Surface functionalization of SBA-15 and a nonordered mesoporous silica with a 1,4-diazabicyclo[2.2.2]octane derivative: study of CuCl2 adsorption from ethanol solution

The equilibrium position of a spherical or prolate spheroidal particle resembling a needle floating at the interface between two immiscible fluids is discussed. A three-dimensional meniscus attached to an a priori unknown contact line at a specified contact angle is established around the particle, imparting to the particle a capillary force due to surface tension that is balanced by the buoyancy force and the particle weight. An accurate numerical solution for a floating sphere is obtained by solving a boundary-value problem, and the results are compared favorably with an approximate solution where the effect of the particle surface curvature is ignored and the elevation of the contact line is computed using an analytical solution for the meniscus attached to an inclined flat plate. The approximate formulation is applied locally around the nearly planar elliptical contact line of a prolate spheroid to derive a nonlinear algebraic equation governing the position of the particle center and the mean elevation of the contact line. The effect of the fluid and particle densities, contact angle, and capillary length is discussed, and the shape of the contact line is reconstructed and displayed from the local solution.     

Synergistic interaction in emulsions stabilized by a mixture of silica nanoparticles and cationic surfactant

Using a range of complementary experiments, a detailed investigation into the behavior of dodecane-water emulsions stabilized by a mixture of silica nanoparticles and pure cationic surfactant has been made. Both emulsifiers prefer to stabilize o/w emulsions. At high pH, particles are ineffective emulsifiers, whereas surfactant-stabilized emulsions become increasingly stable to coalescence with concentration. In mixtures, no emulsion phase inversion occurs although synergism between the emulsifiers leads to enhanced stability at either fixed surfactant concentration or fixed particle concentration. Emulsions are most stable under conditions where particles have negligible charge and are most flocculated. Freeze fracture scanning electron microscopy confirms the presence of particle flocs at drop interfaces. At low pH, particles and surfactant are good emulsifiers alone. Synergism is also displayed in these mixtures, with the extent of creaming being minimum when particles are most flocculated. Experiments have been undertaken in order to offer an explanation for the latter synergy. By determining the adsorption isotherm of surfactant on particles in water, we show that surfactant addition initially leads to particle flocculation followed by re-dispersion. Using suitable contact angle measurements at oil-water-solid interfaces, we show that silica surfaces initially become increasingly hydrophobic upon surfactant addition, as well as surfactant adsorption lowering the oil-water interfacial tension. A competition exists between the influence of surfactant on the contact angle and the tension in the attachment energy of a particle to the interface.     

The role of capillary and surface forces in the crossover behavior of solid nanoparticles at liquid interfaces

We investigate the interaction between a nanoparticle and an oil-water interface with particular emphasis on the particle crossing through the interface. The formation of a three-phase contact line is investigated in two cases, namely in the presence and in the absence of surface forces. We carefully examine the interplay between capillary and surface forces in such systems. Two instabilities of the interface (snap-in/snap-out) as the particle is moved through the interface are identified and quantitatively described. While the snap-in instability was observed in some AFM studies, the precise interface position and configuration relative to the particle at the instability depends on the nature of the surface forces present in the system. After the snap-in, the particle is adsorbed and must overcome an energy barrier due to the interface deformation in order to cross-over to the other liquid. We make quantitative predictions on the interface configuration at the instabilities and the free energy barrier height. The roles of particle size and different interaction parameters characterizing the system in determining the magnitude of the energy barrier for crossing and in the formation of a three-phase contact line are discussed. Ultimately, this study will enable us to make quantitative predictions on capillary effects in nanoparticle-microemulsions mixtures and other colloidal systems. For particles in the micrometer range and larger the capillary forces dominate over the surface forces and dictate how the snap-in occurs. However, the situation becomes different for particle sizes smaller than about 100 nm. The presence of surface forces modifies the interface configuration and the free energy jump at the snap-in instability.     

