Colloidal aggregation in polymer blends

We consider here a low-density assembly of colloidal particles immersed in a critical polymer mixture of two chemically incompatible polymers. We assume that, close to the critical point of the free mixture, the colloids prefer to be surrounded by one polymer (critical adsorption). As result, one is assisted to a reversible colloidal aggregation in the nonpreferred phase, due the existence of a long-range attractive Casimir force between particles. This aggregation is a phase transition driving the colloidal system from dilute to dense phases, as the usual gas-liquid transition. We are interested in a quantitative investigation of the phase diagram of the immersed colloids. We suppose that the positions of particles are disordered, and the disorder is quenched and follows a Gaussian distribution. To apprehend the problem, use is made of the standard phi(4) theory, where the field phi represents the composition fluctuation (order parameter), combined with the standard cumulant method. First, we derive the expression of the effective free energy of colloids and show that this is of Flory-Huggins type. Second, we find that the interaction parameter u between colloids is simply a linear combination of the isotherm compressibility and specific heat of the free mixture. Third, with the help of the derived effective free energy, we determine the complete shape of the phase diagram (binodal and spinodal) in the (Psi,u) plane, with Psi as the volume fraction of immersed colloids. The continuous "gas-liquid" transition occurs at some critical point K of coordinates (Psi(c) = 0.5,u(c) = 2). Finally, we emphasize that the present work is a natural extension of that, relative to simple liquid mixtures incorporating colloids.     

Structure and aggregation of colloids immersed in critical solvents

We consider an ensemble of spherical colloidal particles immersed in a near-critical solvent such as a binary liquid mixture close to its critical demixing point. The emerging long-ranged fluctuations of the corresponding order parameter of the solvent drive the divergence of the correlation length. Spatial confinements of these critical fluctuations by colloidal solute particles, acting as cavities in the fluctuating medium, restrict and modify the fluctuation spectrum in a way which depends on their relative configuration. This results in effective, so-called critical Casimir forces (CCFs) acting on the confining surfaces. Using the available knowledge about CCFs we study the structure and stability of such colloidal suspensions by employing an approach in terms of effective, one-component colloidal systems. Applying the approximation of pairwise additive CCFs we calculate the radial distribution function of the colloids, which is experimentally accessible. We analyze colloidal aggregation due to CCFs and thus allude to previous experimental studies which are still under debate.     

Phase behavior of charge stablized colloid dispersion with added water soluble polymers

Demixing and colloidal crystallization in the mixture of charge stabilized colloidal poly(methyl methacrylate) particles and soluble poly(ethylene oxide) were investigated by means of synchrotron small-angle X-ray scattering (SAXS) technique. Phase diagram of the mixture was obtained based on visual inspection and SAXS results. The phase behavior is determined as a function of the concentration of the polymer as well as the volume fraction of the colloidal particles. The system shows a one phase region when the concentration of the polymer is low, whereas a two-phase region is present when the concentration of the polymer is larger than a critical concentration at certain volume fraction of the colloids. Interestingly, a face centered cubic colloidal crystalline structure was formed under certain conditions, which has been rarely observed in experiments of colloid-polymer mixtures with competing interactions.     

Gold nanoparticles decorated with oligo(ethylene glycol) thiols: protein resistance and colloidal stability

The interactions between proteins and gold colloids functionalized with protein-resistant oligo(ethylene glycol) (OEG) thiol, HS(CH2)11(OCH2CH2)6OMe (EG6OMe), in aqueous solution have been studied by small-angle X-ray scattering (SAXS) and UV-vis spectroscopy. The mean size, 2R, and the size distribution of the decorated gold colloids have been characterized by SAXS. The monolayer-protected gold colloids have no correlations due to the low volume fraction in solution and are stable in a wide range of temperatures (5-70 degrees C), pH (1.3-12.4), and ionic strength (0-1.0 M). In contrast, protein (bovine serum albumin) solutions with concentrations in the range of 60-200 mg/mL (4.6-14.5 vol %) show a pronounced correlation peak in SAXS, which results from the repulsive electrostatic interaction between charged proteins. These protein interactions show significant dependence on ionic strength, as would be expected for an electrostatic interaction (Zhang et al. J. Phys. Chem. B 2007, 111, 251). For a mixture of proteins and gold colloids, the protein-protein interaction changes little upon mixing with OEG-decorated gold colloids. In contrast, the colloid-colloid interaction is found to be strongly dependent on the protein concentration and the size of the colloid itself. Adding protein to a colloidal solution results in an attractive depletion interaction between functionalized gold colloids, and above a critical protein concentration, c*, the colloids form aggregates and flocculate. Adding salt to such mixtures enhances the depletion effect and decreases the critical protein concentration. The aggregation is a reversible process (i.e., diluting the solution leads to dissolution of aggregates). The results also indicate that the charge of the OEG self-assembled monolayer at a curved interface has a rather limited effect on the colloidal stabilization and the repulsive interaction with proteins.     

