Flocculation behavior stability

Figure 8.8. Schematic illustrating the analogy between colloid flocculation behavior and phase behavior of the stabilizer in bulk solution. As density is lowered, separation of solvent from chains in bulk solution resembles separation of solvent from chains on surfaces, which produces flocculation.

The behavior of neutral brushes in good solvent conditions has been generalized to any solvent quality [218,219]. In 0 solvent, the excluded volume, which is the second virial coefficient, is equal to zero. As a consequence, the thickness L results from the balance between the three-body interaction forces (positive and proportional to c3) and the chain elasticity. In a poor solvent, the chains shrink, and the elasticity term is irrelevant. The determination of L thus results from the equilibrium between the two-body and three-body interaction forces. The expression of L as a function of N, a, d, and the second and third virial coefficients is obtained by the authors, who demonstrate that the brush configuration is preserved in both the 0 and the poor solvent with a linear variation of L with N. The brush thickness continuously decreases as the solvent quality decreases. Experimentally, this can be suitably achieved by varying the temperature. The effect of changing solvent quality on the flocculation behavior of emulsions stabilized by copolymers was discussed by March and Napper [154] (Sec. III.B). Even if the random copolymers used in this study are not expected to form brushes, it was clear that flocculation was observed in 0 solvent conditions while it was suppressed in good solvent conditions. 

Figure 6. Schematic illustrating the analogy between colloid flocculation behavior and behavior of the stabilizer in bulk solution.

Polyelectrolytes have been studied extensively because molecular structures can be tailored to allow large conformational changes with pH, temperature, or added electrolytes. Molecular parameters that influence behavior include number, type, and distribution of charged repeat imits on the chain, hydropho-bic/hydrophilic balance, distance of charged moiety from the backbone, and counterion type. Solution properties including phase behavior, hydrodynamic volume, and binding can be altered, offering utilization in flocculation, adhesion, stabilization, compatibilization, viscosification, suspension, etc. 

Stability implies a resistance to change, and may be defined qualitatively in those terms. In the specific case at hand, stability is defined as resistance to molecular or chemical disturbance. This requirement recognizes that a flocculated dispersion may be more stable than a peptized dispersion from the standpoint of its future behavior. A physically stable dispersion is one which will not undergo molecular replacements at the interface between the dispersed solid and the continuous phase. 

For suspensions primarily stabilized by a polymeric material, it is important to carefully consider the optimal pH value of the product since certain polymer properties, especially the rheological behavior, can strongly depend on the pH of the system. For example, the viscosity of hydrophilic colloids, such as xanthan gums and colloidal microcrystalline cellulose, is known to be somewhat pH- dependent. Most disperse systems are stable over a pH range of 4-10 but may flocculate under extreme pH conditions. Therefore, each dispersion should be examined for pH stability over an adequate storage period. Any... 

Understanding how polymer functions as a stabilizer and flocculant is obviously a problem of polymer adsorption and its conformation at the particle/liquid interface(l, 2). The process of polymer adsorption is fairly complicated the behavior depends on many factors, e.g., the nature of the adsorbent, the molecular weight of the polymer, the temperature, the effect of the solvent,... 

A very opaque version has recently been introduced to the market in the USA. This type, which is also yellower than traditional varieties, demonstrates not only excellent rheological behavior but also high flocculation stability and therefore high gloss. It is slightly more weatherfast than more transparent types. The main application is in lead-free automotive finishes for full shades. 

The first observation of depletion flocculation by surfactant micelles was reported by Aronson [3]. Bibette et al. [4] have studied the behavior of silicone-in-water emulsions stabilized by sodium dodecyl sulfate (SDS). They have exploited the attractive depletion interaction to size fractionate a crude polydisperse emulsion [5]. Because the surfactant volume fraction necessary to induce flocculation is always lower than 5%, the micelle osmotic pressure can be taken to be the ideal-gas value ... 

The object of this study was to clarify some aspects of the mechanism of shear-induced flocculation in colloidal dispersions. Vinyl chloride homopolymer and copolymer latices were prepared by emulsion polymerization using sodium dodecyl sulphate as emulsifier. Agglomeration behavior in these latices was studied by measuring the mechanical stability using a high speed stirring test. The latex particle size was measured by an analytical centrifuge. Molecular areas of emulsifier in the saturated adsorption layer at the surface of homopolymer and copolymer latex particles were estimated from adsorption titration data. 

A Brookfield viscometer with a helipath attachment (Stoughton, MA) is a useful rheological instrument for measuring the settling behavior and structure of pharmaceutical suspensions and for characterizing the properties and stability of flocculated suspensions. The viscometer should be properly calibrated to measure the apparent viscosity of the suspension at equilibrium at a given temperature to establish suspension reproducibility. Apparent viscosity, like pH, is an exponential term, and therefore the log-apparent viscosity is an appropriate way of reporting the results. 

---