Orientation of a nanocylinder at a fluid interface

A nanocylinder placed on a fluid interface can assume an end-on or side-on orientation, or it can immerse itself in the surrounding bulk phases. Any of these orientations can satisfy a mechanical force balance when the particle is small enough that gravitational effects are negligible. The orientation is determined by the surface energies of the fluid-solid, fluid-vapor, and vapor-solid surfaces. A comparison of the energy of each state allows phase diagrams to be defined in terms of the scaled aspect ratio x=2L/pir and the contact angle thetao, where L and r denote the nanocylinder length and radius, respectively. Line tension can also influence the orientations by changing the equilibrium contact angle theta and by increasing the energetic cost of the contact line. Phase diagrams accounting for positive line tensions Sigma are also constructed. These phase diagrams can be divided into two classes. In the first, over some range of x and Sigma, nanocylinders can be driven from side-on to end-on orientations with increasing Sigma. This transition terminates at a triple point where the side-on, end-on, and immersed energies are the same. In the second class, there is no triple point and, for a range of Sigma values, nanocylinders of all aspect ratios x prefer an end-on orientation. In all cases, for high enough Sigma, line tension drives a wetting transition similar to that already noted in the literature for spherical particles. The zero line tension predictions are compared favorably to experiment, in which functionalized gold nanowires made by template synthesis are spread at aqueous-gas interfaces, immobilized using a gel-fixation technique, and observed by scanning electron microscopy. The small aspect ratio particles (disks) were in an end-on configuration, while the longer nanowires were in a side-on orientation, in agreement with the theory.     

DNA hybridization-induced reorientation of liquid crystal anchoring at the nematic liquid crystal/aqueous interface

Interactions between DNA and an adsorbed cationic surfactant at the nematic liquid crystal (LC)/aqueous interface were investigated using polarized and fluorescence microscopy. The adsorption of octadecyltrimethylammonium bromide (OTAB) surfactant to the LC/aqueous interface resulted in homeotropic (untilted) LC alignment. Subsequent adsorption of single-stranded DNA (ssDNA) to the surfactant-laden interface modified the interfacial structure, resulting in a reorientation of the LC from homeotropic alignment to an intermediate tilt angle. Exposure of the ssDNA/OTAB interfacial complex to its ssDNA complement induced a second change in the interfacial structure characterized by the nucleation, growth, and coalescence of lateral regions that induced homeotropic LC alignment. Fluorescence microscopy showed explicitly that the complement was colocalized in the same regions as the homeotropic domains. Exposure to noncomplementary ssDNA caused no such response, suggesting that the homeotropic regions were due to DNA hybridization. This hybridization occurred in the vicinity of the interface despite the fact that the conditions in bulk solution were such that hybridization did not occur (high stringency), suggesting that the presence of the cationic surfactant neutralized electrostatic repulsion and allowed for hydrogen bonding between DNA complements. This system has potential for label-less and portable DNA detection. Indeed, LC response to ssDNA target was detected with a lower limit of approximately 50 fmol of complement and was sufficiently selective to differentiate a one-base-pair mismatch in a 16-mer target.     

Synergistic stabilization of emulsions by a mixture of surface-active nanoparticles and surfactant

The stability and rheology of tricaprylin oil-in-water emulsions containing a mixture of surface-active hydrophilic silica nanoparticles and pure nonionic surfactant molecules are reported and compared with those of emulsions stabilized by each emulsifier alone. The importance of the preparation protocol is highlighted. Addition of particles to a surfactant-stabilized emulsion results in the appearance of a small population of large drops due to coalescence, possibly by bridging of adsorbed particles. Addition of surfactant to a particle-stabilized emulsion surprisingly led to increased coalescence too, although the resistance to creaming increased mainly due to an increase in viscosity. Simultaneous emulsification of particles and surfactant led to synergistic stabilization at intermediate concentrations of surfactant; emulsions completely stable to both creaming and coalescence exist at low overall emulsifier concentration. Using the adsorption isotherm of surfactant on particles and the viscosity and optical density of aqueous particle dispersions, we show that the most stable emulsions are formed from dispersions of flocculated, partially hydrophobic particles. From equilibrium contact angle and oil-water interfacial tension measurements, the calculated free energy of adsorption E of a silica particle to the oil-water interface passes through a maximum with respect to surfactant concentration, in line with the emulsion stability optimum. This results from a competition between the influence of particle hydrophobicity and interfacial tension on the magnitude of E.     

