Dispersed Systems: Concepts and Applications

Dispersed Systems: Concepts and Applications

A dispersed system is a system composed of multiple phases dispersed in a dispersion medium. The dispersed particles (DPs) have sizes smaller than 1 micrometer, creating a large specific surface area that influences the system’s properties.

# 1. Basic Concepts

• Microshere: A spherical particle with a homogeneous structure, without a membrane.

• Microcapsule: A particle with a membrane, distinct from a microshere.

• Degree of Dispersion (D): A quantity that characterizes the fineness of a dispersed system. It is calculated as the inverse of the average size of DPs:

• D = 1/d = 1/2r (where d is the diameter, r is the radius of the DP)

• Specific Surface Area (S): The surface area of contact between the dispersed phase and the dispersion medium per unit volume or mass of the dispersed phase.

• S = (Total surface area of particles) / (Volume of dispersed phase) = 3/r

• The specific surface area is inversely proportional to the size of the dispersed particles (d) and directly proportional to the degree of dispersion (D).

# 2. Solid Dispersed Systems

A solid dispersed system is a system consisting of one or more solid drugs dispersed in one or more carriers. The solid phase can exist in two states:

• Amorphous: A structure with short-range order, where atoms are arranged irregularly but tightly packed.

• Crystalline: A structure with long-range order.

Solid solutions are a special type of solid dispersed system where the drug is dispersed at the molecular level in the carrier. The size of DPs is at the molecular or atomic level.

• Distinguishing solid solutions from eutectic mixtures:

• Eutectic mixture: Two components A and B crystallize simultaneously, but component B does not exist in the crystalline structure of A and vice versa. The two crystal lattices are only adjacent to each other.

• Solid solution: Component A is present in the crystalline lattice of B.

Conditions for forming solid solutions:

• Similar physical properties.

• Isomorphism (same crystal structure).

• Similar ratios of structures between atoms/ions.

• Similarity in chemical formula, with both substances having the same bonding characteristics in the molecule.

# 3. Applications of Solid Dispersed Systems

Drugs formulated as solid solutions or solid dispersed systems exhibit greater dissolution than the raw material form.

• Increased drug solubility and dissolution rate:

• Increases the rate and extent of drug absorption.

• Increases the concentration of the drug at the target site.

• Enhances therapeutic efficacy.

Why do drugs in the form of solid solutions or solid dispersed systems have high solubility?

• The drug is in the form of very small particles, resulting in a large total surface area.

• There is no aggregation of particles.

• Increased wettability of the drug particles by the carrier.

• The carrier can create a diffusion layer around the particle surface, which aids in dissolution.

• No aggregation of particles.

Fick’s Law: Describes the rate of dissolution:

• dC/dT = (D.S(Cs-Ct))/n.s

• D: Diffusion coefficient.

• S: Total surface area of particles.

• Cs: Saturation concentration.

• Ct: Concentration at time t.

• n: Viscosity of the solution.

• s: Diffusion thickness.

• The rate of dissolution is directly proportional to:

• Diffusion coefficient.

• Total surface area of particles.

• Concentration gradient.

• The rate of dissolution is inversely proportional to:

• Viscosity and diffusion thickness.

# 4. Other types of dispersed systems

• Solid in gas (S/G): Example: Smoke.

• Liquid in solid (L/S): Example: Fog.

• Gas in liquid (G/L): Example: Foam.

• Gas in solid (G/S): Example: Solid foam.

• Solid in liquid (S/L): Example: Suspension.

• Liquid in solid (L/S): Example: Solid emulsion.

• Solid in solid (S/S): Example: Solid suspension.

• Liquid in liquid (L/L): Example: Emulsion.

Dispersed systems are an important concept in many fields, including pharmaceuticals, chemistry, materials science, and engineering. Understanding dispersed systems helps us to tailor the properties of materials and products, leading to more efficient and safer products.