Combining convective/capillary deposition and AFM oxidation lithography for close-packed directed assembly of colloids

We combine convective/capillary deposition and oxidation lithography by atomic force microscopy to direct the close-packed assembly of colloids on SiOx patterns fabricated on silicon substrates previously functionalized with a hydrophobic monolayer of octadecyltrimethoxysilane. The efficiency of this original generic method, which is well adapted to integrate colloids into silicon devices, is demonstrated for 100 nm colloidal latex nanoparticles and Escherichia coli bacteria in aqueous suspensions. A three-step mechanism involving convective flow and capillary forces appears to be responsible for these close-packed assemblies of colloids onto SiOx patterns.     

Design and elaboration of colloidal molecules: an overview

The concept of colloidal molecules was first evoked by van Blaaderen in 2003 for describing small non-spherical colloids made of the aggregation of a small number of particles. He predicted original properties to the complex assemblies of such colloids, in particular in optics. This critical review deals with the different strategies reported for creating robust clusters of spherical particles which could mimic the space-filling models of simple conventional molecules. These routes concern either the controlled clustering of preformed colloids directed by coalescence, physical routes, chemical routes, or 2-D/3-D geometrical confinement, or strategies starting from a single colloid which is decorated by satellite colloids by taking advantage of controlled phase separation or nucleation and growth phenomena. These routes are compared from the viewpoint of the accessible shapes, their tunability and scalability (146 references).     

Phase behavior of colloidal suspensions with critical solvents in terms of effective interactions

We study the phase behavior of colloidal suspensions the solvents of which are considered to be binary liquid mixtures undergoing phase segregation. We focus on the thermodynamic region close to the critical point of the accompanying miscibility gap. There, due to the colloidal particles acting as cavities in the critical medium, the spatial confinements of the critical fluctuations of the corresponding order parameter result in the effective, so-called critical Casimir forces between the colloids. Employing an approach in terms of effective, one-component colloidal systems, we explore the possibility of phase coexistence between two phases of colloidal suspensions, one being rich and the other being poor in colloidal particles. The reliability of this effective approach is discussed.     

Depletion induced isotropic-isotropic phase separation in suspensions of rod-like colloids

When non-adsorbing polymers are added to an isotropic suspension of rod-like colloids, the colloids effectively attract each other via depletion forces. We performed Monte Carlo simulations to study the phase diagram of such rod-polymer mixture. The colloidal rods were modeled as hard spherocylinders; the polymers were described as spheres of the same diameter as the rods. The polymers may overlap with no energy cost, while the overlap of polymers and rods is forbidden. Large amounts of depletant cause phase separation of the mixture. We estimated the phase boundaries of isotropic-isotropic coexistence both in the bulk and in confinement. To determine the phase boundaries we applied the grand canonical ensemble using successive umbrella sampling [J. Chem. Phys. 120, 10925 (2004)], and we performed a finite size scaling analysis to estimate the location of the critical point. The results are compared with predictions of the free volume theory developed by Lekkerkerker and Stroobants [Nuovo Cimento D 16, 949 (1994)]. We also give estimates for the interfacial tension between the coexisting isotropic phases and analyze its power-law behavior on the approach of the critical point.     

金核银壳纳米粒子薄膜的制备及SERS活性研究

采用柠檬酸化学还原法制备金溶胶, 通过自组装技术在石英片表面制备金纳米粒子薄膜, 在银增强剂混合溶液中反应获得金核银壳纳米粒子薄膜. 用紫外-可见吸收光谱仪和原子力显微镜(AFM)研究了不同条件下制备的金核银壳纳米粒子薄膜的光谱特性和表面形貌, 并以结晶紫为探针分子测量了金核银壳纳米粒子薄膜的表面增强拉曼光谱(SERS). 结果表明, 金纳米粒子薄膜的分布、银增强剂反应时间的长短对金核银壳纳米粒子薄膜的形成均有重要影响. 制备过程中, 可以通过控制反应条件获得一定粒径的、具有良好表面增强拉曼散射活性的金核银壳纳米粒子薄膜.     

