8 mg/L H2O2 to sterilize bacteria attached to the membrane surface. This method not only changed the characteristics of the EPS, but also inactivated bacteria by disinfection with H2O2, thereby reducing the amount of EPS. The proposed method provided a long-term stable operation guarantee for ultrafiltration of UBACF effluent.

Measurement of Van der Waals force using oscillating optical tweezers

We employ oscillating optical tweezers as a probe to measure the surface forces between polystyrene and silica. Thus, we modulate a trapped polystyrene particle with an external sinusoidal force in close proximity (∼80 nm) of a silica surface. The particle motion is influenced by several factors which include an increased drag force according to Faxen's correction, a spurious force that comes into play due to the diffusion coefficient of the medium becoming position dependent, and finally, the London-Van der Waals (LVdW) force which becomes substantial when the particle approaches the surface. By accounting for the other forces from the analytically known results, we are able to directly quantify the LVdW force from the experimentally measured amplitude of the oscillating particle. Thereby, we determine the Hamaker constant H for the LVdW force between polystyrene and silica, and obtain a good agreement with the value reported in the literature. Our method is general in nature and can be extended toward probing other surface effects or other interaction forces using oscillating optical tweezers.

Multicomponent polyurethane–carbon black composite as piezoresistive sensor

Composite of carbon black (CB) dispersed into a multicomponent polyurethane matrix combines the good mechanical properties of the polymer, such as elasticity, with the electrical property of the conducting particles. Electrical current–voltage (I × V) analysis found the percolation threshold through the sample surface to be between 0.6 and 0.8 vol%. Below the percolation threshold concentration (pc), the effect of the Coulomb potential is accentuated on the CB dispersion, while in the region over pc, the London–van der Waals potential is important. The repeatability in the piezoresistive behavior was observed under the application of several loading cycles. The gauge factors obtained were 4.9 and 2.0 for samples with 0.8 and 1.0 vol% of CB, respectively. The results indicate that the material can be used as a piezoresistive sensor.

Van der Waals Density Functional Theory vdW-DFq for Semihard Materials

There are a large number of materials with mild stiffness, which are not as soft as tissues and not as strong as metals. These semihard materials include energetic materials, molecular crystals, layered materials, and van der Waals crystals. The integrity and mechanical stability are mainly determined by the interactions between instantaneously induced dipoles, the so called London dispersion force or van der Waals force. It is challenging to accurately model the structural and mechanical properties of these semihard materials in the frame of density functional theory where the non-local correlation functionals are not well known. Here, we propose a van der Waals density functional named vdW-DFq to accurately model the density and geometry of semihard materials. Using β -cyclotetramethylene tetranitramine as a prototype, we adjust the enhancement factor of the exchange energy functional with generalized gradient approximations. We find this method to be simple and robust over a wide tuning range when calibrating the functional on-demand with experimental data. With a calibrated value q = 1.05 , the proposed vdW-DFq method shows good performance in predicting the geometries of 11 common energetic material molecular crystals and three typical layered van der Waals crystals. This success could be attributed to the similar electronic charge density gradients, suggesting a wide use in modeling semihard materials. This method could be useful in developing non-empirical density functional theories for semihard and soft materials.

Influence of electrical surface charges on thermodynamics of wettability during low salinity water flooding on limestone reservoirs

Wettability is an important factor in controlling the displacement efficiency and ultimate oil recovery in oil-wet naturally fractured reservoirs. The scope of this study extends from the experimental investigation to analytical approach based on the DLVO theory to thin film stability evaluation. The zeta potentials on limestone rock/brine and oil/brine interfaces, and also, the electrical double layer, the London–van der Waals, and the structural forces are measured to model the disjoining pressure isotherms with the brine solutions containing monovalent and divalent ions. Then, the stability of the wetting film on the limestone surface is compared with IFT measurements, contact angle and core flooding experiments. Based on results, strong negative charges at rock/brine and oil/brine interfaces were occurred during sea water injection and increase the repulsive forces at the rock/oil interface to release the oil film from the limestone surface. Also, the oil recovery in case of Na+ in the presence of SO42− is 50% higher than it for both cases of Na+/K+ and Cl−. From the disjoining pressure isotherms, a more stable water film observes from the low salinity brine containing the Mg2+ in the presence of Cl− compared to Mg2+ in the presence of SO42−.

London-van der Waals Force Field of a Chemically Patterned Surface To Enable Selective Adhesion.