Nature of disclination cores in liquid crystals

A disclination is an orientation symmetry-breaking defect and its strength corresponds to the number of rotations (in multiples of 2π) of the director over a path encircling the disclination. The line core plays a considerable role in the disclination energy balance and mobility, however, the detailed nature of the core structure is largely unknown. Here we demonstrate different core structures for different disclinations by using transmission electron microscopy and atomic force microscopy coupled with the nanostripe decoration technique. We show that the molecular distribution in the core of (+1) defects changes from the prolate planar to oblate homeotropic, resembling but not identical to 'escape into the third dimension'. The homeotropic (-1) defect appears to possess an isotropic core, contrary to theoretical predictions.     

Effect of dispersion pH on the formation and stability of Pickering emulsions stabilized by layered double hydroxides particles

Using positively charged plate-like layered double hydroxides (LDHs) particles as emulsifier, liquid paraffin-in-water emulsions stabilized solely by such particles are successfully prepared. The effects of the pH of LDHs aqueous dispersions on the formation and stability of the emulsions are investigated here. The properties of the LDHs dispersions at different pHs are described, including particle zeta potential, particle aggregation, particle contact angle, flow behavior of the dispersions and particle adsorption at a planar oil/water interface. The zeta potential decreases with increasing pH, leading to the aggregation of LDHs particles into large flocs. The structural strength of LDHs dispersions is enhanced by increasing pH and particle concentration. The three-phase contact angle of LDHs also increases with increasing pH, but the variation is very small. Visual observation and SEM images of the interfacial particle layers show that the adsorption behavior of LDHs particles at the planar oil/water interface is controlled by dispersion pH. We consider that the particle-particle (at the interface) and particle-interface electrostatic interactions are well controlled by adjusting the dispersion pH, leading to pH-tailored colloid adsorption. The formation of an adsorbed particle layer around the oil drops is crucial for the formation and stability of the emulsions. Emulsion stability improves with increasing pH and particle concentration because more particles are available to be adsorbed at the oil/water interface. The structural strength of LDHs dispersions and the gel-like structure of emulsions also influence the stability of the emulsions, but they are not necessary for the formation of emulsions. The emulsions cannot be demulsified by adjusting emulsion pH due to the irreversible adsorption of LDHs particles at the oil/water interface. TEM images of the emulsion drops show that a thick particle layer forms around the oil drops, confirming that Pickering emulsions are stabilized by the adsorbed particle layers. The thick adsorbed particle layer may be composed of a stable inner particle layer which is in direct contact with the oil phase and a relatively unstable outer particle layer surrounding the inner layer.     

Wetting behavior of spherical nanoparticles at a vapor-liquid interface: a density functional theory study

The wetting behavior of spherical nanoparticles at a vapor-liquid interface is investigated by using density functional theory, and the line tension calculation method is modified by analyzing the total energy of the vapor-liquid-particle equilibrium. Compared with the direct measurement data from simulation, the results reveal that the thermodynamically consistent Young's equation for planar interfaces is still applicable for high curvature surfaces in predicting a wide range of contact angles. The effect of the line tension on the contact angle is further explored, showing that the contact angles given by the original and modified Young's equations are nearly the same within the region of 60° < θ < 120°. Whereas the effect is considerable when the contact angle deviates from the region. The wetting property of nanoparticles in terms of the fluid-particle interaction strength, particle size, and temperature is also discussed. It is found that, for a certain particle, a moderate fluid-particle interaction strength would keep the particle stable at the interface in a wide temperature range.     