Quantification of the velocity acceleration factor for colloidal transport in porous media using NMR

Nuclear magnetic resonance (NMR) techniques were used to quantify the transport of colloids through porous media. This was achieved via the application of chemically-resolved pulsed field gradient (PFG) methods, hence probing the displacement (probability distribution) propagators of both the colloidal and continuous liquid phase. A dilute decane-in-water emulsion was used with flow through a random glass sphere packing being considered. The acquired propagators allowed for quantification of both colloidal entrapment and the velocities of both the continuous phase and the flowing colloids. The flowing colloids were found to experience a velocity acceleration factor (VAF) increase of 1.08 relative to the continuous phase. This was found to be independent of displacement observation time or flowrate. It was speculated to be a consequence of radial exclusion due to the finite size of the colloids. Simulations of the colloidal transport were also performed using a lattice Boltzmann platform and a Lagrangian particle-tracking algorithm which incorporated colloidal radial exclusion. Reasonable agreement was observed between the simulation and the experimental data.     

Interaction between colloids with grafted diblock polyampholytes

The interaction between composite colloidal particles composed of a spherical core and grafted AB-diblock polyampholytes (diblock copolymers with oppositely charged blocks) are investigated by using a coarse-grained model solved with Monte Carlo simulations. The B block is end-grafted onto the core of the colloid and its linear charge density is varied, whereas the linear charge density of the A block is fixed. The brush structure of a single colloid, the mean force between two colloids, and the structure of solutions of such colloids have been determined for different linear charge densities of the B blocks and block lengths. Many features of the present system are controlled by the charge of the B blocks. In the limit of uncharged B blocks, (i) the grafted chains are stretched and form an extended polyelectrolyte brush, (ii) a strong repulsive force is operating between two colloids, (iii) and the solution is thermodynamic stable and displays strong spatial correlation among the colloids. In the limit where the charges of the two types of blocks exactly compensate each other, (i) the chains are collapsed and form a polyelectrolyte complex surrounding the cores, (ii) an attractive force appears between two colloids, and (iii) strong colloid clustering appears in the solution. These features become more pronounced as the length of the polymer blocks is increased, and a phase instability occurs at sufficiently long chains. A comparison with properties for other related colloidal particles is also provided.     

Azo polymer colloidal spheres containing different amounts of functional groups and their photoinduced deformation behavior

In this work, colloidal spheres composed of azo polymers with different chromophore loading densities were prepared, and their photoinduced deformation behavior was studied. The colloids were constructed by using a series of amphiphilic epoxy-based random copolymers containing 4-carboxylazobenzene functional groups with different degrees of functionalization (DFs). The colloidal spheres were fabricated through gradual hydrophobic aggregation of the polymeric chains in tetrahydrofuran-H2O dispersion media, which was induced by gradually adding water into the systems. The colloidal spheres were characterized by using transmission electron microscopy and dynamic light scattering. The photoinduced deformation behavior was studied by irradiating the colloidal spheres with a linearly polarized Ar+ laser beam. Results showed that the critical water content (CWC) for the colloid formation is related to the DF of the polymers, and CWC increases with the increase of DF. The hydrodynamic diameter of the colloidal spheres is also related to the DF of the polymers. When the DF of the polymers increases, the average size of the colloids gradually decreases. The hydrodynamic diameter of the colloidal spheres increases as the water dropping rate decreases. When the dropping rate is below 20 microL/s, the size of the colloidal spheres increases abruptly as the dropping rate further decreases. Upon the linearly polarized Ar+ laser beam irradiation, the colloids composed of polymers with different DFs can all be elongated along the polarization direction of the laser beam. As DF increases, the deformation degree characterized by the axial ratio (l/d) almost linearly increases. These observations can give some insight into the photoinduced deformation mechanism and can be used to construct colloids with different sizes and photoresponsive ability.     

ATR-FTIR observations of water structure in colloidal silica: implications for the hydration force mechanism

Attenuated total internal reflection Fourier-transform infrared spectroscopy (ATR-FTIR) was used to probe the change in water structure in silica colloids as a function of particle density. The absorption index (k) spectra were calculated from the ATR spectra using the subtractive Kramers-Kronig transform in order to avoid the effects of the density-dependent refractive index on the raw spectra and allow direct comparison of the different chemical environments. Normalized difference spectra were obtained by subtracting the k spectrum of bulk water from those of the silica colloids. At low particle densities, these difference spectra reveal the presence of a strongly hydrogen-bonded hydration layer at the surface of the colloidal particles. At higher particle densities, the hydrogen-bonding network is increasingly disrupted. The results provide direct experimental evidence of hydrogen-bond breaking as the mechanism for the hydration force, which provides the extraordinary stability of colloidal silica.     

