Hamaker Constant Research Articles

Aggregation of TiO2–graphene nanocomposites in aqueous environment: Influence of environmental factors and UV irradiation

The aggregation kinetics of TiO2–graphene nanocomposites in aqueous solution affected by solution pH, salt types (NaCl, CaCl2) and concentrations of electrolytes, and stability induced by UV irradiation was investigated in this study. The zeta potentials and hydrodynamic diameter of the nanoparticles were used as bases to assess the aggregation behavior, and stability of nanocomposites exposed to UV irradiation was expressed in terms of supernatant concentration. The aggregation of TiO2–graphene nanoparticles in aqueous media followed the colloidal theory. TiO2–graphene nanoparticles were significantly aggregated in the presence of a diavalent cation compared with monovalent cation because the former was more capable of effective charge screening and neutralization. The calculated Hamaker constant of the TiO2–graphene nanocomposites in aqueous solution prepared in the lab was 2.31×10−20J. The stability of this composite nanoparticles was between those of pure TiO2 and graphene. A known intensity of UV irradiation was beneficial in the formation of TiO2–graphene nanoparticle aggregates. However, prolonged UV irradiation may stabilize the nanoparticles. These results provided critical information about the colloidal properties of the new TiO2–graphene nanocomposites and were useful in predicting the fate and transport of TiO2–graphene nanocomposites in natural water environments.

An enhanced centrifuge-based approach to powder characterization: Particle size and Hamaker constant determination

Many types of manufacturing processes involve powders and are affected by powder behavior. During the development of new powder processes, it is common that only very small quantities of powder are available. It is highly desirable to implement tools that allow the behavior of bulk powder to be predicted based on the behavior of these small quantities. If the powder can be well-characterized, it is possible to improve the processing performance. In this work, an enhancement of the centrifuge technique was proposed as a means of powder characterization by using specially designed substrates with hemispherical indentations within the centrifuge. Simulations of the centrifuge technique were performed to test the viability of applying this enhancement experimentally. These simulations predicted the percentage of particles remaining as a function of centrifuge rotational speed, indentation size, and particle size, for spherical silica particles against indented substrates made from silicon. Particles that are smaller than an indentation complement that indentation, leading to an increase in the adhesion force. In comparison, particles that are larger than an indentation have a smaller relative adhesion force. These two types of particle–indentation interactions generate a unique set of residual adhering percentage (RAP) curves that can be used to determine the particle size distribution and Hamaker constant of the system. These simulation results for the specifically-designed substrates demonstrate the capability of the enhanced centrifuge technique for advanced powder characterization.

Orthokinetic aggregation of charged colloidal particles in the presence of repulsive double layer force: A trajectory analysis with the solution of non-linear Poisson–Boltzmann equation

Abstract We studied turbulent coagulation rates of well-characterized carboxyl-terminated latex particles as a function of pH at different KCl concentrations. The negative charge of the studied particles increases and thus develops the electrical double layer repulsion with increasing pH. The pH-dependent coagulation rate was analyzed on the basis of the trajectory analysis with the Derjaguin, Landau, Verwey, and Overbeek theory using the solution of non-linear Poisson–Boltzmann (PB) equation under the charge regulation boundary condition, in which the electrostatic boundary conditions such as surface potential depends on surface separation distance. It should be noted that this is the first attempt to compare the calculated results with the experimental data of the effect of surface charge on turbulent coagulation rates. The calculation using the trajectory analysis can well describe the experimental data. However, to obtain the good agreement between the experimental data and the calculation, we need to decrease the Hamaker constant A H with increasing KCl concentration. The dependence of A H on salt concentration is probably ascribed to the fact that the origin of the van der Waals attraction is essentially electrostatic dipole–dipole interaction between molecules. That is, we consider that the van der Waals attraction is increasingly screened by the charge of ions with increasing salt concentration.

Cohesive Hamaker Constants and Dispersive Surface Energies of RDX, PETN, TNT, and Ammonium Nitrate‐Based Explosives

AbstractInverse gas chromatography (IGC) was used to characterize dispersive surface energies γSDand cohesive Hamaker constantsAiifor RDX, PETN, TNT, ammonium nitrate (AN), and AN‐based explosives at 303 K. The γSDfor RDX at 303 K is compared to previous studies and generally found to be in good agreement, substantiating the use of NESTT training materials to characterize explosives via IGC. Additionally, the effect of the amount of fuel in the AN mixtures on γSDis examined using simple linear regression. Finally, the IGC‐predictedAiivalues are compared to Lifshitz estimations forAiiof RDX, PETN, TNT, and AN.

