No abstract

In colloidal suspensions containing a binary mixture of hard spheres depletion forces occur which substantially contribute to the interaction of the larger spheres among themselves and a wall, respectively. We investigated the depletion force acting on a large colloidal polystyrene sphere immersed in a solution of small, noncharged polymer coils close to a flat glass surface by means of total internal reflection microscopy. When the distance between the polystyrene sphere and the wall is smaller than the diameter of the polymer coils, an attractive potential acting on the sphere is observed which depends strongly on the polymer concentration. Our results are in agreement with theoretical predictions.[S0031-9007 (98) The stability of colloidal mixtures consisting of larger and smaller particles is well known to be strongly influenced by entropic depletion forces. Accordingly, those forces are essential in understanding phase separation phenomena and flocculation of binary hard-sphere mixtures, colloids in the presence of micelles, and of colloid polymer mixtures [1,2]. Very recently, it was suggested that depletion forces even may play an important role in the shape changes of phospholipid vesicles [3].The principal phenomenon of depletion interaction is easily understood when, e.g., a hard sphere of radius R suspended in a fluid containing smaller spheres of radius r (the latter are referred to as macromolecules in the following) in front of a wall at distance z is considered (see Fig. 1). If z decreases below the diameter of the macromolecules they are expelled from the region between the sphere and the wall. Consequently, the concentration of macromolecules becomes depleted in this region compared to that of the bulk, and an effective osmotic pressure causing a net attraction between the sphere and the wall occurs. Such an attractive force is also observed when the wall is replaced by another hard sphere.The first quantitative explanation of this effect was given by Asakura and Oosawa [4]. According to their calculation the change in the Helmholtz free energy DF of a single sphere positioned at distance z from a wall and suspended in a fluid of macromolecules can be written as

The effective interaction energy of a colloidal sphere in a suspension containing small amounts of non-ionic polymers and a flat glass surface has been measured and calculated using total internal reflection microscopy (TIRM) and a novel approach within density functional theory (DFT), respectively. Quantitative agreement between experiment and theory demonstrates that the resulting repulsive part of the depletion forces cannot be interpreted entirely in terms of entropic arguments but that particularly at small distances ( 100 nm) attractive dispersion forces have to be taken into account.82.70. Dd, The stability of systems like polydisperse mixtures of colloids, natural rubbers, micelles or polymer coils is known to be strongly influenced by depletion forces. In addition, such forces may also play an important role in biological systems, e.g., in promoting the aggregation of red blood cells [1,2]. The understanding of these forces being responsible for spontaneous structure formation, phase separation or flocculation in such systems is highly demanding both from the experimental and the theoretical point of view. A first approximate explanation of depletion forces was given by Asakura and Oosawa (AO), who recognized that the presence of small hard spheres (referred to as macromolecules in the following) can mediate effective forces between two larger objects if their distance is sufficiently small [3]. This can be easily understood by considering, e.g., a hard sphere of radius R suspended in a hard-sphere fluid of macromolecules with radius r and bulk number density ρ b in front of a wall (or another big sphere) at distance z, measured between the surfaces of the wall and the spheres. For z < 2r, the macromolecules are expelled from a region of excluded volume overlap, i.e., their density is depleted at that hemisphere of the large particle facing the wall compared to the opposite side which faces the bulk liquid. Such density gradients give rise to an effective osmotic pressure, causing the big sphere to be pushed towards the wall thereby allowing the entropy of the macromolecules to increase. Accordingly, these depletion forces result from an asymmetric density distribution of the macromolecules around the big sphere. Quantitative calculations of depletion forces require the knowledge of the density distribution ρ(r) of the macromolecules. Within the crudest approximation ρ(r) is considered to be constant (corresponding to the above mentioned AO-approximation), so that depletion forces can be easily calculated in terms of excluded volume arguments and are predicted to be entirely attractive for z < 2r. For sufficiently low macromolecule densities this approximation is in agreement with experimental results [4,5]. If, however, at high concentrations, the structural correlation effects in the macromolecular liquid become important, ρ(r) displays an oscillatory behavior at small values of z generating repulsive parts of the depletion interactions as observed recently [6]. By means of virial coefficient expansio...

We report a fabrication method for producing interference-based electrooptic phase gratings that switch between diffracting and transparent states. The phase grating consists of a hexagonal closepacked array of monodisperse emulsion drops of nematic liquid crystal, embedded in a polymer matrix. Monodisperse droplet size allows for fast switching at low electric fields.

Small‐angle X‐ray scattering (SAXS) was applied for the structural analysis of an industrial polymer dispersion in water (synthetic latex). For the preparation of the spherical latex particles under investigation, butadiene, styrene, and acrylic acid were used as monomers in a seeded emulsion polymerization process. The product is widely being used as a film‐forming agent for coatings in the paper‐making industry. It is demonstrated that by measuring the SAXS curves at different contrasts the overall particle size, mass density, polydispersity, and degree of heterogeneity can be estimated with a good accuracy, even without the necessity of a detailed fitting procedure. In particular, one obtains information about the spatial distribution of the various monomer units within the particles. Five isoscattering points could be observed in the contrast variation measurements and the scattering curves at all contrasts could be modeled with a fully consistent set of fit parameters. It is thus safe to conclude that the contrast agent sucrose does not affect the particle structure. A quantitative fitting of the experimental data set revealed that a significant amount of the poly(acrylic acid) is preferentially located in a thin shell of ca. 2 nm thickness around the core of the particles, that is mainly formed by poly(styrene‐co‐butadiene). The obtained results are fully consistent with additional measurements of the particle mass density, of the hydrodynamic particle radius by dynamic light scattering, and of the particle surface charge by potentiometric titration. It is concluded that SAXS is a highly useful tool for characterizing the structure of industrial latexes.SAXS data of a BAYSTAL® latex measured at various contrasts, the weight fraction of sucrose in the dispersion medium is given in the legend.magnified imageSAXS data of a BAYSTAL® latex measured at various contrasts, the weight fraction of sucrose in the dispersion medium is given in the legend.