Adsorption of microstructured particles at liquid-liquid interfaces

The solid particles are adsorbed at interfaces and form self-assembled structures when the particles have suitable wettability to both liquids. Here, we show theoretically how the microstructure on the particle surface affects their adsorption properties. The physical properties of the interface adsorbing a particle will be described by taking into account the surface roughness due to the microstructure. The microstructure on the surface changes drastically the wettability and the equilibrium position of the adsorbed particle. Therefore, the contact angle of the particle at the three-phase contact line shifts with the particle surface area, because the surface roughness enhances the interfacial properties of the particle surface. Moreover, the range of the interfacial tensions at which the particle is adsorbed becomes narrower with the increase of the surface roughness. The effect of the particle shape on the adsorption properties is also studied. In the case of disk-shaped particles, the energy changes discontinuously when the plane surface of the particle contacts the liquid-liquid interface. The adsorbing position does not change with the surface roughness. The orientation of a parallelepiped particle at the liquid-liquid interface is governed by the aspect ratio and the surface area of the particle. On the other hand, the particle which is partially covered with the microstructured surface is adsorbed firmly at the interface in an oriented state. We should consider not only the interfacial tensions but also the surface structure and the particle shape to control the adsorption behavior of the particle.     

Tilt angle change of nematic liquid crystal as a function of the number of graphite layers

The tilt angle of a nematic liquid crystal on a graphite flake was observed to change with increasing numbers of graphite layers. A portion of the substrate that induced homeotropic alignment was covered with graphite flakes, which induced a planar alignment. Nematic liquid crystals placed on the graphite deviated from vertical orientation to the polar angle. The angle of deviation appeared to be proportional to the number of layers and reached a limit, with almost planar alignment, at about 7–8 graphite layers. Although the main contributing factor to the tilt angle change was considered to be the result of van der Waals forces, it was seen that other long-range interaction forces needed to be considered to explain the experimental results obtained.     

A model for investigating the behaviour of non-spherical particles at interfaces

This paper introduces a simple method for modelling non-spherical particles with a fixed contact angle at an interface whilst also providing a method to fix the particles orientation. It is shown how a wide variety of particle shapes (spherical, ellipsoidal, disc) can be created from a simple initial geometry containing only six vertices. The shapes are made from one continuous surface with edges and corners treated as smooth curves not discontinuities. As such, particles approaching cylindrical and orthorhombic shapes can be simulated but the contact angle crossing the edges will be fixed. Non-spherical particles, when attached to an interface can cause large distortions in the surface which affect the forces acting on the particle. The model presented is capable of resolving this distortion of the surface around the particle at the interface as well as allowing for the particle's orientation to be controlled. It is shown that, when considering orthorhombic particles with rounded edges, the flatter the particle the more energetically stable it is to sit flat at the interface. However, as the particle becomes more cube like, the effects of contact angle have a greater effect on the energetically stable orientations. Results for cylindrical particles with rounded edges are also discussed. The model presented allows the user to define the shape, dimensions, contact angle and orientation of the particle at the interface allowing more in-depth investigation of the complex phenomenon of 3D film distortion around an attached particle and the forces that arise due to it.     

On the kinetics of nanoparticle self-assembly at liquid/liquid interfaces

We investigate the concentration and size dependent self-assembly of cadmium selenide nanoparticles at an oil/water interface. Using a pendant drop tensiometer, we monitor the assembly kinetics and evaluate the effective diffusion coefficients following changes in the interfacial tension for the early and late stages of nanoparticle adsorption. Comparison with the coefficients for free diffusion reveals the energy barrier for particle segregation to the interface. The formation of a nanoparticle monolayer at the oil/water interface is characterised by transmission electron microscopy.     