Decoupling Substrate Surface Interactions in Block Polymer Thin Film Self-Assembly

We report a highly predictive approach to capturing the major substrate–polymer interactions that can dominate nanoscale ordering and orientation in block polymer (BP) thin films. Our approach allows one to create designer BP thin films on modified substrates while minimizing the need for extensive parameter space exploration. Herein, we systematically and quantitatively examined the influence of substrate surface energy components (dispersive and polar interactions) on thin film self-assembly, and our analysis demonstrates that although total surface energy plays a dominant role in substrate wetting, individual contributions from the dispersive and polar components of the surface energy influence the composite through-film behavior. Additionally, long-range forces as described by the Hamaker constant are under-recognized factors in thin film assembly and can alter expected wetting behavior by affecting thermodynamic stability. This more inclusive interpretation of surface energy effects, including the Hamaker constant, on BP thin films was supported by studies of interfacial and through-film behavior as gleaned from temporal island/hole measurements via in situ optical microscopy during thermal annealing. The formalism correctly predicted experimental wetting and hole formation sizes over a wide range of substrate surface energies when employing the appropriate relationships based on decoupled dispersive and polar components. Our results indicate a promising and more universal approach for matching desired BP thin film self-assembly with chemically tailored substrate modifications.

Methodology for Disjoining Pressure of Free Water Nanofilms

The disjoining pressure has a significant influence on the stability of thin films, which is important in various biological, environmental, and industrial processes. However, due to its complicated nature, especially in nanoscale, a convenient way is still lack. Here, a methodology derived from film thermodynamics was developed to determine the disjoining pressure of thin films using molecular dynamics (MD) simulations. The obtained disjoining pressure was purely attractive for pure water films, and compared with that from the Hamaker constant by Lifshitz-van der Waals theory. This method can be readily applied to similar aqueous films, which will enhance the interpretation of laboratory experiments and extend previous theoretical framework.

Interaction Forces and Aggregation Rates of Colloidal Latex Particles in the Presence of Monovalent Counterions.

Force profiles and aggregation rates involving positively and negatively charged polystyrene latex particles are investigated in monovalent electrolyte solutions, whereby the counterions are varied within the Hofmeister series. The force measurements are carried out with the colloidal probe technique, which is based on the atomic force microscope (AFM), while the aggregation rates are measured with time-resolved multiangle light scattering. The interaction force profiles cannot be described by classical DLVO theory, but an additional attractive short-ranged force must be included. An exponential force profile with a decay length of about 0.5 nm is consistent with the measured forces. Furthermore, the Hamaker constants extracted from the measured force profiles are substantially smaller than the theoretical values calculated from dielectric spectra. The small surface roughness of the latex particles (below 1 nm) is probably responsible for this deviation. Based on the measured force profiles, the aggregation rates can be predicted without adjustable parameters. The measured absolute aggregation rates in the fast regime are somewhat lower than the calculated ones. The critical coagulation concentration (CCC) agrees well with the experiment, including the respective shifts of the CCC within the Hofmeister series. These shifts are particularly pronounced for the positively charged particles. However, the consideration of the additional attractive short-ranged force is essential to quantify these shifts correctly. In the slow regime, the calculated rates are substantially smaller than the experimental ones. This disagreement is probably related to surface charge heterogeneities.

Improved Parametrization for Extended Derjaguin, Landau, Verwey, and Overbeek Predictions of Functionalized Gold Nanosphere Stability

Surface chemistry variations on alkanethiol-modified gold nanoparticles (i.e., packing density, tilt angle, and composition) influence their function in nanotechnology-based applications. Accurate theoretical predictions of the stability of functionalized nanoparticles enable guided design of their properties but are often limited by the accuracy of the parameters used as model inputs. These parametrization limitations for the extended Derjaguin, Landau, Verwey, and Overbeek (xDLVO) theory are overcome using a size-dependent Hamaker constant for gold, interfacial surface potentials, and the tilt angles of self-assembled monolayers (SAMs), which improve the predictive power of xDLVO theory for modeling nanoparticle stability. Measurements of the electrical properties of gold nanoparticles functionalized with a series of thiolated acids of differing ligand lengths and SAM tilt angles validate the predictions of xDLVO theory using these new parametrizations, illustrating the potential for this approach to im...

