Self-assembly of gears at a fluid/air interface

This paper describes a dynamic system-a system that develops order only when dissipating energy-comprising millimeter to centimeter scale gears that self-assemble into a simple machine at a fluid/air interface. The gears are driven externally and indirectly by magnetic interactions; they are made of poly(dimethylsiloxane) (PDMS) or magnetically doped PDMS, and fabricated by soft lithography. Transfer of torque between gears can take place through three different mechanisms: mechanical interaction, hydrodynamic shear, and capillarity/overlap of menisci. Interplay between these forces allows interactions and motions that are not possible with conventional systems of gears.     

Electrodipping force acting on solid particles at a fluid interface

We report experimental results which show that the interfacial deformation around glass particles (radius, 200-300 microm) at an oil-water (or air-water) interface is dominated by an electric force, rather than by gravity. It turns out that this force, called for brevity "electrodipping," is independent of the electrolyte concentration in the water phase. The force is greater for oil-water than for air-water interfaces. Under our experimental conditions, it is due to charges at the particle-oil (instead of particle-water) boundary. The derived theoretical expressions, and the experiment, indicate that this electric force pushes the particles into water. To compute exactly the electric stresses, we solved numerically the electrostatic boundary problem, which reduces to a set of differential equations. Convenient analytical expressions are also derived. Both the experimental and the calculated meniscus profile, which are in excellent agreement, exhibit a logarithmic dependence at long distances. This gives rise to a long-range electric-field-induced capillary attraction between the particles, detected by other authors. Deviation from the logarithmic dependence is observed at short distances from the particle surface due to the electric pressure difference across the meniscus. The latter effect gives rise to an additional short-range contribution to the capillary interaction between two floating particles. The above conclusions are valid for either planar or spherical fluid interfaces, including emulsion drops. The electrodipping force, and the related long-range capillary attraction, can engender two-dimensional aggregation and self-assembly of colloidal particles. These effects could have implications for colloid science and the development of new materials.     

Particle motion near a deformable fluid interface

The present paper reports on one aspect of our recent studies of the hydrodynamic interactions of two deformable particles in a viscous fluid. This general hydrodynamics problem represents an initial step toward a fundamental invertigation of particle/ drop or droplet/droplet interactions in processes such as coalescence and flotation where both hydrodynamic and colloidal effects may be important. Here we consider only the limiting problem of translation of a rigid sphere with constant velocity normal to the plane of an initially flat interface. The Reynolds number is assumed to be vanishingly small; however, no restriction is imposed on the magnitude of the interface deformation.A primary focus of our research has been the qualitative dependence of the mode of interface deformation on the viscosity ratio, and on appropriate non-dimensional measures of interfacial tension and the density difference across the interface. In some instances, the deformation is relatively small and a so called “film drainage” configuration is attained as the particle passes across the plane of the undeformed interface. In other cases. however, the particle passes well into the domain of the second fluid while still surrounded by a layer of the first fluid that is connected to its original domain by a thin column (or “tail”) of fluid behind the sphere. In these latter cases, the rate of thinning of the tail is greate than the rate of thinning of the fluid layer around the particle; thus suggesting a second mode of particle “breakingthrough”, in addition to that associated with the film drainage configuration.     

DNA separation at a liquid-solid interface

We demonstrate that it is possible to separate a broad band of DNA on a solid substrate without topological obstacles. The mobility was found to scale with molecular size (N) as N(-0.25), while the resolution scaled as N(0.75) indicating that diffusivity on this substrate was minimal. By varying the buffer concentration we were able to show that the mobility for a given chain length scaled with the persistent length (p) as p(1/2). This could be shown to be related to the Gaussian conformation of the chains adsorbed on the surface. A two-dimensional corrugated surface of nonporous silica beads was produced using a self-assembling process at the air/water interface. Even though the surface corrugations were comparable to persistence length we show that they do not affect the mobility, indicating that surface friction rather than topological constraints are the predominant mechanism of separation on a surface.     

Interactions between particles with an undulated contact line at a fluid interface: capillary multipoles of arbitrary order

A colloidal particle adsorbed at a fluid interface could have an undulated, or irregular contact line in the presence of surface roughness and/or chemical inhomogeneity. The contact-line undulations produce distortions in the surrounding liquid interface, whose overlap engenders capillary interaction between the particles. The convex and concave local deviations of the meniscus shape from planarity can be formally treated as positive and negative "capillary charges," which form "capillary multipoles." Here, we derive theoretical expressions for the interaction between two capillary multipoles of arbitrary order. Depending on the angle of mutual orientation, the interaction energy could exhibit a minimum, or it could represent a monotonic attraction. For undulation amplitudes larger than 5 nm, the interaction energy is typically much greater than the thermal energy kT. As a consequence, a monolayer from capillary multipoles exhibits considerable shear elasticity, and such monolayer is expected to behave as a two-dimensional elastic solid. These theoretical results could be helpful for the understanding of phenomena related to aggregation and ordering of particles adsorbed at a fluid interface, and for the interpretation of rheological properties of particulate monolayers. Related research fields are the particle-stabilized (Pickering) emulsions and the two-dimensional self-assembly of microscopic particles.     

