Colloidal Dispersions Overview

Colloidal Dispersions

• Definition: Sol is a general term for solid substance dispersed in a liquid, solid, or gas. Prefixes (e.g., hydro- for water, alco- for alcohol) specify the dispersion medium. Aerosols are solid/liquid dispersed in gas.

• Colloidal Particles: Particles sized between 1 nm and 0.5 µm are considered colloidal. They are larger than atoms, molecules, or ions.

• Lyophilic vs. Lyophobic: Lyophilic colloids (solvent-loving) have strong attractions with the dispersion medium, are more stable, and easier to prepare. Lyophobic colloids (solvent-hating) have weak attractions, require special preparation methods, and are less stable.

• Association/Amphiphilic Colloids: Formed by molecules with both lyophilic and lyophobic properties.

• Gels: Lyophilic colloids at high concentrations form gels, which are semisolid or solid dispersions. Their structure arises from a network of the disperse phase that entraps the dispersion medium. They can revert to sols (liquids) with temperature changes or agitation (thixotropy).

Types of Gels

• Classification Schemes: Gels can be classified as inorganic or organic, or as hydrogels (water-based) or organogels (non-aqueous).

• Hydrogels: Contain hydrophilic colloids (e.g., silica, bentonite, tragacanth, sodium alginate, methylcellulose, CMC), and can be natural or synthetic.

• Organogels: Include hydrocarbons, animal/vegetable fats, or soap base greases.

• Inorganic Gels & Magmas: Some form from precipitation or direct hydration of inorganic chemicals. Examples include aluminum hydroxide gel and bentonite magma.

Preparation and Properties of Magmas and Gels

• Preparation Methods: Some are prepared by precipitation, others by hydration of the chemical.

• Additives: Propylene glycol and hydroxypropyl cellulose are thickening agents commonly used.

• Stability: Magmas and gels are generally uniform on standing due to strong attraction between phases, though a supernatant layer can develop over time. Shaking is necessary for uniform dose.

• Usage: These systems are commonly administered orally due to the value of the dispersed phase, but also topically for certain products.

Gelling Agents & Their Properties

• Gelling Agents: Substances used to thicken and stabilize liquid solutions, creating an internal structure for a gel-like appearance.

• Examples: Alginic acid, carbomer resins, colloidal silicon dioxide, gelatin, magnesium aluminum silicate (Veegum), methylcellulose, Bentonite, Acacia, Xanthan gum, Tragacanth gum, Povidone, and Carboxymethylcellulose (CMC).

Gel Formulation Considerations

• Dissolution: Powdered polymers can form temporary gels with slow dissolution in water.

• Thermal Effects: Change in temperature affects various physical properties, including the ability of polymers to undergo reactions (e.g., cooling of a colloidal gelatin dispersion results in the gelatin molecules associating due to dipole-dipole interactions).

• Inorganic Salts: Salts can act as competitors for water in gel systems, affecting gelation at lower concentrations (often reversible).

• Hydration: Aqueous polymer solutions need time (48 hrs) for full hydration and optimal viscosity/clarity.

Specific Examples of Magmas and Gels

• Bentonite Magma: A suspension of bentonite (colloidal hydrated aluminum silicate) used as a suspending agent. Important to consider its alkaline pH.

• Aluminum Hydroxide Gel: A gelatinous precipitate of insoluble aluminum hydroxide and aluminum oxide. It neutralizes acid, coats the stomach, and is used to treat hyperacidity and peptic ulcers. A disadvantage is its constipating effects. Has interaction with tetracycline and other drugs.

• Milk of Magnesia: Suspension of magnesium hydroxide, neutralizing stomach acid but can cause diarrhea. Often combined with aluminum hydroxide to balance this effect.

• Starch Glycerite: A topical vehicle/protectant.

Administration Considerations

• Oral: Measured by spoon/dropper/devices.

• Reconstituted oral suspensions require shaking prior to use and storage considerations.

• Important to be aware of potential drug-drug interactions.