Obtaining electrostatically bound CdS-SiO2 aggregates from electrophoretic concentrates of CdS nanoparticles

Nonaqueous electrophoresis reveals that the electrokinetic potential of CdS nanoparticles increases slightly (85–120 mV) along with the concentration (0–5 × 10−3 M) of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in n-decane, while negatively charged SiO2 particles acquire positive charge (switching from −75 up to +135 mV). The energies of interparticle interactions in CdS-CdS and CdS-SiO2 systems are calculated from these parameters and the literature values of the Hamaker constants according to the Deryaguin-Landau-Verwey-Overbeek (DLVO) theory. It is concluded that the presence of a minimum (2.5 kBT) on the potential dependences of the CdS-SiO2 system indicates the formation of CdS-SiO2 aggregates electrostatically bound by heterocoagulation at low concentrations of AOT. The luminescent properties of the obtained ultrafine CdS-SiO2 powders depend on the CdS content.

Surface Thermodynamics Approach to Mycobacterium tuberculosis (MTB) – Human Sputum Interactions

This research work presents the surface thermodynamics approach to M-TB/HIV-Human sputum interactions. This involved the use of the Hamaker coefficient concept as a surface energetics tool in determining the interaction processes, with the surface interfacial energies explained using van der Waals concept of particle interactions. The Lifshitz derivation for van der Waals forces was applied as an alternative to the contact angle approach which has been widely used in other biological systems. The methodology involved taking sputum samples from twenty infected persons and from twenty uninfected persons for absorbance measurement using a digital Ultraviolet visible Spectrophotometer. The variables required for the computations with the Lifshitz formula were derived from the absorbance data. The Matlab software tools were used in the mathematical analysis of the data produced from the experiments (absorbance values). The Hamaker constants and the combined Hamaker coefficients were obtained using the values of the dielectric constant together with the Lifshitz equation. The absolute combined Hamaker coefficients A132abs and A131abs on both infected and uninfected sputum samples gave the values of A132abs = 0.21631×10-21 Joule for M-TB infected sputum and A132abs = 0.18825×10-21 Joule for M-TB/HIV infected sputum. The significance of this result is the positive value of the absolute combined Hamaker coefficient which suggests the existence of net positive vanderwaals forces demonstrating an attraction between the bacteria and the macrophage. This however, implies that infection can occur. It was also shown that in the presence of HIV, the interaction energy is reduced by 13% conforming adverse effects observed in HIV patients suffering from tuberculosis.

Prediction and Verification of Compatibility of MMT Nanofiller in PA6 Matrix

This contribution deals about study of mechanical properties and compatibility between PA6 polymer as matrix and modified and unmodified montmorillonite clay nanofiller Cloisite. For this purposes in the first part of study the Hamaker constants, Adhesion work and B parameter for systems PA6/Cloisite 30B, PA6/Cloisite 93A and Pa6/Cloisite Na+ were calculated and compared. The second part of article consists of mechanical tests (tensile test, impact test) of moulded samples and Scanning Electron Microscope (SEM) study of these samples.

Direct determination of the local Hamaker constant of inorganic surfaces based on scanning force microscopy.

The energetics involved in the bonding fluctuations between nanometer-sized silicon dioxide (SiO2) probes and highly oriented pyrolytic graphite (HOPG) and molybdenum disulfide (MoS2) could be quantified directly and locally on the submicron scale via a time-temperature superposition analysis of the lateral forces between scanning force microscopy silicon dioxide probes and inorganic sample surfaces. The so-called "intrinsic friction analysis" (IFA) provided direct access to the Hamaker constants for HOPG and MoS2, as well as the control sample, calcium fluoride (CaF2). The use of scanning probe enables nanoscopic analysis of bonding fluctuations, thereby overcoming challenges associated with larger scale inhomogeneity and surface roughness common to conventional techniques used to determine surface free energies and dielectric properties. A complementary numerical analysis based on optical and electron energy loss spectroscopy and the Lifshitz quantum electrodynamic theory of van der Waals interactions is provided and confirms quantitatively the IFA results.

Capillary contact angle in a completely wet groove.

