Question

Explain how each of the following factors helps determine the stability or instability of a colloidal dispersion: (a) particulate mass, (b) hydrophobic character, (c) charges on colloidal particles.

Short answer

The stability of a colloidal dispersion is influenced by particulate mass, hydrophobic character, and charges on colloidal particles. Larger particles with greater mass sediment faster, leading to instability, whereas smaller particles remain dispersed and provide stability. Hydrophobic character promotes particle aggregation and coalescence, destabilizing the dispersion, while surfactants can counteract this effect. Charged particles contribute to electrostatic repulsion, resulting in enhanced stability by counterbalancing attractive van der Waals forces and preventing aggregation or coalescence.

Step-by-step solution

Factor 1: Particulate mass

The particulate mass plays a crucial role in the stability of a colloidal dispersion. Larger particles tend to settle out due to gravity, causing the dispersion to be unstable. In contrast, smaller particles have lower settling rates and can remain dispersed for longer durations, leading to the stability of the colloidal dispersion. The sedimentation rate of particles can be determined using Stokes' Law: \[ v = \frac{2r^2(g - 1)g}{9\eta} \] where v is the sedimentation rate, r is the particle radius, g is the gravitational acceleration, η is the fluid viscosity, and (g-1)g represents the difference in the density of the particle and the fluid. From Stokes' Law, it can be deduced that as the particulate mass (and hence radius) increases, the sedimentation rate also increases, leading to a greater potential for the dispersion to become unstable.

Factor 2: Hydrophobic character

The hydrophobic character of the colloidal particles can also affect the stability of the dispersion. A higher hydrophobic character means that the particles will be more attracted to each other due to aggregation and coalescence, leading to the instability of the colloidal dispersion. To counteract this issue, surfactants are commonly used to provide stability to the dispersions. These surfactants adsorb onto the surfaces of the colloidal particles and reduce the interfacial tension between the particles and the surrounding fluid. This creates a repulsive force between the particles, preventing aggregation or coalescence and thus improving the stability of the colloidal dispersion.

Factor 3: Charges on colloidal particles

The charges present on the surfaces of colloidal particles can influence their stability. Charged particles create an electrostatic repulsion between them, which can significantly enhance the stability of the colloidal dispersion. Two main factors contribute to the charges on colloidal particles: 1) Ionization: Ionizable functional groups present on the surface of the particles can lose or gain ions, leading to charged particles. 2) Adsorption of charged species: Colloidal particles can also acquire charge by the adsorption of charged species from the surrounding fluid, such as ions or ionic surfactants. The repulsive electrostatic forces between charged particles counterbalance the attractive van der Waals forces, imparting stability to the colloidal dispersion. In particular, if the repulsion is sufficiently large, it can lead to the formation of an energy barrier that prevents particles from coming close enough to aggregate or coalesce, thus maintaining the stability of the colloidal dispersion.