Colloidal wettability probed with X-ray microscopy

Colloids (colloidal particles or nanoparticles) and their in-situ characterizations are important topics in colloid and interface science. In-situ visualization of colloids with X-ray microscopy is a growing frontier. Here, after a brief introduction on the method, we focus on its application for identifying nanoscale wettability of colloidal particles at fluid interfaces, which is a critical factor in colloidal self-assembly. We discuss a quantitative study on colloidal wettability with two microscopic methods: (i) X-ray microscopy by visualizing natural oil–water interfaces and (ii) confocal microscopy by visualizing fluorescently-labeled interfaces. Both methods show consistent estimation results in colloid–fluid interfacial tensions. This comparison strongly suggests a feasibility of X-ray microscopy as a promising in-situ protocol in colloid research, without fluorescent staining. Finally, we address a prospect of X-ray imaging for colloid and interface science.     

Colloidal crystals from surface-tension-assisted self-assembly: a novel matrix for single-molecule experiments

In this work, we develop a new method of creating colloidal crystals with cavities for the entrapment and long-term observation of single biomolecules. Colloidal crystals are first fabricated using surface-tension-assisted self-assembly. Surface tension helps to reduce the interparticle distance between dispensed colloids. Subsequently, the colloids are used as a matrix in which single fluorescently tagged molecules can be tracked using fluorescence microscopy. This method has a high efficiency of self-assembly for small volumes (4 microL) of colloidal suspensions (polystyrene colloids with diameters of 1000, 500, 200, and 100 nm) at low concentration (1% w/w). The spatial hindrance effect on the diffusion of molecules and their entrapment is discussed on the basis of fluorescence correlation spectroscopy results from the diffusion of molecules with different hydrodynamic radii in the cavities of colloidal crystals formed from micrometer- to nanometer-sized polystyrene spheres. Single horseradish peroxidase molecules turning over fluorescent products are tracked over a few seconds. This shows that colloidal crystals can be used to test the function of single molecules of enzymes and protein under controlled spatial confinement.     

Depletion-induced phase separation in colloid-polymer mixtures

Phase separation can be induced in a colloidal dispersion by adding non-adsorbing polymers. Depletion of polymer around the colloidal particles induces an effective attraction, leading to demixing at sufficient polymer concentration. This communication reviews theoretical and experimental work carried out on the polymer-mediated attraction between spherical colloids and the resulting phase separation of the polymer-colloid mixture. Theoretical studies have mainly focused on the limits where polymers are small or large as compared to the colloidal size. Recently, however, theories are being developed that cover a wider colloid-polymer size ratio range. In practical systems, size polydispersity and polyelectrolytes (instead of neutral polymers) and/or charges on the colloidal surfaces play a role in polymer-colloid mixtures. The limited amount of theoretical work performed on this is also discussed. Finally, an overview is given on experimental investigations with respect to phase behavior and results obtained with techniques enabling measurement of the depletion-induced interaction potential, the structure factor, the depletion layer thickness and the interfacial tension between the demixed phases of a colloid-polymer mixture.     

Influence of poly(ethylene oxide) brushes on the rheological properties of MgO colloidal suspensions in water

A combined experimental and multiscale simulation study of the influence of polymer brush modification on interactions of colloidal particles and rheological properties of dense colloidal suspensions has been conducted. Our colloidal suspension is comprised of polydisperse MgO colloidal particles modified with poly(ethylene oxide) (PEO) brushes in water. The shear stress as a function of shear rate was determined experimentally and from multiscale simulations for a suspension of 0.48 volume fraction colloids at room temperature for both bare and PEO-modified MgO colloids. Bare MgO particles exhibited strong shear thinning behavior and a yield stress on the order of several Pascals in both experiments and simulations. In contrast, simulations of PEO-modified colloids revealed no significant yielding or shear thinning and viscosity only a few times larger than solvent viscosity. This behavior is inconsistent with results obtained from experiments where modification of colloids with PEO brushes formed by adsorption of PEO-based comb-branched chains resulted in relatively little change in suspension rheology compared to bare colloids over the range of concentration of comb-branch additives investigated. We attribute this discrepancy in rheological properties between simulation and experiment for PEO-modified colloidal suspensions to heterogeneous adsorption of the comb-branch polymers.     

Shear thickening in a model colloidal suspension

We study the rheology of model colloidal suspensions using molecular-dynamics simulations. We relate the onset of shear thickening to the transition from a low-viscosity regime, in which the solvent facilitates the flow of colloids, to a high-viscosity regime associated with jamming of the colloids and the formation of chains of colloids. In the low-viscosity regime, the colloidal particles are, on average, surrounded by two layers of solvent particles. On the contrary, in the high-viscosity regime, the solvent is expelled from the interstice between the jammed colloids. The thickening in suspensions is shown to obey the same criterion as in simple fluids. This demonstrates that jamming, even without the divergence of lubrication interactions, is sufficient to observe shear thickening.