We consider the phase equilibria of a fluid confined in a deep capillary groove of width L with identical side walls and a bottom made of a different material. All walls are completely wet by the liquid. Using density functional theory and interfacial models, we show that the meniscus separating liquid and gas phases at two phase capillary coexistence meets the bottom capped end of the groove at a capillary contact angle θ(cap)(L) which depends on the difference between the Hamaker constants. If the bottom wall has a weaker wall-fluid attraction than the side walls, then θ(cap) > 0 even though all the isolated walls are themselves completely wet. This alters the capillary condensation transition which is now first order; this would be continuous in a capped capillary made wholly of either type of material. We show that the capillary contact angle θ(cap)(L) vanishes in two limits, corresponding to different capillary wetting transitions. These occur as the width (i) becomes macroscopically large, and (ii) is reduced to a microscopic value determined by the difference in Hamaker constants. This second wetting transition is characterized by large scale fluctuations and essential critical singularities arising from marginal interfacial interactions.

Characteristics of an evaporating thin film of a highly wetting liquid in a groove

Abstract The behavior of an evaporating thin film of a highly wetting liquid filled in a rectangular groove was studied theoretically. Following the previous studies, the liquid film was divided into an adsorbed film region, a micro region and an intrinsic meniscus region. A theoretically based method was employed to estimate the Hamaker constant. The Marangoni effect due to the thickness change of a very thin liquid film was included in the basic equation. The fourth order differential equation for the liquid film thickness was solved by the finite difference method. Three sets of the combination of boundary conditions at both ends of the micro region were examined and the set of boundary conditions which gave the most realistic solution was decided. The solutions were obtained for a wide range of the micro region length. The principle of maximum entropy production was introduced to estimate the most probable solution. The results were compared with previous results obtained by the shooting method and the effect of the solution method on the numerical results was discussed.

Prediction of the soil-water characteristic curve based on the specific surface area of fine-grained soils

This paper presents a new, mathematical expression for describing the soil-water characteristic curve (SWCC) over a range of water contents where fine-grained soils exhibit plastic properties. The finding that the relationship between the soil suction and the water content can be expressed in terms of the specific surface area of soils was based on experimentally determined relationships between the water content and the soil’s specific surface area, as well as between the thickness of the adsorbed water layer on the external surfaces of clay minerals and the quantity of free-pore water for the water content between the liquid and plastic limits. The double-porosity model for the pore-space geometry was considered, as well as that all the water in the clay-aggregates is adsorbed, and that the adsorption mechanism is dominated by the van der Waals forces. The validity and applicability of the proposed equation for the SWCC estimation was verified on three samples in which the SWCC was measured, as well as the specific surface area, the mineralogical and chemical compositions, the grain size distribution, and the Atterberg limits. Despite the fact that good correlations were found between the calculated parts of the SWCC and the measured, the practical applicability of the proposed equation remains problematic due to the values of Hamaker constant, which are not yet well defined for different minerals.

Predicting and measuring repulsive van der Waals forces for a Teflon AF™–solvent–α-alumina system

The force interaction of an α-alumina particle against Teflon AF™ in the presence of several non-polar and polar solvents was quantified. Solvents were chosen based on calculation of a negative Hamaker constant, indicating a repulsive van der Waals interaction. These results were compared to those predicted by theoretical models for van der Waals forces. Measured forces for the non-polar solvent interactions are comparable to theoretical predictions, whereas the polar solvent interactions show repulsive forces between one and two orders of magnitude smaller than those predicted. In the case of water, surface charges were large enough to completely counteract the repulsive van der Waals forces and lead to an attractive force interaction. This minimization of repulsion, and even attraction, can be predicted by DLVO theory.

Wrinkling and folding of nanotube-polymer bilayers.

The influence of a polymer capping layer on the deformation of purified single-wall carbon nanotube (SWCNT) networks is analyzed through the wrinkling of compressed SWCNT-polymer bilayers on polydimethylsiloxane. The films exhibit both wrinkling and folding under compression and we extract the elastoplastic response using conventional two-plate buckling schemes. The formation of a diffuse interpenetrating nanotube-polymer interface has a dramatic effect on the nanotube layer modulus for both metallic and semiconducting species. In contrast to the usual percolation exhibited by the pure SWCNT films, the capped films show a crossover from "composite" behavior (the modulus of the SWCNT film is enhanced by the polymer) to "plasticized" behavior (the modulus of the SWCNT film is reduced by the polymer) as the SWCNT film thickness increases. For almost all thicknesses, however, the polymer enhances the yield strain of the nanotube network. Conductivity measurements on identical films suggest that the polymer has a modest effect on charge transport, which we interpret as a strain-induced polymer penetration of interfacial nanotube contacts. We use scaling, Flory-Huggins theory, and independently determined nanotube-nanotube and nanotube-polymer Hamaker constants to model the response.

