The Importance of Ointment Bases
An ointment base is the vehicle into which medicinal substances are incorporated for topical application to the skin or mucous membranes. Beyond simply holding the active ingredient, the base itself plays a critical role in the therapeutic outcome. The properties of the base—including its greasiness, ability to absorb water, and washability—determine how the medication is released, how much moisture is retained on the skin, and the overall feel and patient acceptance of the product. Selecting the right base is therefore a crucial step in formulating an effective and appropriate topical medication.
1. Hydrocarbon (Oleaginous) Bases
Hydrocarbon bases are the classic, greasy, water-repellent ointments. They are composed entirely of hydrocarbons, derived primarily from petroleum.
Composition and Properties
- Composition: These bases are anhydrous, meaning they contain no water. They are made from ingredients like petrolatum (petroleum jelly), mineral oil, and various waxes.
- Water Content: Anhydrous (water-free).
- Washability: Extremely difficult to wash off with water due to their hydrophobic (water-repellent) nature.
- Occlusiveness: Highly occlusive, meaning they form a continuous, protective film on the skin that prevents moisture evaporation. This makes them excellent for moisturizing very dry or chapped skin.
- Drug Release: Tend to release drugs slowly because the oily base does not readily mix with or dissolve many active ingredients.
Common Uses
- Protecting and moisturizing dry, chapped, or irritated skin, such as with petroleum jelly.
- Protecting wounds and burns.
- Providing a long-lasting emollient effect for conditions like eczema.
2. Absorption Bases
Absorption bases are anhydrous but differ from hydrocarbon bases in one key aspect: they can absorb water to form a water-in-oil (W/O) emulsion. There are two types of absorption bases.
Composition and Properties
- Type 1 (Anhydrous Absorption Bases): Consist of a hydrocarbon base with a w/o emulsifying agent.
- Examples: Hydrophilic petrolatum, anhydrous lanolin.
- Type 2 (Water-in-Oil Emulsions): These are already formed emulsions.
- Examples: Hydrous lanolin, cold cream.
- Water Content: Initially anhydrous but can absorb aqueous solutions.
- Washability: Difficult to wash off with water, similar to hydrocarbon bases.
- Occlusiveness: Highly occlusive, retaining moisture on the skin.
- Drug Incorporation: Allows for the incorporation of small amounts of water-soluble drugs into the formulation.
Common Uses
- Emollients for hydrating the skin.
- Vehicles for incorporating aqueous solutions of drugs.
- Protective barriers for sensitive or irritated skin.
3. Water-Removable Bases (Emulsion Bases)
Also known as creams or oil-in-water (O/W) emulsions, these bases are readily removable from the skin with water. This is because their external phase is aqueous.
Composition and Properties
- Composition: Composed of oil droplets dispersed in a continuous water phase, stabilized by an emulsifying agent.
- Water Content: Hydrous (contain water).
- Washability: Easily washed from the skin with water.
- Occlusiveness: Less occlusive than hydrocarbon or absorption bases, allowing for a more breathable application.
- Feel: Lighter and less greasy than oil-based ointments.
Common Uses
- Cosmetic applications, such as vanishing creams.
- Vehicle for drugs intended for superficial skin conditions.
- When patient preference dictates a less greasy, easy-to-remove product.
4. Water-Soluble Bases
Water-soluble bases are greaseless and contain no oleaginous components. They are fully washable with water and are primarily composed of polyethylene glycols (PEGs).
Composition and Properties
- Composition: Contain only water-soluble ingredients, often a mixture of liquid and solid PEGs. The viscosity depends on the molecular weight of the PEG mixture.
- Water Content: Can be anhydrous or contain water.
- Washability: Completely soluble and washable with water.
- Occlusiveness: Non-occlusive and offer little emollient effect.
- Drug Incorporation: Suitable for incorporating solid materials and aqueous solutions.
Common Uses
- As a greaseless vehicle for water-soluble drugs.
- For topical applications where an emollient or occlusive effect is not desired.
- In certain wound care preparations.
Comparing the 4 Ointment Bases
| Feature | Hydrocarbon (Oleaginous) Bases | Absorption Bases | Water-Removable (Emulsion) Bases | Water-Soluble Bases |
|---|---|---|---|---|
| Composition | Anhydrous, greasy hydrocarbons | Anhydrous or W/O emulsion, contains emulsifying agent | O/W emulsion (cream), contains emulsifying agent | Anhydrous or hydrous, water-soluble polymers (PEGs) |
| Water Content | Anhydrous | Can incorporate water | Contains water | Can contain water or be anhydrous |
| Washability | Poor, not washable with water | Poor, not washable with water | Good, washable with water | Excellent, fully water-soluble |
| Occlusiveness | High (forms protective barrier) | High (forms protective barrier) | Low | None |
| Feel | Greasy | Greasy | Non-greasy (cream-like) | Greaseless |
| Drug Release | Poor | Poor (but better than oleaginous) | Fair to good | Good |
| Uses | Protectant, emollient, for dry skin | Emollient, incorporates aqueous solutions | Emollient, cleansing cream, general vehicle | Vehicle for water-soluble drugs |
| Examples | White Petrolatum, White Ointment | Hydrophilic Petrolatum, Lanolin, Aquaphor® | Hydrophilic Ointment, Vanishing Cream | Polyethylene Glycol (PEG) Ointment |
Selecting the Appropriate Ointment Base
Choosing the right ointment base is a critical step in pharmaceutical compounding and depends on several factors. For instance, a physician prescribing a topical medication for severely dry, cracked skin would likely choose a highly occlusive hydrocarbon base, such as petrolatum, to lock in moisture effectively. In contrast, for a condition on the scalp or a larger body area where easy removal is important, a patient-friendly water-removable or water-soluble base might be preferred.
Other considerations include the characteristics of the active drug itself. A drug that is highly sensitive to moisture would be incompatible with a hydrous base and would require an anhydrous base, like a hydrocarbon or absorption base. The intended depth of penetration and the desired drug release rate also influence the decision. Ointments maximize potency by enhancing drug retention on the skin. Ultimately, understanding the unique properties of each of the four bases allows for a more effective and targeted therapeutic outcome.
For more detailed compounding techniques, educational resources from university pharmacy programs can provide valuable insights, such as those found on The University of North Carolina at Chapel Hill's Pharmlabs website.
Conclusion
Ointment bases are far more than simple carriers; they are integral to the function and patient experience of topical medications. The four classifications—oleaginous, absorption, water-removable, and water-soluble—each offer a distinct set of characteristics tailored for specific dermatological conditions and drug properties. The high occlusiveness of oleaginous and absorption bases is ideal for severe dryness, while the washability of water-removable and water-soluble bases improves patient compliance and is suitable for different skin types. Knowledge of these differences enables pharmacists to select the most appropriate base for compounding, ensuring optimal stability, efficacy, and application of the final product.
