Steric stabilization attractive interaction

Influence of Addition of Electrolyte and Increase of Temperature Addition of electrolyte or increase of temperature at a given electrolyte concentration to a sterically stabilized dispersion may result in its flocculation at a critical concentration or temperature, which in many cases coincides with the theta point for the stabilizing chain. At the theta point the mixing term in the steric interaction is zero and any yield value measured should correspond to the residual van der Waals attraction. The energy arising from van der Waals attraction may be calculated from the following approximate relationship,... 

In many practical instances (see Vignette 1.5), electrostatic repulsion is not a convenient option. In such cases, a suitable polymer that adsorbs on the particle surfaces may be added to the dispersion. The resulting polymer layer masks the attraction and may also provide a repulsive force, partly due to pure steric effect, when the polymer layers on two interacting particles attempt to overlap with each other. This is what is known as polymer-induced stability. Polymer-induced stability is often referred to as steric stability for the above... 

FIGURE 7.7 Schematic Baxter (—), Van der Waals attraction (—), and steric stabilization (.) potential curves describing the interaction between two DMDBTDM A aggregates. Rc is the radius of the polar core, Rhs is the hard-sphere radius, and (8-Rhs) represents the distance of the effective attractive interaction. (From L. Martinet, Organisation Supramoleculaire des Phases Qrganiques de Malonamides du Precede d Extraction DIAMEX. PhD thesis. Rapport CEA-R-6105, 2005. With permission.)... 

The electrostatic stabilization theory was developed for dilute colloidal systems and involves attractive van dcr Waals interactions and repulsive double layer interactions between two particles. They may lead to a potential barrier, an overall repulsion and/or to a minimum similar to that generated by steric stabilization. Johnson and Morrison [1] suggest that the stability in non-aqueous dispersions when the stabilizers are surfactant molecules, which arc relatively small, is due to scmi-stcric stabilization, hence to a smaller ran dcr Waals attraction between two particles caused by the adsorbed shell of surfactant molecules. The fact that such systems are quite stable suggests, however, that some repulsion is also prescni. In fact, it was demonstrated on the basis of electrophoretic measurements that a surface charge originates on solid particles suspended in aprotic liquids even in the absence of traces of... 

The phase behavior of mixtures of sterically stabilized dispersions and free polymer molecules is determined by the interparticle potential. In such systems, the interplay between the repulsive steric forces and the attractive forces due to the presence of free polymer molecules and van der Waals interaction between the particles determines the onset of instability. 

The proposed transition state is illustrated in Fig. 3. The two tartrate ester units prefer to occupy the position axial to the dioxaborolidine unit. The stabilized dipole-dipole interaction between the carbonyl carbon ((5+ a-bromoacrolein) and the proximate ester carbonyl oxygen, together with the attractive interaction of the jr-basic benzyl ring and the jr-acidic dienophile in the s-cis conformation, locks the dienophile in the s-cis conformation. Approach of the diene from the less sterically hindered side (opposite the aryl ring) gives the cycloadducts in good enantioselectivity. 

Fig. 4 The total potential energy of interaction Vt as a function of distance of surface separation H for two similar oh droplets in an oil-in-water emulsion. (A) Electrostatic stabilization by a monolayer of ionic surfactant. (B) Steric stabilization by a monolayer of non-ionic surfactant. V van der Waals attractive force Vr electrostatic repulsive force Vs steric repulsive force.

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]. 

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