Small solid particles and liquid lenses at fluid/fluid interfaces

The behaviour of small solid particles and liquid droplets at fluid interfaces is of wide interest, in part because of the roles they play in the stability of foams and emulsions. Here we focus on solid particles at liquid interfaces, both singly and in highly structured monolayers. We briefly mention small oil lenses on water in connection with the determination of line tension, τ. Particles are surface-active in the sense that they often adhere quite strongly to liquid surfaces, although of course they are not usually amphiphilic. The three-phase contact line around a particle at an interface is associated with an excess free energy resulting in a tendency of the line to contract (positive τ, which is a 1D analogue of surface tension) or to expand (negative τ). Positive line tension acts so as to push the contact angle of a particle with the fluid interface further away from 90°, i.e. to force the particle towards the more “wetting” of the two bulk phases. It also leads to activation barriers to entry and departure of particles from an interface. The behaviour of particle monolayers at octane/water interfaces is also discussed . It is found that, for monodisperse spherical polystyrene particles containing ionisable sulphate groups at the surface, highly ordered monolayers are formed. This appears to result from very long range electrostatic repulsion mediated through the oil phase. Surface pressure–surface area isotherms are discussed for particle monolayers and it is shown, using light microscopy, that at monolayer “collapse” particles are not expelled from the monolayers but rather the monolayer folds, remaining intact. This has an important bearing on methods, involving the use of the Langmuir trough, for the experimental determination of contact angles and line tensions in particulate systems. Received: 18 July 1999/Accepted: 30 August 1999     

Adsorption of colloidal particles to curved interfaces

As a simple model for a Pickering emulsion droplet, we consider the adsorption of spherical particles to a spherical liquid-liquid interface in order to investigate the curvature effect on the particle adsorption. By taking into account both the surface and the volume energies due to the presence of a particle, we show that the equilibrium contact angle is determined by the classical Young's equation although the adsorption energy depends on the curvature. We also calculate the partitioning of the colloidal particles among the two liquids and the interface. The distribution of colloidal particles is expressed in terms of the interfacial curvature as well as the relative wettability of the particle.     

Dynamic Surface Tension and Adsorption/Desorption Kinetics for Mixtures on Planar Surfaces: I. Under Nonflow Conditions

The adsorption/desorption kinetics for individual polymers and polymer mixtures of the water-soluble associative polymers with molecular weights of 12, 62, and 100 kg/mol onto SiO2 planar substrate have been studied by ellipsometry at room temperature under nonflow conditions. Equations were derived to predict behaviors of the adsorption/desorption kinetics and dynamic surface tension onto planar surfaces for any times. It is shown that the desorption kinetics of the water-soluble associative polymers onto planar SiO2 surface is an irreversible process due to strong interaction between polymer molecules and an interface. Copyright 1999 Academic Press.     

A mesoscale modelling study of nematic liquid crystals confined to ellipsoidal domains

Director configurations of nematic liquid crystalline molecules packed in ellipsoidal domains have been investigated using mesoscale modelling techniques. Interactions between the directors were described by the Lebwohl-Lasher potential. Four different ellipsoidal shapes (sphere, oblate spheroid, prolate spheroid, and ellipsoid) were studied under homogeneous and homeotropic surface anchoring conditions. The model has been characterized by computing thermodynamic and structural properties as a function of ellipsoidal shape (prolate and oblate) and size. The predicted director configuration in ellipsoids resulting from homeotropic surface anchoring is found to be very different from that in spherical domains. The bipolar configuration involving homogeneous surface anchoring is nearly identical in the four cases. The effect of an external electric field, applied at different orientations with respect to the major axis of the ellipsoid, has been probed as a function of the magnitude of the field and the ellipsoidal size and shape. The orientation of directors is most easily accomplished parallel and perpendicular to the major axis for the oblate and prolate spheroids, respectively, for homeotropic anchoring, and along the bipolar symmetry axis for homogeneous anchoring. In domains with homeotropic surface anchoring, the oblate spheroid and elongated ellipsoid are predicted to be the most efficient geometries for PDLC applications; for homogeneous anchoring conditions, the prolate spheroid and elongated ellipsoid are predicted to be the most efficient.     

