Interfacial Properties of Suspended Particles

Introduction:

Interfacial properties play a critical role in the behavior and stability of suspended particles within pharmaceutical formulations. Despite their significance, the thermodynamic conditions at solid surfaces are still not fully understood. This article delves into the interfacial phenomena governing particle stability, flocculation, and aggregation within suspensions, providing insights into the factors influencing their behavior.

Thermodynamic Stability of Suspended Particles:

The process of reducing solid particles into smaller sizes and dispersing them within a liquid medium requires energy. However, this process results in a large surface area, leading to a thermodynamically unstable system characterized by high surface free energy. This instability drives particles to regroup, either forming fluffy conglomerates known as floccules or stronger aggregates under specific conditions.

Equilibrium and Interfacial Tension:

The reduction of surface free energy is crucial for achieving stability within the suspension. Equilibrium is reached when the change in surface free energy (ΔG) approaches zero. This equilibrium can be attained by either reducing interfacial tension or decreasing the interfacial area. The addition of surfactants can reduce interfacial tension, but it typically remains finite, leading to particle flocculation due to residual attractive forces.

Forces at the Particle Surface:

Interactions at the particle surface involve both attractive forces, such as London–van der Waals forces, and repulsive forces arising from the electric double layers surrounding each particle. The balance between these forces determines the degree of flocculation and agglomeration within the suspension, influencing its stability and behavior.

DLVO Theory and Particle Stability:

The Derjaguin and Landau, Verwey and Overbeek (DLVO) theory provides a framework for understanding the stability of colloidal dispersions, including pharmaceutical suspensions. This theory considers both attractive van der Waals forces and repulsive electrostatic forces to predict particle behavior. Computer programs, such as those developed by Schneider et al., facilitate calculations of repulsion and attraction energies in pharmaceutical suspensions, aiding in understanding and optimizing suspension stability.

Implications of Particle Flocculation and Aggregation:

Flocculated particles exhibit weak bonding, settling rapidly, but are easily re-suspended. In contrast, deflocculated particles settle slowly, forming sediments that eventually aggregate into a hard cake, challenging re-suspension. Understanding these behaviors is crucial for formulating stable suspensions with desirable properties and performance characteristics.

Conclusion:

Interfacial properties play a pivotal role in determining the stability and behavior of suspended particles within pharmaceutical suspensions. By elucidating the thermodynamic principles and forces governing particle interactions, researchers and formulators can optimize suspension formulations, ensuring stability, efficacy, and patient safety. Continued advancements in understanding interfacial phenomena will contribute to the development of innovative suspension-based drug delivery systems, enhancing therapeutic outcomes across various medical applications.