The London-van der Waals (L-vdW) force between a particle and a surface strongly depends on the topography and the chemical properties of the interacting surfaces. Although a great deal of work has been done to understand the effect of topographical heterogeneity on the L-vdW adhesion, the role of chemical heterogeneity has been discussed only rarely. This study makes an attempt to quantify the magnitude and range of the L-vdW force acting on a spherical particle in the vicinity of a chemically patterned surface. Specifically, an ideal system of a smooth spherical particle approaching a surface composed of parallel stripes of chemically distinct materials with different Hamaker constants is considered. The L-vdW forces for such systems are determined by solving the London dispersion potential for the entire volumes of the adhering bodies from first principles, using Hamaker's microscopic approach. The computational results elucidate that a chemical interface can apply a tangential L-vdW force, in addition to the normal L-vdW force, on nearby particles. This can cause lateral motion of particles neighboring a chemically inhomogeneous surface. The magnitude of the tangential L-vdW force is found to be maximum when the particle is centered at the interface and shows a gradual drop as it moves away from this location. The magnitude and range of the tangential L-vdW force can be large for large colloidal particles in close contact with a chemically patterned surface whose materials have distinct Hamaker constants. This study suggests that the tangential L-vdW force field generated by a chemical interface can be utilized as a tool to manipulate the path of an approaching particle to facilitate selective adhesion.

Anesthetic Action and "Quantum Consciousness": A Match Made in Olive Oil.

Anesthetic Action and "Quantum Consciousness": A Match Made in Olive Oil.

Thermocapillary migration of droplets under molecular and gravitational forces

We study the thermocapillary migration of two-dimensional droplets of partially wetting liquids on a non-uniformly heated surface. The effect of a non-zero contact angle is imposed through a disjoining–conjoining pressure term. The numerical results for two different molecular interactions are compared: on the one hand, London–van der Waals and ionic–electrostatics molecular interactions that account for polar liquids; on the other hand, long- and short-range molecular forces that model molecular interactions of non-polar fluids. In addition, the effect of gravity on the velocity of the drop is analysed. We find that for small contact angles, the long-time dynamics is independent of the molecular potential, and the footprint of the droplet increases with the square root of time. For intermediate contact angles we observe that polar droplets are more likely to break up into smaller volumes than non-polar ones. A linear stability analysis allows us to predict the number of droplets after breakup occurs. In this regime, the effect of gravity is stabilizing: it reduces the growth rates of the unstable modes and increases the shortest unstable wavelength. When breakup is not observed, the droplet moves steadily with a profile that consists in a capillary ridge followed by a film of constant thickness, for which we find power law dependencies with the cross-sectional area of the droplet, the contact angle and the temperature gradients. For large contact angles, non-polar liquids move faster than polar ones, and the velocity is proportional to the Marangoni stress. We find power law dependencies for the velocity for the different regimes of flow. The numerical results allow us to shed light on experimental facts such as the origin of the elongation of droplets and the existence of saturation velocity.

Fractional ionization and size of cetyltrialkyl ammonium bromide and hydroxide micelles as a function of head-group lipophilicity and temperature

We have measured the diffusion coefficients, D, of aqueous micelles formed by cetyltriethyl-, cetyltripropyl- and cetyltributyl-ammonium bromides (CTEABr, CTPABr and CTBABr, respectively) and cetyltriethyl- and cetyltripropyl-ammonium hydroxides (CTEAOH and CTPAOH, respectively) by dynamic light scattering (DLS) at several temperatures from 15 to 55 °C and a range of surfactant (0.01–0.05 M) and salt (0.02–0.06 M NaBr; 0.05–0.3 M NaOH) concentrations. From values of D, we derived the respective fractional ionization values of micellar surfaces. For surfactants with bromide counterion we obtained fits of the diffusivity data using the linear interaction/DLVO approach, thus yielding estimates of the micellar hydrodynamic radius, Rh, and the micellar fractional ionization, α, which ranged from 0.26 to 0.35. For CTEAOH and CTPAOH, the fits appeared to be poorly sensitive to changes in the London-Van der Waals interactions, as expressed by the Hamaker constant, and only a large fractional ionization could account for the observed diffusivities.

Molecular features of nonionic detergents involved in the binding kinetics and solubilization efficiency, as studied in model (Langmuir films) and biological (Erythrocytes) membranes.