Thermodynamic model of surfactant adsorption on soft liquid crystal interfaces

The Gibbs adsorption isotherm for planar liquid crystal/fluid interfaces is derived using the anisotropic Gibbs-Duhem equation. The Gibbs adsorption isotherm for planar interfaces is used to analyze the adsorption-driven orientation transition in aqueous solutions of anionic surfactants in contact with rodlike uniaxial nematic liquid crystal films. In qualitative agreement with experiments, the model predicts that, as the surfactant concentration increases, the tangential (planar) average molecular orientation of the liquid crystal with respect to the interface undergoes a transition to a normal (homeotropic) orientation. The anchoring coefficient or strength of anisotropic component of the interfacial tension is shown to depend on the surfactant's concentration. Analyzing the response to addition of a co-cation, the model reveals that, as the fractional coverage of the surfactant's chains increases, the interpenetration of liquid crystal molecules between the adsorbed surfactant tails promotes the orientation transition; at even higher surfactant chain concentrations, interpenetration is hindered because of lack of available space and a random surface orientation emerges. Thus, for aqueous surfactant solutions in contact with nematic liquid crystals, increasing the surfactant concentration leads to the following interfacial liquid crystal orientation transition cascade, planar orientation --> homeotropic orientation --> random orientation, which can lead to new sensor capabilities and surface structuring processes.     

A mesoscale modelling study of nematic liquid crystals confined to ellipsoidal domains

Director configurations of nematic liquid crystalline molecules packed in ellipsoidal domains have been investigated using mesoscale modelling techniques. Interactions between the directors were described by the Lebwohl-Lasher potential. Four different ellipsoidal shapes (sphere, oblate spheroid, prolate spheroid, and ellipsoid) were studied under homogeneous and homeotropic surface anchoring conditions. The model has been characterized by computing thermodynamic and structural properties as a function of ellipsoidal shape (prolate and oblate) and size. The predicted director configuration in ellipsoids resulting from homeotropic surface anchoring is found to be very different from that in spherical domains. The bipolar configuration involving homogeneous surface anchoring is nearly identical in the four cases. The effect of an external electric field, applied at different orientations with respect to the major axis of the ellipsoid, has been probed as a function of the magnitude of the field and the ellipsoidal size and shape. The orientation of directors is most easily accomplished parallel and perpendicular to the major axis for the oblate and prolate spheroids, respectively, for homeotropic anchoring, and along the bipolar symmetry axis for homogeneous anchoring. In domains with homeotropic surface anchoring, the oblate spheroid and elongated ellipsoid are predicted to be the most efficient geometries for PDLC applications; for homogeneous anchoring conditions, the prolate spheroid and elongated ellipsoid are predicted to be the most efficient.     

UV Polymerisation of Surfactants Adsorbed at the Nematic Liquid Crystal–Water Interface Produces an Optical Response

We have investigated the changes in crossed polariser optical textures produced by adsorption and UV polymerisation of a range of polymerisable surfactants at the interface between a nematic liquid crystal and water. Similar to non‐polymerisable surfactants, the adsorption of polymerisable surfactants with sufficiently long hydrophobic tail groups produces a transition from planar to homeotropic anchoring. UV polymerisation of surfactants with a polymerisable group located in the hydrophobic tail region changes the anchoring from homeotropic back to planar. Polymerisation in the hydrophilic headgroup region does not produce an optical transition. We demonstrate that these systems can be used to “write with light” in the interfaces and that they form the basis of a UV sensor device in which the optical response is visible to the naked eye.