Two-dimensional Colloids Research Articles

Mesoscopic Tapes Formed by Amphiphilic Two-Dimensional Colloids on Water Surface

A two-dimensional (2D) analogue of 3D fibrous supramolecular assemblies was formed spontaneously on the water surface by a short chain carboxyazobenzene derivative. It is a mesoscopic tape 1 nm thick, 80 nm wide, and can be as long as 0.1 mm. The tapes are obtained as Langmuir−Blodgett (LB) films on mica and examined by atomic force microscopy (AFM). AFM images exhibit complex internal structures, indicating that the tapes are not single crystals. They are not produced by LB processes, such as compression and lifting, and are not induced by the mica surface. The mesoscopic tape is self-contained and exists independent of surface pressure whose equilibrium value is nearly zero. UV−vis reflection spectroscopy on the water surface shows a peak at 305 nm. Increasing the water temperature to above 30 °C or leaving the film on the water surface for 14 h caused the tapes to disintegrate into small clusters and the UV−vis peak to shift to 365 nm. The concentration at which the solvent evaporates controls the tape formation. At low concentrations, the tapes coexist with the domains of an ordinary phase. Then, there is a critical aggregation concentration at 8 nm2/molecule that the tape fraction increases abruptly. These results suggest that the tape is closely related to associations of 2D clusters.

Experimental Study of Laser-Induced Melting in Two-Dimensional Colloids

We study the phase behavior of a charged two-dimensional colloidal system in the presence of a periodic light field composed of two interfering laser beams. Above a certain light intensity, the light field initiates a liquid-solid transition. This effect is known as light-induced freezing. When the light field is increased above a critical value, however, the induced crystal is predicted to melt into a modulated liquid again. In this paper we report the first direct experimental observation of this reentrance phenomenon.

Colloid crystal self-organization and dynamics at the air/water interface

The properties of two-dimensional arrays of micrometre-sized particles are of interest in relation to a wide range of phenomena, including self-organization and phase behaviour in colloid science and condensed-matter physics1,2,3, the behaviour of dusty plasmas4 and the templating of ordered structures for photonic applications5. Most studies have used pre-existing particles such as monodisperse latex spheres, which may be manipulated with electric or magnetic fields. In contrast, we report here an inorganic solution that spontaneously precipitates a self-organized two-dimensional colloid crystal at the air/water interface. A solution of calcium hydroxide exposed to air reacts with dissolved carbon dioxide to precipitate microcrystals of calcium carbonate in the form of calcite. These aggregate at the surface to form a disordered gel mat with fractal characteristics6. We find, however, that in aged solutions a second population of charged microcrystals with the ‘dogtooth spar’ morphology appears on the surface. These crystallites, which can be observed by optical microscopy, become organized into a regular triangular lattice. The competition between electrostatic and capillary forces between particles leads to lattice spacings of the order of 125 to 175 µm, 5 to 7 times the diameter of the particles. These structures are stable for around 24 h, but eventually aggregate with the fractal gel. The mechanism of their self-organization, as yet incompletely understood, might provide some insights into similar phenomena in colloid science2,3,7,8,9.

Dynamic properties of heterogeneous surface films: Multiple scattering of capillary waves

The phenomenon of multiple scattering of surface waves by two-dimensional viscoelastic particles is investigated. A method to evaluate the scattering amplitude is proposed and a simple equation which describes a nonlinear dependency of the effective wave number of transverse surface waves on the concentration of a two-dimensional colloid system is obtained. These results are used thereupon to investigate the propagation of capillary waves in a liquid with a heterogeneous surface film. For equilibrium films, effects of multiple scattering due to fluctuations of the surface tension can be neglected. However, for nonequilibrium systems which contain a heterogeneous solidlike surface phase, the nonlinear dependency of the real part of the effective wave number on the concentration of scatterers can be observed. The inhomogeneity of the dynamic surface elasticity can significantly influence the characteristics of surface waves only for condensed films with two-dimensional “bubbles” of the gaseous phase.

