Colloids, steric stabilization

FIG. 4 Sterically stabilized colloidal particles are coated with short polymer brushes. A hard sphere-like interaction arises. 

The hard sphere (HS) interaction is an excellent approximation for sterically stabilized colloids. However, there are other interactions present in colloidal systems that may replace or extend the pure HS interaction. As an example let us consider soft spheres given by an inverse power law (0 = The energy scale Vq and the length scale cr can be com-... 

From a technical standpoint, it is also important to note that colloids display a wide range of rheological behavior. Charged dispersions (even at very low volume fractions) and sterically stabilized colloids show elastic behavior like solids. When the interparticle interactions are not important, they behave like ordinary liquids (i.e., they flow easily when subjected to even small shear forces) this is known as viscous behavior. Very often, the behavior falls somewhere between these two extremes the dispersion is then said to be viscoelastic. Therefore, it becomes important to understand how the interaction forces and fluid mechanics of the dispersions affect the flow behavior of dispersions. 

I.V. Rao and E. Ruckenstein Phase behaviour of mixtures of sterically stabilized colloidal dispersions and free polymer, JOURNAL OF COLLOID AND INTERFACE SCIENCE 108 (1985) 389-402. 

Phase Behavior of Mixtures of Sterically Stabilized Colloidal Dispersions and Free Polymer... 

Recent experimental studies (1-3), on systems of sterically stabilized colloidal particles that are dispersed in polymer solutions, have highlighted the role played by the free polymer molecules. These experiments are particularly relevant because the systems chosen are model dispersions in which the particles can be well approximated as monodisperse hard spheres. This simplifies the interpretation of the data and leads to a better understanding of the intcrparticle forces. DeHek and Vrij (1, 2) have added polystyrene molecules to sterically stabilized silica particles dispersed in cyclohexane and observed the separation of the mixtures into two phases—a silica-rich phase and a polystyrene-rich phase—when the concentration of the free polymer exceeds a certain limiting value. These experimental results indicate that the limiting polymer concentration decreases with increasing molecular weight of... 

Here a mixture of sterically stabilized colloidal particles, solvent, and free polymer molecules in solution is considered. When two particles approach one another during a Brownian collision, the interaction potential between the two depends not only on the distance of separation between them, but also on various parameters, such as the thickness and the segment density distribution of the adsorbed layer, the concentration and the molecular weight of the free polymer. The various types of forces that are expected lo contribute to the interaction potential are (i) forces due to the presence of the adsorbed polymer, (ii) forces due to the presence of the free polymer, and (iii) van der Waals forces. It is assumed here that there are no electrostatic forces. A brief account of the nature of these forces as... 

The presence of free polymer in sterically stabilized colloidal dispersions is shown to be responsible for the occurrence of phase sepa-ration/flocculation in dilute dispersions. The theoretical calculations show that the limiting polymer concentration for the onset of phase separation decreases with increasing molecular weight of the free polymer and with increasing particle concentration, as observed experimentally. The two limiting cases of no penetration and total penetration of the free polymer into the adsorbed layers represent extreme ends with the real situation being somewhat intermediary between the extreme cases. 

In our earlier work 113], the stability of mixtures of sterically stabilized colloidal particles and free polymer molecules in solution was examined theoretically by considering the total intcrparticle potential as the sum of contributions due to (i) the presence of the adsorbed polymer, (ii) the presence of free polymer in solution, and (iii) van der Waals attraction. 

Flomogeneous solution Growing polymer particle Sterically stabilized colloid... 

A. A. Rigos and G. Wilemski,/. Phys. Chem., 96,3981 (1992). Brownian Dynamics Simulations of An Order-Disorder Transition in Sheared Sterically Stabilized Colloidal Suspension. 

A variety of organic colloids including emulsions and polymer latexes have been dispersed in carbon dioxide in the presence of surfactants (3,13). In most cases, owing to the lower interfacial tension of the former as explained shortly it is easier to form organic-in-C02 emulsions than water-in-C02, emulsions. Sterically stabilized colloids are stable above the critical flocculation density (CFD) and precipitate below this density. In some cases the CFD occurs at the upper critical solution density of the steric stabilizer, that is, the density at which the stabilizer phase separates from CO2, as has been shown by theory (14,15) and experiment (16). So-called ambidextrous surfactants have been designed to allow polymer latexes produced in CO2 to be transferred to an aqueous solution to form a dispersion (17,18). 