The effect of the nonionic detergents Brij-98 and Brij-58 over human erythrocytes was studied through quantitative hemolysis and in Langmuir films. Hemolytic tests revealed that Brijs are stronger membrane solubilizers than Triton X-100 (TX-100), with effective detergent/lipid ratios of 0.18 and 0.37 for Brij-98 and Brij-58, respectively. Experiments with Langmuir films provided significant information on the kinetics and thermodynamics of detergent-membrane interaction. The adsorption (ka) and desorption (kd) rate constants of Brijs were lower than those of TX-100. In the case of ka, that is probably due to their larger hydrophilic head (with twice (20) the oxyethylene units of TX-100). As for the thermodynamic binding constant, the linear and longer hydrophobic acyl chains of Brijs favor their stabilization in-between the lipids, through London van der Waals forces. Consequently, Kb,m values of Brij-98 (12,500 M-1) and Brij-58 (19,300 M-1) resulted higher than TX-100 (7500 M-1), in agreement with results from the hemolytic tests. Furthermore, Brij-58 binds with higher affinity than Brij-98 to bilayers and monolayers, despite its shorter (palmitic) hydrocarbon chain, showing that unsaturation restrains the detergent insertion into these environments. Our results provide significant information about the mechanism of interaction between Brijs and membranes, supporting their distinct solubilization effect.

On adiabatic pair potentials of highly charged colloid particles

Generalizing the Debye–Hückel formalism, we develop a new mean field theory for adiabatic pair potentials of highly charged particles in colloid dispersions. The unoccupied volume and the osmotic pressure are the key concepts to describe the chemical and thermodynamical equilibrium of the gas of small ions in the outside region of all of the colloid particles. To define the proper thermodynamic quantities, it is postulated to take an ensemble averaging with respect to the particle configurations in the integrals for their densities consisting of the electric potential satisfying a set of equations that are derived by linearizing the Poisson–Boltzmann equation. With the Fourier integral representation of the electric potential, we calculate first the internal electric energy of the system from which the Helmholtz free energy is obtained through the Legendre transformation. Then, the Gibbs free energy is calculated using both ways of the Legendre transformation with respect to the unoccupied volume and the summation of chemical potentials. The thermodynamic functions provide three types of pair potentials, all of which are inversely proportional to the fraction of the unoccupied volume. At the limit when the fraction factor reduces to unity, the Helmholtz pair potential turns exactly into the well known Derjaguin–Landau–Verwey–Overbeek repulsive potential. The Gibbs pair potential possessing a medium-range strong repulsive part and a long-range weak attractive tail can explain the Schulze–Hardy rule for coagulation in combination with the van der Waals–London potential and describes a rich variety of phenomena of phase transitions observed in the dilute dispersions of highly charged particles.

DLVO THEORY OF COLLOID STABILITY

The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory [1,2] has made it possible to discuss the stability of lyophobic colloids quantitatively. This theory is based on the interaction due to overlapping of electrical double layers and London–van der Waals forces between colloid particles.

Recycling of activated carbon filter backwash water using ultrafiltration: Membrane fouling caused by different dominant interfacial forces

The reuse of activated carbon filter backwash water was performed using an ultrafiltration (UF) system and the membrane fouling mechanism was investigated using a model involving XDLVO theory and hydrodynamic interaction. Pretreatment by micro-flocculation/sedimentation removed most particulate pollutants; thus, residual organic colloids determined the details of the UF membrane fouling. The membrane fouling was divided into two periods. Membrane fouling in the initial period was determined by interfacial forces between the membrane and colloids. The calculated interfacial forces showed that permeation drag (PD) force dominated the total interfacial force at long-range, while London-van der Waals and short-range Lewis acid-base (AB) forces governed membrane fouling at short-range (< 3nm). The electrostatic double layer force, reaching its energy peak at around 10nm, favored mitigation of membrane fouling. The initial membrane fouling could be alleviated by reducing the PD force, which was mainly associated with permeate flux. In the second membrane fouling period, a cake layer formed mostly of hydrophobic colloids retained at the membrane surface. The increasing AB force contributed mainly to the interfacial forces responsible for membrane fouling at short-range, greatly aggravating membrane fouling. The pre-ozonation of the UF influent was involved in mitigating membrane fouling in the second period. Pre-ozonation enhanced the hydrophilic properties of both cake layer and organic colloids, which clearly decreased the AB force. The pre-ozonation also narrowed the colloid size range and decreased the PD force. The use of interval pre-ozonation during the filtration process was a promising way to alleviate membrane fouling, and involved a chemical cleaning cycle extending from 15 to 18 days.