Stability, colloid . . . 107 , layer

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. 

Polymer molecules are often employed to stabilize colloids [1]. In most theoretical treatments of the effect of polymer adsorption [2-5], only the steric force is taken into account, and the steric force and the traditional double-layer force for particles devoid of hairs are assumed to be additive. The steric force is a short-range interaction which acts only when the chains on the surfaces of the two particles interpenetrate [6-8]. However, in addition to this short-range interaction, a hairy surface can also generate another effect, because it can change the dielectric constant in the vicinity of the surface. 

The asymptotic approach has not yet been extended to interactions between stretched layers in theta and poor solvents, which control the incipient flocculation of polymerically-stabilized colloidal dispersions. Application of the mean-field theory to poor solvents produces an attractive minimum only for —nll2v/w112 > new112/l, when layers begin to interpenetrate rather than... 

The interaction potentials described in previous sections for adsorbing homopolymer and terminally anchored layers in good solvents clearly indicate the ability of polymers to stabilize colloidal dispersions against flocculation due to van der Waals dispersion forces. Indeed, the practice preceeded the analyses by centuries in some cases and decades in others, since the use of adsorbing polymers dates to ancient times, and block copolymer stabilizers emerged from industrial laboratories in the 1960s (Napper, 1983). 

Energy can also be stored in other ways on a microscopic scale, e.g., by electrical charges being forced near each other in colloidal systems and by emulsion drops being distorted from the spherical shape. In this case, the surface tension gives them stabilizing surfactant layers on dispersed particles being pressed into each other. 

Steric stabilization is another well-established method of stabilizing colloidal suspensions of submicron to micron size [23]. The particles are coated with a layer of adsorbed or grafted polymer chains that provides a steric repulsion of entropie origin and helps disperse the particles by counterbalancing van der Waals attraction (Fig. la). The polymeric nature of the adsorbed or grafted layer softens the interparticle interactions and makes the particles intrinsically deformable. Many polymer chain/particle combinations have been synthesized and studied, and are described in the literature. Several popular colloidal systems consist of silica particles covered with various polymers such as polydimethylsiloxane [24], stearyl alcohol [25], alkyl chains [26], and polyethylene oxide [27]. Polymethylmethacrylate and polystyrene particles grafted with polymer chains have also been used extensively. For a review on the impressive literature on the subject we refer the interested reader to Vlassopoulos and Fytas [2]. 

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. 

It should be noted that the polishing mechanism is not just simply abrasive polishing but includes a chemical reaction component also. Alkali stabilized colloidal silica is used and it can be shown that the polishing rate increases with the pH of the sol. This is thought to indicate that the alkali reacts with the silicon to form an alkaline silicate at the surface and that one function of the colloidal silica is to remove the silicate layer so that fresh surface can be exposed. 

In Sects. 1.2.1 and 1.2.2 we shall first qualitatively consider double layer and Van der Waals interactions, the two contributions to the DLVO potential (Sect. 1.2.3), and then discuss (polymeric) steric stabilization by end-attached polymer in Sect. 1.2.4. While not further discussed here we mention that adsorbing polymers, proteins or particles can also be used to protect colloids against flocculation. For protein adsorption, often used for instance in food emulsions, we refer to [28]. Using particles to stabilize colloids is referred to as Ramsden-Pickering stabilization [29]. Finally, the depletion interaction will be treated in Sect. 1.2.5. 

In the above descriptions we concentrated on situations where a polar background solvent was implicitly assumed. In apolar solvents double layer repulsion is diflhcult to achieve because dissociation, leading to charged surface groups, is less likely to occur and it becomes essential to stabilize colloids with polymers as to prevent instabilities. In the first decades after the establishment of the DLVO theory most papers on forces between colloidal particles focused on Van der Waals and double layer interactions. Forces of other origin such as polymeric steric stabilization [17], depletion [40] or effects of a critical solvent mixture [41] gained interest at a later stage. 

These core-shell nanoparticles are extremely soluble and stable (up to several month) in water (or phosphate buffered saline solutions, PBS) in which they maintain an outstanding monodispersity. The strength of this strategy is mainly being a one-pot method, in which very cheap and basically non-toxic compruients are used even if the synthesis pertains to functionalized nanoparticles. Moreover, the PEG shell boosts the performances of the colloidal system looking at in vivo and in vitro bio-analytical applications. The PEG shell provides a stabilizing stealth layer [97] and as a matter of fact in simulated physiological or bio-analytical protocols work-up conditions (PBS lx, bovine serum albumin up to 10 wt%) these colloidal systems retain their stability and mono-dispersion. 

Steric stabilization of a colloidal dispersion is achieved by attaching long-chain molecules to colloidal particles (Fig. 3.6). Then when colloidal particles approach one another (for example due to Brownian motion), the limited interpenetration of the polymer chains leads to an effective repnlsion which stabilizes the dispersion against flocculation. Steric stabilization has several advantages compared to charge stabilization. First, the interparticle repulsion does not depend on electrolyte concentration, in contrast to charge-stabilized colloids where the electric double-layer thickness is very sensitive to ionic strength. Second, steric stabilization is effective in both... 

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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