Orientation of a monoclonal antibody adsorbed at the solid/solution interface: a combined study using atomic force microscopy and neutron reflectivity

Conformational orientations of a mouse monoclonal antibody to the beta unit of human chorionic gonadotrophin (anti-beta-hCG) at the hydrophilic silicon oxide/water interface were investigated using atomic force microscopy (AFM) and neutron reflectivity (NR). The surface structural characterization was conducted with the antibody concentration in solution ranging from 2 to 50 mg.L(-1) with the ionic strength kept at 20 mM and pH = 7.0. It was found that the antibody adopted a predominantly "flat-on" orientation, with the Fc and two Fab fragments lying flat on the surface. The AFM measurement revealed a thickness of 30-33 A of the layer formed in contact with 2 mg.L(-1) antibody in water, but, interestingly, the flat-on antibody molecules formed small nonuniform clusters equivalent to 2-15 antibody molecules. Parallel AFM scanning in air revealed even larger surface clusters, suggesting that surface drying induced further aggregation. The AFM study thus demonstrated that the interaction between protein and the hydrophilic surface is weak and indicated that surface aggregation can be driven by the attraction between neighboring protein molecules. NR measurements at the solid/water interface confirmed the flat-on layer orientation of adsorbed molecules over the entire concentration range studied. Thus, at 2 mg.L(-1), the adsorbed antibody layer was well represented by a uniform layer with a thickness of 40 A. This value is thicker than the 30-33 A observed from AFM, suggesting possible layer compression caused by the tip tapping. An increase in the antibody concentration to 10 mg.L(-1) led to increasing surface adsorption. The corresponding layer structure was well represented by a three-layer model consisting of an inner sublayer of 10 A, a middle sublayer of 30 A, and an outer sublayer of 25 A, with the protein volume fractions in each sublayer being 0.22, 0.42, and 0.10, respectively. The structural transition can be interpreted as a twisting and tilting of segments of the adsorbed molecules, driven by an electrostatic repulsion between them that increases with the surface packing density. Hindrance of antigen access to antibody binding sites, resulting from the change in surface packing, can account for the decrease in antigen binding capacity (AgBC) with increasing surface density of the antibody that is observed.     

Orientation of the 3-methylpyridine molecules at the liquid–vapour interface of their aqueous solution

The orientation of the 3-methylpyridine (3MPy) molecules at the liquid–vapour interface of their 3 mol% solution is analysed in detail at 298 K on the basis of a Monte Carlo simulation. The 3MPy molecules are found to have a strong tendency for being adsorbed at the interface, exhibiting a dual orientational preference. At the vapour side of the interface they are preferentially aligned perpendicular to the interface, in such a way that the para carbon atom of the pyridine ring points straight to the vapour phase, whereas the C–Me bond declines by about 60° from the interface normal. On the other hand, at the liquid side of the interface the preferred orientation of the 3MPy molecules is close to the parallel alignment with the plane of the interface.     

Expression of a supramolecular complex at a multivalent interface

The multivalent binding of a supramolecular complex at a multivalent host surface by combining the orthogonal beta-cyclodextrin (CD) host-guest and metal ion-ethylenediamine coordination motifs is described. As a heterotropic, divalent linker, an adamantyl-functionalized ethylenediamine derivative was used. This was complexed with Cu(II) or Ni(II). The binding of the complexes to a CD self-assembled monolayer (SAM) was studied as a function of pH by means of surface plasmon resonance (SPR) spectroscopy. A heterotropic, multivalent binding model at interfaces was used to quantify the multivalent enhancement at the surface. The Cu(II) complex showed divalent binding to the CD surface with an enhancement factor higher than 100 relative to the formation of the corresponding divalent complex in solution. Similar behavior was observed for the Ni(II) system. Although the Ni(II) system could potentially be trivalent, only divalent binding was observed at the CD SAMs, which was confirmed by desorption experiments.     

Varying the counterions at a charged interface

The influence of different counterions on the adsorption behavior of the ionic soluble surfactant dodecyl-dimethylammonium-pyridimium bromide is investigated. The addition of potassium halogenides to aqueous solutions of the surfactant modifies the surface activity of the amphiphile and has a profound influence on the surface tension isotherms. The measured critical micelle concentration follows the order of the periodic table of elements which is in strong contrast to the surface excess. The number density of the adsorbed surfactants at the cmc does not depend on the amount of counterions in the solution but on the nature of the counterion. Furthermore, evidence is provided that the surface region is depleted on fluoride ions. Surface second harmonic generation and ellipsometry have been used to gain direct structural information which complement the thermodynamic considerations. The combination of both optical techniques yields the number density of the condensed counterions within the compact layer. A strategy to retrieve selected parameters of the ion binding model of Radke et al. is presented. The analysis of the optical data reveal the existence of a phase transition towards a surface condensed state with increasing salt condensation.     

