Colloidal dispersions osmotic pressure, effect

Another unique phenomenon involving colloidal dispersions stabilized by low molecular weight, weakly adsorbed polymer chains is the depletion flocculation mechanism [41], as shown in Figure 2.12. When an isolated pair of the particles approach each other, the weakly adsorbed polymer chains are squeezed out of the overlap volume due to the greatly reduced space available for these polymer chains. This then results in the imbalance of the local osmotic pressure that is, the concentration of the adsorbed polymer is lower than that in the continuous bulk phase. Thus, water molecules are forced to diffuse out of the overlap region to counterbalance the osmotic pressure effect. The net effect is that the particles are pulled together and flocculation takes place. 

The theory of Asakura a Asakura and Oosawa (1954) were the first to recognize that depletion effects could give rise to the flocculation of colloidal dispersions. They considered the specific example of two parallel flat plates immersed in solutions of rigid molecules, either spherical or rod shaped. When the distance between the plates is smaller than the diameter of the solute molecules, assumed for the purposes at hand to be spherical, none of these molecules can enter the domain between the plates. This region is then composed entirely of solvent. The solution outside the plates retains its bulk concentration of solute and so it exerts an inward force, arising from its osmotic pressure, on the plates. 

Electrically stabihzed colloidal dispersions are very sensitive to the addition of electrolytes. If the concentration of ions in the solution increases, decreases as a result of both entropic and electrical screening effects, leading to a reduction in the repulsive potential. On the other hand, colloid particles dispersed in organic media (low dielectric constant) cannot be effectively stabilized by charges because is extremely short. In these cases, steric stabilization is recommended. Steric stabihzation is imparted by nonionic amphiphilic molecules (usually polymeric molecules). The lyophobic moiety will adsorb onto the surface of the colloidal particles, while its lyophilic moiety will be extended in the continuous phase. When two sterically stabihzed particles approach each other, the concentration of the lyophilic segments in the portion of the continuous phase between the particles is increased. This higher local concentration results in an osmotic pressure that... 

Properties of Colloidal Systems.—Colloidal solutions resemble true solutions in that they exert an osmotic pressure and have a lower freezing point than that of the solvent. Neither effect is as marked as in the true solution owing to the relatively greater size and smaller concentration of the dispersed particles. The osmotic pressure of the plasma proteins, however, is an important factor in maintaining the volume of blood, and in man has a normal value of 305-307 mm. HjO (0-03 atmospheres) at 22° C. In addition to these general properties, colloidal sj tems, especially suspensoids, exhibit special properties due to the surface and surface charge of the dispersed particles. These properties are —... 

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