Structural properties of colloidal suspensions

Structural properties of three- and two-dimensional colloids composed by hard spheres and/or by Yukawa particles, which can have different diameters and charges, are studied by solving the Ornstein-Zernike equation, together with Percus-Yevick, hypernetted chain and Rogers-Young approximations. From the partial radial distribution functions g ij ( r) the partial structure factors S ij ( k) are determined, and with them the compressibility structure factor S x ( k), the measured structure factor S M ( k) and the Bhatia-Thornton structure factors S NN ( k), S NQ ( k) and S QQ ( k). As an effect of diameter and/or charge polydispersity on the structure of binary mixtures, the position and height of the main peak of S M ( k), and its value at k = 0, change non-monotonously with the composition. In the case of binary mixtures of hard and Yukawa spheres the structure is given by two different scales. A liquid-solid phase transition induced by a change in the dimensionality was found for monodisperse systems.

Two-dimensional colloids. The linear thermodynamics of monolayers

A variety of two-dimensional microscopic structures resembling emulsions and other colloidal forms have been reported in spread and adsorbed monolayers at fluid interfaces. These two-dimensional colloids may be described as the mutual dispersion of first-order co-existing surface phases associated with the linear tension between the phases, in a manner analogous to the role of surface tension in the dispersion of three-dimensional phases. A formal description of linear tension in monolayers is given, and several relationships, notably the one-dimensional analogues of the Gibbs adsorption isotherm and Kelvin equation, are discussed in the context of potential new experiments. The discussion is primarily for spread insoluble monolayers. The limitations of the utility of the linear tension concept are also considered.

Formation of two-dimensional colloid crystals in liquid films under the action of capillary forces

When two similar small particles are attached to a liquid interface they attract each other due to a lateral capillary force. This force appears because the gravitational potential energy of the floating particles decreases when they are approaching each other. This force is proportional to R6 (R is the particle radius), so it decreases very fast with particle size and becomes negligible for R<10 mu m. We found that the situation is quite different when the particles (instead of being freely floating) are partially immersed in a liquid layer on a substrate. In this case the energy of capillary attraction is proportional to R2 and turns out to be much larger than kT even with particles of diameter about 10 nm. The effect is related to the particle three-phase contact angle, i.e. to the intermolecular forces, rather than to gravity. The experiments show that the lateral capillary forces can bring about the formation of a two-dimensional array (2D-crystal) from both micrometre-size and submicrometre particles: latex spheres, protein globules, etc.

Formation of ordered two-dimensional gold colloid lattices by electrophoretic deposition

The preparation of ordered two-dimensional (2D) gold colloid lattices using an electrophoretic technique is reported. The lattices can be readily observed by electron microscope and the lattice constants determined by electron diffraction. Using image processing, it can be shown that the gold particles condense into a hexagonal lattice and also that the crystallographic axes of the individual gold particles are randomly oriented. The equilibrium distance between the particles corresponds approximately to the size of the adsorbed stabilizers. The structures are therefore stabilized by short-range steric repulsion and not by a diffuse layer electrostatic barrier, as is normally the case for latex sols. The method is of general interest as a means to prepare monolayer films of nanosized metal or semiconductor particles. © 1993 American Chemical Society.

Optical properties of silver films deposited at low temperatures

The reflectance spectrum of quench-condensed silver films was studied as a function of film temperature and thermal history, using an uv-visible stress-modulator ellipsometer. Films deposited at low temperatures were found to exhibit an anomalous absorption in the visible region of the spectrum. This absorption was attenuated and receded to lower wavelengths as the film was annealed. The spectra were successfully reproduced by assuming that films deposited at low temperatures were rough and that the roughness could be modeled by means of a two-dimensional metallic colloid whose optical conduction resonance accounted for the anomalous absorption. The relationship between the surface morphology of these low-temperature condensed films and their ability to give rise to surface-enhanced Raman scattering is discussed.