Liquid Interfacial Properties Research Articles

Contact Angle Assessment of Hydrophobic Silica Nanoparticles Related to the Mechanisms of Dry Water Formation

Dry water is a very convenient way of encapsulating a high amount of aqueous solutions in a powder form made of hydrophobic silica nanoparticles. It was demonstrated in previous studies that both solid and liquid interfacial properties influence the quality of the final product resulting occasionally in mousse formation. To explain this behavior, contact angles of silica nanoparticles have been measured for water and water/ethanol solution by means of liquid intrusion experiments. It was found that the quality of the final product correlates with the contact angle, i.e., contact angle close to 105 degrees leads to mousse formation whereas a slightly higher value of approximately 118 degrees allows dry water formation. The proposed explanation was based on the energy of immersion and adhesion defined as the energy needed for a spherical particle to respectively penetrate into the liquid or attach at the liquid/air interface. Significantly lower energy of immersion calculated for lower contact angle might account for particle penetration into the liquid phase during processing, leading to continuous network aggregation, air entrapment, and finally mousse formation.

Interfacial properties of selected binary mixtures containing n-alkanes

We report the correlation and prediction of the subcritical vapour–liquid interfacial properties of three asymmetric binary mixtures composed of long n-alkanes in equilibria with a smaller solvent: hexane + decane, carbon dioxide + decane, and ethane + eicosane. The interfacial region is described by the complementary use of the Square Gradient Theory (SGT) and molecular dynamics (MD) simulations that allow the prediction of both the macroscopic and molecular level properties of the binary fluid mixtures. Calculations with SGT rely on the description of the vapour–liquid equilibria (VLE) by means of the Statistical Associated Fluid Theory (SAFT) equation of state. MD simulations are performed in the canonical ensemble using united-atom potentials to probe coexisting phases and the accompanying interface simultaneously. In addition to the phase equilibrium compositions and interfacial tensions, other interfacial properties, such as concentration profiles along the interfacial region, surface activities, and relative Gibbs adsorption isotherms at the interfaces have been obtained from a combination of SGT and MD. The entropy and surface enthalpy change of surface formation are also calculated for the case of ethane + eicosane mixture, where we report three different isothermal conditions. While pure component data are used to fit model parameters, mixture results are predictions. When possible, results are compared to available experimental data and quantitative agreement is observed throughout. The particularly high excess adsorption in the interface of the smaller solvents, CO 2 and ethane, is noted.

Modeling of a transition to diffusionless dendritic growth in rapid solidification of a binary alloy

Abstract Diffusionless growth of dendritic crystals results in microsegregation-free microstructures with an initial (nominal) chemical composition of solidifying systems. Normally, a transition from chemically partitioned growth to diffusionless solidification is accompanied by the morphological transition in crystal shape with the appearance of nonlinearity in the kinetic behavior of growing crystals. This phenomenon is discussed using a model of local non-equilibrium rapid solidification. Considering the transition from the solute diffusion-limited growth to purely thermally controlled growth of dendritic crystals, the model predicts the abrupt change of growth kinetics with the break points in the “dendrite tip velocity-undercooling” and “dendrite tip radius-undercooling” relationships. It is shown that the abrupt change of growth kinetics occurs with the ending of the transition to purely thermally controlled growth and the onset of diffusionless solidification. To predict the dendrite growth kinetics in a whole region of undercooling, numeric analysis shows that the model has to take into account both anisotropies of solid–liquid interfacial properties. These are anisotropy of surface energy and anisotropy of atomic kinetics of solidification.

All-Atom Force Field for the Prediction of Vapor−Liquid Equilibria and Interfacial Properties of HFA134a

A new all-atom force field capable of accurately predicting the bulk and interfacial properties of 1,1,1,2-tetrafluoroethane (HFA134a) is reported. Parameterization of several force fields with different initial charge configurations from ab initio calculations was performed using the histogram reweighting method and Monte Carlo simulations in the grand canonical ensemble. The 12-6 Lennard-Jones well depth and diameter for the different HFA134a models were determined by fitting the simulation results to pure-component vapor-equilibrium data. Initial screening of the force fields was achieved by comparing the calculated and experimental bulk properties. The surface tension of pure HFA134a served as an additional screening property to help discriminate an optimum model. The proposed model reproduces the experimental saturated liquid and vapor densities, and the vapor pressure for HFA134a within average errors of 0.7%, 4.4%, and 3.1%, respectively. Critical density, temperature, vapor pressure, normal boiling point, and heat of vaporization at 298 K are also in good agreement with experimental data with errors of 0.2%, 0.1%, 6.2%, 0%, 2.2%, respectively. The calculated surface tension is found to be within the experimental range of 7.7-8.1 mN.m(-1). The dipole moment of the different models was found to significantly affect the prediction of the vapor pressure and surface tension. The ability of the HFA134a models in predicting the interfacial tension against water is also discussed. The results presented here are relevant in the development of technologies where the more environmentally friendly HFA134a is utilized as a substitute to the ozone depleting chlorofluorocarbon propellants.