Casimir forces from conductive silicon carbide surfaces

Samples of conductive silicon carbide (SiC), which is a promising material due to its excellent properties for devices operating in severe environments, were characterized with the atomic force microscope for roughness, and the optical properties were measured with ellipsometry in a wide range of frequencies. The samples show significant far-infrared absorption due to concentration of charge carriers and a sharp surface phonon-polariton peak. The Casimir interaction of SiC with different materials is calculated and discussed. As a result of the infrared structure and beyond to low frequencies, the Casimir force for SiC-SiC and SiC-Au approaches very slowly the limit of ideal metals, while it saturates significantly below this limit if interaction with insulators takes place (${\text{SiC-SiO}}_{2}$). At short separations (10 nm) analysis of the van der Waals force yielded Hamaker constants for SiC-SiC interactions lower but comparable to those of metals, which is of significance to adhesion and surface assembly processes. Finally, bifurcation analysis of microelectromechanical system actuation indicated that SiC can enhance the regime of stable equilibria against stiction.

Filtration of fullerene and copper oxide nanoparticles using surface-modified microfilters.

This study evaluated the filtration of engineered nanoparticles of fullerene and copper oxide (CuO) from water by using surface-modified microsized filters. The surfaces of microsized filters of cellulose acetate and glass fibers were coated with cationic and anionic surfactants to give them positively and negatively charged surfaces, respectively. Uncoated microfilters removed 30% of a fullerene suspension, while no nanosized CuO suspension was removed. Cationic surfactant-coated filters enhanced the removal efficiency up to 70% for the fullerene suspension, while the anionic surfactant-coated filters could not remove fullerene at all. The positively charged filters with cationic surfactant coating could easily adsorb negatively charged fullerenes on their surfaces. However, none of the surfactant-coated filters removed the CuO nanoparticles because the nanoparticles were not affected by the electrical charge of the filtration medium. The Hamaker constants of nanoparticles interacting with the filter materials in water were calculated to study these interactions. The Hamaker constant of fullerene interacting with cellulose acetate in water, 4.68E - 21 J, was higher than that of interacting with quartz in water, 2.59E - 21 J. However, the Hamaker constants of CuO interacting with quartz and cellulose acetate in water were both negative values, implying repulsive van der Waals interactions. The curves of potential energy of interaction between nanoparticles and the various filter media implied that the nanoparticles were very stable in water, and so, natural deposition of nanoparticles on the filters would not occur. Therefore, electrical bonding and hydrophobic interactions were the forces dominating fullerene removal by positively charged filters.

Muscovite mica and koalin slurries: Yield stress–volume fraction and deflocculation point zeta potential comparison

Mica slurries were found to be flocculated over the whole pH range of between 2 and 12. These slurries of different solid loading appeared to obey the yield stress–DLVO force model producing a single value for the deflocculation point zeta potential or the critical zeta potential (at point of flocculated-disperse state transition) of ~48mV. Such a high value indicates the presence of an additional attractive force in addition to the van der Waals force. This force is attributed to edge-face unlike charge attraction. The use of pyrophosphate additive to minimize edge–face interactions via positive edge charge neutralization caused a large reduction in the critical zeta potential, to 22mV. With this new value, the Hamaker constant of mica in water was determined to be ~13zJ. This falls within the range of values reported for mica. Low Ca(II) kaolin suspension displayed a lower critical zeta potential of 41mV and the use of pyrophosphate additive at 0.2 and 0.4dwb% appeared not to decrease the magnitude of the critical zeta potential. This pyrophosphate-influence critical zeta potential is also independent of Ca(II) content in the kaolin. The established model of edge–face attraction for kaolin suspension at low pH may not be completely correct. A face–face interaction model may be more appropriate. A power law model relating the maximum yield stress with volume fraction showed a very high exponent value of 8 for mica slurries, only 3.1 for the low Ca(II) kaolin and 3.6–3.9 for the high Ca(II) kaolin suspensions. This exponent value may reflect the predominant particle–particle interaction configuration in the clay suspensions.