Colloidal systems, destabilization electrolyte

The potential in the diffuse layer decreases exponentially with the distance to zero (from the Stem plane). The potential changes are affected by the characteristics of the diffuse layer and particularly by the type and number of ions in the bulk solution. In many systems, the electrical double layer originates from the adsorption of potential-determining ions such as surface-active ions. The addition of an inert electrolyte decreases the thickness of the electrical double layer (i.e., compressing the double layer) and thus the potential decays to zero in a short distance. As the surface potential remains constant upon addition of an inert electrolyte, the zeta potential decreases. When two similarly charged particles approach each other, the two particles are repelled due to their electrostatic interactions. The increase in the electrolyte concentration in a bulk solution helps to lower this repulsive interaction. This principle is widely used to destabilize many colloidal systems. 

In true solutions at a concentration c = 102 mol m 3 (n = 6xl025 particles per m 3) the osmotic pressure reaches 6xl025 m 3 xl.38xl0 23 JxK 1 x 298 K = 2.4 x 105 Pa (2.4 atm). For lyophobic colloidal systems the number of particles per m3 does not as a rule exceed 1021, and hence the osmotic pressure is not greater than a fraction a millimeter of water column. Besides the experimental difficulties in measuring values that are so small, the presence of electrolytes introduces a considerable source of errors. The complete removal of electrolytes may lead to the destabilization of a colloidal system (see Chapter VIII). The presence of electrolytes leads to high osmotic pressures and in the presence of membranes results in the establishment of membrane (Donnan) equilibrium. 

In hydrophobic colloidal systems, the water molecules have a higher affinity for one another than for the colloidal particles. Consequently, the particles would stick together on each encounter, nnless they repel each other. It was discovered by H. Schulze in 1883 that addition of electrolyte destabilizes hydrophobic colloidal dispersions, and W.D. Hardy in 1900 showed that the destabilization is accompanied by a reduction in the electrophoretic mobility of the particles. From this, it was inferred that colloidal stability is maintained by electrostatic repulsion between charged particles. 

Not affected by presence of electrolytes Equally good in aqueous and non-aqueous dispersions Equally good in dilute and dense colloid systems Often results in reversible flocculation Good stability with temperature changes Electrolytes result in destabilization Effective especially for aqueous colloidal dispersions Best for dilute colloidal dispersions Often irreversible coagulation is obtained Coagulation often upon freezing... 

In conclusion, the way polymers influence the stability of lyophobic colloids is far more complicated than the way low molecular weight electrolytes do. Whether polymers stabilize or destabilize, the dispersion is delicately determined by properties and composition of the system (adsorption affinity, solvent quality, particle size, degree of polymerization, charge densities on the particle and the polymer, particle-polymer ratio, ionic strength, presence of divalent ions, and so on) and external conditions, such as the temperature. 

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