Investigation of vapour—liquid nucleation properties for spherical and chain-like fluids by density functional theory

The excess Helmholtz free energy functional for nonpolar chain-like molecules is formulated in terms of a weighted density approximation (WDA) for short-range interactions and a Weaks–Chandler–Andersen (WCA) approximation and a Barker–Henderson (BH) theory for long-range attraction. Within the framework of density functional theory (DFT), vapour–liquid interfacial properties including density profile and surface tension, and vapour–liquid nucleation properties including density profile, work of formation and number of particles are investigated for spherical and chainlike molecules. The obtained vapour–liquid surface tension and the number of particles in critical nucleus for Lennard–Jones (LJ) fluids are consistent with the simulation results. The influences of supersaturation, temperature and chain length on vapour–liquid nucleation properties are discussed.

Liquid–liquid interfacial properties of mixed nanoparticle–surfactant systems

In this study the effect of particles of nanometric dimensions on the interfacial properties of liquid–liquid systems is addressed. The nanoparticle transfer and the consequent attachment to the fluid interface are mainly governed by their hydrophobic/lipophilic character. For this reason a model particle–surfactant system has been investigated, namely a nanometric colloidal silica dispersion plus CTAB, where the role of the cationic surfactant is varying the particle surface properties adsorbing on them. The interfacial properties of the dispersion against hexane were determined as a function of the surfactant concentration, measuring the interfacial tension and the dilational viscoelasticity versus frequency. These results were then crossed with a study on the behaviour of micrometric oil droplets inside the same dispersions, as well as on the morphology of the respective nanoparticle stabilised emulsions. By this multiple investigation it was possible to find out that diffusion transport of particles from the bulk to the interface plays an important role, as well as the reorganisation of the mixed particle-surfactant layer. Moreover from these results the irreversible aspect of the nanoparticle attachment to the fluid interface was also evidenced which, under particular conditions, can provide the formation of a solid-like layer.

Chemical synthesis and microstructural analysis of superconducting YBa2Cu3O7-δ ink deposited by drop-on-demand ink-jet printing on silver substrates

The basis for the development of computer controlled printing techniques for YBa2Cu3O7-δ (YBCO) coated conductors is discussed in detail. This method of continuous deposition of YBCO material potentially enables non-vacuum formation of complex multi-layer superconducting patterns on silver substrates. Sol–ink processes were used to print single droplets of YBCO on polycrystalline silver substrates under varying printing conditions. Droplet sizes were controlled from a range of 300 μm–1200 μm. The formation and the stability of the sol–ink with respect to pH and concentration has been studied and discussed herein. For optimum results, the pH of the ink had to regulated to 6.5. Printing and substrate parameters that alter solid–liquid interface properties were also investigated such as ink concentration and surface roughness, the best results produced were with an ink concentration of 0.025 M on a polished silver surface. In order to control the flow properties of the sol media dynamic viscosities were determined. Characterisation by X-Ray Diffraction, Differential Thermal Analysis, Thermal Gravitometry and Scanning Electron Microscopy was used to examine the sol–ink samples, whilst AC susceptibility was used to assess the superconductivity of the printed samples where they were discovered to have a characteristic Tc of ∼83 K.

Adsorption of surfactants on minerals for wettability control in improved oil recovery processes

Chemical-flooding schemes for recovering residual oil have been in general less than satisfactory due to loss of chemicals by adsorption on reservoir rocks, precipitation, and resultant changes in rock wettability. Adsorption and wettability changes are determined mainly by the chemical structure and mix of the surfactants, surface properties of the rock, composition of the oil and reservoir fluids, nature of the polymers added and solution conditions such as salinity, pH and temperature. The mineralogical composition of reservoir rocks plays an important role in determining interactions between reservoir minerals and externally added reagents (surfactants/polymers) and their effects on solid–liquid interfacial properties such as surface charge and wettability. Some of the reservoir minerals can be sparingly soluble causing precipitation and changes in wettabilty as well as drastic depletion of surfactants/polymers. Most importantly, the effect of surfactants on wettability depends not only how much is adsorbed but also on how they adsorb. A water wetted rock surface that is beneficial for displacement of oil can be obtained by manipulating the orientation of the adsorbed layers. New surfactants capable of tolerating harsh conditions created by extremes of pH, temperature or inorganics and capable of interacting favorably with inorganics and polymers are promising for enhanced oil recovery. In this regard, such surfactants as sugar based ones and pyrrolidones are attracting attention, as they are also biodegradable. In many cases, mixed surfactants perform much better than single surfactants due to synergetic effects and ability to alleviate precipitation. Also, addition of inorganics such as silicates, phosphates and carbonates and polymers such as lignins can be used to control the adsorption and the wettability. In this paper, use of specialty surfactants and their mixtures is discussed along with the mechanisms involved.

