Free polymer effect, emulsion stability

Stability in mixtures of colloidal particles and polymer molecules, dispersed in a solvent, has been the subject of experimental and theoretical investigations for a long time and it has applications in diverse fields such as paint technology, wastewater treatment, emulsion polymerization, biology etc. It has now been well recognized that polymer molecules can be used to induce either stabilization or flocculation (phase separation) in colloidal dispersions. It is important to distinguish between polymers which are adsorbed on the particle surface and those that are free in solution because the two situations usually lead to qualitatively different effects. Stability imparted by adsorbed polymers is known as steric stabilization and the flocculation or phase separation caused by the free polymer is due... 

Thus, K-casein in its native stabilizing role exists, probably as small disulphide linked polymers, bound to the micellar surface (the ill-defined boundary between the hydrophobic interior of the micelle and the aqueous phase). C-terminal polypeptides (61 residues) of the protein project from the surface into the solution. In this position, the macropeptide moiety of the protein is conformationally free (H), constrained only by its interactions with its neighbours (J ), and the bond 105-106 of the protein is held in a particularly advantageous position for attack by enzymes such as chymosin ( ). The importance of this will be apparent when emulsions stabilized by K-casein are being discussed. The enzymic action has a relatively small but detectable effect on the hydrodynamic diameters of the particles, and a large effect on their electrophoretic mobilities, which decrease by between one-third and one-half, depending on the solution conditions ( ). 

Theoretical work on depletion interactions and their effects on macroscopic properties such as phase stability commenced along various routes. First, Vrij [40] considered the depletion interaction between hard spheres due to dilute non-ad-sorbing polymers such as penetrable hard spheres (see Sect. 1.2.5 and Sect. 2.1). Vrij [40] referred to the work of Vester [82], Li-In-On et al. [55] and preliminary experiments at the Van t Hoff Laboratory on micro-emulsion droplets mixed with free polymer [40] for experimental evidence of depletion effects. 

Emulsion polymerization and suspension polymerization are the preferred industrial processes. Either process is carried out in a closed, stirred reactor, which should be glass-lined and jacketed for heating and cooling. The reactor must be purged of oxygen, and the water and monomer must be free of metallic impurities to prevent an adverse effect on the thermal stability of the polymer. 

It was discovered that the addition of 1,3-cyclohexadiene to the Rh -catalyzed reactions increased the rate of butadiene polymerization by a factor of over 20 [20]. Considering the reducing properties of 1,3-cyclohexadiene, this effect could be due to the reduction of Rh to Rh and stabilization of this low oxidation state by the diene ligands. With neat 1,3-cyclohexadiene, Rh is reduced to the metallic state. These emulsion polymerizations are sensitive to the presence of Lewis basic functional groups. A stoichiometric amount of amine (based on Rh) is sufficient to inhibit polymerization completely. It was also discovered that styrene could be polymerized using the Rh catalyst. However, the atactic nature of the polymer, along with the kinetic behavior of the reaction, indicated that a free-radical process, rather than a coordination-insertion mechanism, was operative. 

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