Pharmaceutics 1: Colloidal Stability Overview

Introduction to Pharmaceutics 1: Colloidal Stability

• Pharmaceutics 1 lecture on colloidal stability focuses on suspensions.
• The lecture aims to explain physical instability in disperse systems.
• It describes the electrical double layer surrounding particles in aqueous electrolyte solutions and the forces governing physical instability.
• The lecture covers how electrolyte concentrations affect electrical double layers, repulsive forces, and instability in disperse systems.
• It also discusses the influence of electrolyte valency on flocculation in disperse systems.

Disperse Systems and Colloids

• Colloids are disperse systems where one phase consists of tiny particles or droplets.
• Liquid-in-liquid colloids are called emulsions.
• Solid-in-liquid colloids are called suspensions.

Physical Instability in Suspensions

• Physical instability in suspensions is observed as sedimentation (particles settling due to density differences) and caking (high-density sediment forming).
• Redispersing a solid-liquid colloid often requires shaking.

Flocculation

• Flocculation involves particles/droplets clustering together with maintained individual identity (unlike coagulation).
• Flocculation can be redispersed by shaking.
• Flocculation is desirable in formulations to prevent caking.

Coagulation

• Coagulation involves strong attractive forces between particles and droplets.
• Coagulated particles form small aggregates that cannot be redispersed by shaking, resulting in permanent medication failure (caking).

Forces Acting on Particles in Disperse Systems

• Particles less than 2 microns in diameter move due to Brownian motion, kinetic energy influenced by temperature and external forces.
• As particles approach, they experience repulsive electrostatic interactions (VR) and attractive van der Waals forces (VA).
• Steric forces (Vs) also arise if non-ionic surfactants are present.
• The total potential energy (VT) is determined by the combined interactions: VT = VA + VR + Vs

DVLO Theory

• DVLO theory describes the balance between electrostatic repulsion and attractive forces in charged particles.
• In the case of charged particles, steric forces (Vs) are insignificant, simplifying the equation to VT = VA + VR

Forces and Particle Separation

• The attractive and repulsive forces change with the distance between particles (H).
• Potential diagrams visualize force (V) plotted against particle separation (H).
• The attractive force (VA) inversely depends on the particle separation distance(1/H).

Repulsive Forces

• Colloidal particles are often charged.
• Charge arises from ion dissolution (e.g., drug uneven dissolution).
• Ion adsorption (e.g., surfactants) can also cause surface charge.
• Electrostatic repulsion (VR) occurs between particles with similar charges.
• VR is related to particle separation and charge density as defined by; VR = 2πεαψ2(exp(-кH)).
• VR decreases rapidly with increasing particle separation distance.
• VR also decreases with increasing Debye-Huckel Constant (к)

Debye-Huckel Length and Electrolyte Concentration

• The Debye-Huckel length (1/к) signifies the extent of the electrical double layer.
• High electrolyte concentrations reduce the Debye-Huckel length, increasing the Debye-Huckel Constant, and thus decreasing VR.
• Low electrolyte concentrations increase the Debye-Huckel length, decreasing the Debye-Huckel Constant, and thus increasing VR
• Intermediate electrolyte concentrations result in both attractive and repulsive forces, leading to a stable suspension

Total Potential Energy

• The total potential energy (VT) combines attractive and repulsive forces.
• Different electrolyte concentrations lead to diverse VT profiles and thus differing colloidal stability.
• High electrolyte concentration creates a situation where the primary maximum is absent, resulting in an unstable colloidal suspension.
• All kinetic energy levels will lead to coagulation.
• Intermediate electrolyte concentration results in both attractive forces and a secondary minimum, fostering colloidal stability.

Effect of Multivalent Ions

• DLVO theory simplifies the interactions that determine colloidal stability.
• The Schulze-Hardy rule highlights the significant flocculating power of multivalent ions (proportionally related to their charge to the power of 6).
• The addition of divalent or trivalent ions can negatively impact stability.

Summary

• Attractive and repulsive forces influence colloidal stability and physical instability.
• Electrolyte concentration changes the thickness of the electrical double layer and ultimately impacts colloidal stability.
• The addition of multivalent ions must be avoided for many disperse systems in order to maintain colloidal stability.

This quiz covers key concepts from the Pharmaceutics 1 lecture on colloidal stability, focusing on the physical instability of disperse systems such as suspensions. It explores the role of the electrical double layer, electrolyte concentrations, and their impacts on stability. Prepare to test your understanding of colloids and the mechanisms behind their behavior in various solutions.