Protein and surfactant foams: linear rheology and dilatancy effect

We report results on the dependence of foam rheology on the nature of its components (gas, liquid and surfactant). We have varied the liquid bulk properties, the gas–liquid interfacial properties, which implies also different thin liquid films properties, and the gas. Microscopic characterizations are also performed to find correlations between the different length scales, and to interpret the macroscopic rheological behavior (oscillations and creep). We have found that the elastic modulus G′, once normalized by the Laplace pressure, does not depend on the foam components, whereas the loss modulus G″ shows some dependence with the interfacial properties. We show that the gas is an important factor for these moduli because of its role in the foam aging, via coarsening. We have also evidenced a dilatancy effect taking place under continuous shear, by using a special foam sample shape, in which only a foam band is subjected to shear. We have observed and measured an increase of the liquid volume fraction ɛ in that band.

A nonadditive methanol force field: Bulk liquid and liquid-vapor interfacial properties via molecular dynamics simulations using a fluctuating charge model

We study the bulk and interfacial properties of methanol via molecular dynamics simulations using a CHARMM (Chemistry at HARvard Molecular Mechanics) fluctuating charge force field. We discuss the parametrization of the electrostatic model as part of the ongoing CHARMM development for polarizable protein force fields. The bulk liquid properties are in agreement with available experimental data and competitive with existing fixed-charge and polarizable force fields. The liquid density and vaporization enthalpy are determined to be 0.809 g/cm3 and 8.9 kcal/mol compared to the experimental values of 0.787 g/cm3 and 8.94 kcal/mol, respectively. The liquid structure as indicated by radial distribution functions is in keeping with the most recent neutron diffraction results; the force field shows a slightly more ordered liquid, necessarily arising from the enhanced condensed phase electrostatics (as evidenced by an induced liquid phase dipole moment of 0.7 D), although the average coordination with two neighboring molecules is consistent with the experimental diffraction study as well as with recent density functional molecular dynamics calculations. The predicted surface tension of 19.66+/-1.03 dyn/cm is slightly lower than the experimental value of 22.6 dyn/cm, but still competitive with classical force fields. The interface demonstrates the preferential molecular orientation of molecules as observed via nonlinear optical spectroscopic methods. Finally, via canonical molecular dynamics simulations, we assess the model's ability to reproduce the vapor-liquid equilibrium from 298 to 423 K, the simulation data then used to obtain estimates of the model's critical temperature and density. The model predicts a critical temperature of 470.1 K and critical density of 0.312 g/cm3 compared to the experimental values of 512.65 K and 0.279 g/cm3, respectively. The model underestimates the critical temperature by 8% and overestimates the critical density by 10%, and in this sense is roughly equivalent to the underlying fixed-charge CHARMM22 force field.

Detecting solid–liquid interface properties with mechanical slip modelling for quartz crystal microbalance operating in liquid

Quartz crystal microbalances (QCMs) provide sensitive probes for changes at solid–solid or solid–liquid interfaces. It is essential to obtain a physical insight into the details of the interface loading mechanism to interpret the observed behaviour leading to fresh applications of AT-cut quartz resonators. In this work, a mechanical slip model of the interface between a quartz plate and a viscoelastic liquid is presented to replace the continuous displacement assumption. The electrical impedance of a compounded quartz crystal resonator is expressed as a function of the properties of liquid, and the quartz and the strength of contact attraction between the solid and liquid. The interfacial slip parameter between the solid and liquid, which is defined as the displacement transmission from solid particles to liquid bottom particles, is explicitly calculated from the complex attraction strength between the liquid and solid. Comparisons of the physical slip model with other interfacial modes used in the QCM are presented, including the continuous mode and the transmission mode based on the friction force interface. The explicit expression of the slip parameter is presented, and the influence of interfacial slip on QCM measurements is discussed with numerical results. A detailed physical description of the solid–liquid interfacial is useful for exploring fresh ideas for the use of the QCM in biological industry. A new approach by using the slip parameter measured with QCM is proposed to determine the attraction strength between the particles of a viscous liquid and solid particles. The experimental data in the literatures for a hydrophilic-coated sensor and a hydrophobic-coated sensor are used for the numerical examples. It is found that the imaginary part of the interactive strength of these two types of sensor is almost the same. The real part of the interactive strength contributes significantly to distinguish the different interface conditions for these two types of sensor.