The boehmite system (y-AlOOH), originally studied by Zocher and Torok [63] and Bugosh [64] was further developed by Lekkerkerker and coworkers [65]. They extended the hydrothermal preparation pioneered by Bugosh [64] by starting from an aqueous aluminum alkoxide mixture acidified with hydrochloric acid [65a]. They studied the phase behavior of both charge stabilized aqueous dispersions of colloidal boehmite rods [65b,c] as well as sterically stabilized colloidal boehmite rods in an organic solvent (cyclohexane) [65d-f]. 

It is convenient from a conceptual viewpoint to delineate three domains of close approach for sterically stabilized colloidal particles (Evans and Napper, 1973a). These zones delimit domains in which different phenomenology is clearly recognizable. They are determined by the relative sizes of the spans of the attached polymer chains (assumed for simplicity to be monodisperse) and the separation between the particles. The span can be defined in this context as specifying the conformational average distance normal to the surface that the furthermost segment meanders from the surface. The span thus sets the absolute limit to the average conformational thickness of the steric layers. 

Parenteral administration (injection), which is the immediate option for orally undeliverable drugs, has advanced greatly in recent years for systemic and local drug delivery. " The novel drug delivery system has metamorphosed from simple polymer and antibody conjugates to sterically stabilized colloidal systems. Liposomes and nanoparticles can improve pharmacokinetic-pharmacody-... 

Another (better) option is to polymerize the OM directly on a latex (in fact, S. Jasne from Polaroid had proposed this route in the mid 1980s [55], finally without practical success, and it was again recently proposed by the Intch Company DSM [56]), or in a sterically stabilized colloidal form [57]. Both concepts are based on the idea that an OM polymer blend should be a dispersed system, but that unquestionable idea could get around the very complicate dispersion task by starting with colloidal particles. This is not basically wrong however, it was not taken into account that... 

Sterically Stabilized Colloidal Boehmite Rods + Polymer... 

Recently, DHBCs have been used as a good stabilizer for the in-situ formation of various metal nanocolloids and semiconductor nanocrystals such as Pd, Pt [328-330], Au [280,328-330], Ag [331], CdS [332], and lanthanum hydroxide [333]. PAA-fe-PAM and PAA-fc-PHEA were used as stabihzer for the formation of hairy needle-Uke colloidal lanthanum hydroxide through the complexation of lanthanum ions in water and subsequent micelhzation and reaction [333]. The polyacrylate blocks induced the formation of starshaped micelles stabilized by the PAM or PHEA blocks. The size of the sterically stabilized colloids was controlled by simply adjusting the polymer-to-metal ratio, a very easy and versatile synthesis strategy for stable colloids in aqueous environment [333]. The concept of induced micelhzation of anionic DHBCs by cations was also apphed in a systematic study of the direct synthesis of highly stable metal hydrous oxide colloids of AP+, La +, Ni +, Zn ", Ca ", or Cu " via hydrolysis and inorganic polycondensation in the micelle core [334,335]. The AP+ colloids were characterized in detail by TEM [336], and the intermediate species in the hydrolysis process by SANS, DLS, and cryo-TEM [337]. 

Wood mannans have also been shown to sterically stabilize colloidal wood resin droplets. Applied studies at our laboratory showed that it was mainly the molar mass of the mannan that governed the stabilization of wood resin - the larger the polymer, the better was its stabilizing properties (59). 

Repulsive interactions are important when molecules are close to each other. They result from the overlap of electrons when atoms approach one another. As molecules move very close to each other the potential energy rises steeply, due partly to repulsive interactions between electrons, but also due to forces with a quantum mechanical origin in the Pauli exclusion principle. Repulsive interactions effectively correspond to steric or excluded volume interactions. Because a molecule cannot come into contact with other molecules, it effectively excludes volume to these other molecules. The simplest model for an excluded volume interaction is the hard sphere model. The hard sphere model has direct application to one class of soft materials, namely sterically stabilized colloidal dispersions. These are described in Section 3.6. It is also used as a reference system for modelling the behaviour of simple fluids. The hard sphere potential, V(r), has a particularly simple form ... 

In principle, if sterically stabilized colloid particles collide and the adsorbed layers do not interpenetrate, the stability of the colloidal dispersion will be increased by an elastic effect. This arises because the compression of one layer of polymers by another will restrict the number of conformations available to each polymer chain. This decreases the entropy and so increases... 

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