Protruding chain at a sharp penetrable interface

A single protruding chain at a sharp discontinuity of the quality of the surrounding medium was shown by Monte Carlo (MC) simulations to be a good model representing semiquantitatively the major macroscopic features of polymer interfaces such as the interface thickness. This is in accord with early original theoretical suggestions. Compared to multichain simulations of a polymer/polymer interface it is clear that the protruding chain is not completely adequate to represent a diffuse multichain interface. The results on a protruding chain, however, indicate the proper interfacial thickness as obtained by MC simulations in multichain systems. The microscopic results (such as deformation, orientation of coils at the interface and chain end segregation into the interface) are similar to those obtained in multichain polymer interfaces. None of the investigated interface properties exhibits a dependence on the total chain length of the protruding chain.     

Protein stability at a carbon nanotube interface

The interactions of proteins with solid surfaces occur in a variety of situations. Motivated by the many nanoengineering applications of protein-carbon nanotube hybrids, we investigate the conformational transitions of hen egg white lysozyme adsorbed on a carbon nanotube. Using a C(α) structure-based model and replica exchange molecular dynamics, we show how the folding/unfolding equilibrium of the adsorbed protein varies with the strength of its coupling to the surface. The stability of the native state depends on the balance between the favorable entropy and unfavorable enthalpy change on adsorption. In the case of a weakly attractive surface when the former dominates, the protein is stabilized. In this regime, the protein can fold and unfold while maintaining the same binding fraction. With increasing surface attraction, the unfavorable enthalpic effect dominates, the native state is destabilized, and the protein has to extensively unbind before changing states from unfolded to folded. At the highest surface coupling, the entropic penalty of folding vanishes, and a folding intermediate is strongly stabilized. In this intermediate state, the α-domain of lysozyme is disrupted, while the β-sheet remains fully structured. We rationalize the relative stability of the two domains on the basis of the residue contact order.     

Adsorption of styrene at a binary solution-gas interface

The surface tension isotherms of styrene solutions in 2-propanol-water solvents are obtained experimentally. Isotherms of excess and absolute adsorption at the liquid-gas interfaces are also obtained. A thermodynamic model for the adsorption of styrene from 2-propanol-water binary solvent at the solutiongas interfaces is proposed on the basis of the stoichiometric approach. The isosteric adsorption heats are calculated using the experimental isotherms of surface tension and excess adsorption of styrene from 2-propanol-water binary solvent, and it is shown that the formation of the surface layer occurs according to a mechanism combining the processes of displacement adsorption between molecules of the components of the solvent, styrene, and 2-propanol.     

Self-assembled structure of nanoparticles at a liquid-liquid interface

Pickering emulsions are used as a template to investigate the multiphase interactions and self-assembled structure of nanoparticles at a trichloroethylene-water interface. The dodecanethiol-capped silver nanoparticles of 1-5 nm form randomly distributed multilayers at the liquid/liquid interface, with an interparticle distance varying from close contact to approximately 25 nm. This report offers the first direct observation of nanoparticles in a liquid medium using the environmental transmission electron microscope, as well as the first work revealing the detailed self-assembled structure of nanoparticles at a liquid/liquid interface when the size of the nanoparticles is comparable to the molecular dimension of the liquids.     

To the problem of first order phase transition at the fluid–fluid interface

The thermodynamic analysis is developed to describe both the surface freezing and the surface melting in various systems. The key physico-chemical parameters determining the shift of the freezing temperature in boundary phases in comparison to the temperature of corresponding bulk phase transition are discussed.     

Shape of the capillary meniscus around an electrically charged particle at a fluid interface: comparison of theory and experiment

Here, we consider in detail the problem of the shape of the capillary meniscus around a charged colloidal particle, which is attached to a fluid interface: oil/water or air/water. The meniscus profile is influenced by the electric field created by charges at the particle/nonpolar fluid boundary. We digitized the coordinates of points from the meniscus around silanized glass spheres (200-300 mum in radius) attached to the tetradecane/water interface. The theoretical meniscus shape is computed in three different ways that give numerically coincident results. It is proven that for sufficiently small particles the meniscus profile can be expressed as a superposition of pure electric and gravitational deformations. Special attention is paid to the comparison of theory and experiment. A procedure for data processing is developed that allows one to obtain accurate values of the contact angle and surface charge density from the fit of the experimental meniscus profile. For all investigated particles, excellent agreement between theory and experiment is achieved. The results indicate that the electric field gives rise to an interfacial deformation of medium range and considerable amplitude.     

Supercritical fluid chromatography/mass spectrometry using a simple capillary-direct interface

Summary A simple method for interfacing capillary supercritical fluid chromatography with a commercial quadrupole mass spectrometer (SFC/MS) has been developed that yields good chromatographic peak shapes and good sensitivity. No modification of the mass spectrometer is required, and a single instrument can be used for both SFC/MS and GC/MS with conversion between modes requiring less than 20 min. SFC/MS separations and chemical ionization mass spectra of wax components, a triazine pesticide metabolite, abietic acid, and high molecular weight polycyclic aromatic hydrocarbons are reported.

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Kombination von Chromatographie mit überkritischen fluiden Phasen und Massenspektrometrie unter Verwendung eines einfachen direkten Capillar-Interface