Vapor-liquid interfacial properties of mutually saturated water/1-butanol solutions.

Adsorption and ordering at the vapor-liquid interfaces of mutually saturated water/1-butanol solutions at a temperature of 298.15 K were investigated using configurational-bias Monte Carlo simulations in the Gibbs ensemble and compared to the surface properties of neat water and 1-butanol liquids. A dense 1-butanol monolayer is observed at the surface of the water-rich phase, which results in a substantial decrease of its surface tension. In contrast, there is no enrichment of water molecules at the surface of the butanol-rich phase, and its surface tension is not significantly changed. Analysis of the interfacial structures reveals that these systems exhibit orientational ordering and composition heterogeneity. Analysis of the hydrogen-bonding distributions suggests that the formation of the 1-butanol monolayer is driven by an excellent match between water and the primary alcohol; that is, additional hydrogen bonds are formed between the excess free hydrogens of surface water and the excess hydrogen-bond acceptor sites of 1-butanol.

Enhanced amphiphile adsorption at liquid–liquid interfaces in a micellar solution model

We analyze in mean-field approximation the liquid–liquid interfacial properties of a micellar solution model designed to describe amphiphiles in polar solvents. Under specific conditions the amphiphile concentration profiles become non-monotonous and entail enhanced interfacial adsorption. We find that this behavior is related to the occurrence of asymmetry of the closed-loop coexistence curve for dilute and concentrated micellar liquids above a lower critical solution temperature T c . We obtain quantitative agreement with ellipsometric studies of partially miscible mixtures of non-ionic amphiphiles in water. The model allows for elimination of the degrees of freedom within the aggregates, and this leads to an equivalence with the ordinary Ising model but with temperature and concentration dependent field and coupling. The latter is identified with the effective intermicellar interaction.

Molecular dynamics simulations of CCl4–H2O liquid–liquid interface with polarizable potential models

The results from molecular dynamics simulations of the equilibrium properties of the CCl4–H2O liquid–liquid interface at room temperature are presented. The interactions between H2O–H2O, H2O–CCl4, and CCl4–CCl4 are described using the polarizable potential models developed in our laboratory. To our knowledge, this work is the first molecular dynamics simulations of the liquid–liquid interfacial equilibrium properties that explicitly includes nonadditive polarization effects. Molecular dynamics results of a 300 ps simulation following an extensive equilibration process indicate that the liquid interface is very stable, the density profile of H2O is very smooth, while that of CCl4 exhibits some oscillations. It is found that locally there is a sharp transition from one liquid phase to the other, but the overall interface is broadened by thermal fluctuations as indicated by the liquid density profiles. Calculated radial distribution functions suggest that the local structures of CCl4 and H2O remain unchanged from the bulk liquid to the interface. However, the interface does induce orientational order of H2O and CCl4 molecules. To study the polarization effects on the liquid–liquid interfacial equilibrium properties, we have calculated the total and induced dipole moments of H2O and CCl4 molecules as a function of the distance normal to the interface. The calculated dipole moments of the water molecules near the interface are close to their gas phase values, while water molecules far from the interface have dipole moments corresponding to the bulk values. This behavior can be attributed to the changes of the hydrogen bonding patterns and the orientation of water molecules near the interface. The induced dipole moments of the CCl4 molecules near the interface, on the other hand, are significantly enhanced. This is due in part to the strong local field induced by the water molecules at the interface. The calculated electric potentials using the dipole moment approach help us to analyze the orientations of water and CCl4 molecules at the interface.

Effect of adsorption of non-ionic surfactant and non-ionic—anionic surfactant mixtures on silica—liquid interfacial properties

Adsorption of non-ionic surfactants and non-ionic—anionic surfactant mixtures is shown to have marked effects on silica—liquid interfacial properties such as hydrophobicity and zeta potential. The hydrophobicity of silica surface is found to be dependent on both adsorption density and structure of the non-ionic surfactants. At relatively low adsorption densities, non-ionic surfactant with long ethoxyl chain (C 8φEO 40) is observed to impart hydrophobicity to silica surface, but further adsorption restored hydrophilicity to the silica surface. Such changes in hydrophobicity are not observed with non-ionic surfactant with shorter ethoxyl chain and this difference in hydrophobicity effects is related to the conformational behavior of the adsorbed molecules. Anionic surfactant, which by itself does not adsorb on silica, shows significant adsorption in the presence of the non-ionic surfactant and markedly changes the zeta potential of silica particles. Surfactant adsorption mechanisms responsible for changes in silica—liquid interfacial properties are discussed in this paper.