Petroleum jelly is a common household product often used as a skin protectant. Many people question its fundamental composition, but the answer is simple: it is not water-based. Petroleum jelly is an entirely oil-based product, which defines its function and effectiveness. This composition is key to understanding how it interacts with moisture and provides protective benefits to the skin.
The Hydrocarbon Composition
Petroleum jelly, also known as petrolatum, is fundamentally a mixture of mineral oils and waxes. These components are derived from refining crude oil, which establishes its oil-based nature. Chemically, the substance consists of long chains of molecules called hydrocarbons, compounds made solely of hydrogen and carbon atoms. The blend of liquid mineral oils and solid microcrystalline waxes gives petroleum jelly its characteristic semi-solid, waxy texture.
Physical Interaction with Water
The hydrocarbon structure of petroleum jelly dictates its behavior when it encounters water. The molecules are non-polar, meaning they lack the electrical charge imbalance found in polar molecules like water. Due to this non-polar nature, the substance is completely insoluble in water. This behavior follows the chemical principle that “like dissolves like,” meaning non-polar substances do not mix with polar ones.
When applied to the skin, petroleum jelly immediately forms a continuous, hydrophobic barrier. This layer physically repels external water, preventing it from penetrating the skin. The inability of water to dissolve or pass through the jelly makes it effective at creating a water-protective seal. This immiscibility allows it to remain intact on the skin, even when exposed to moisture.
How Petroleum Jelly Moisturizes Skin
The oil-based nature of petroleum jelly is directly responsible for its moisturizing action, which operates through a process called occlusion. Occlusion involves creating a physical seal over the skin’s surface. The resulting film is highly effective at minimizing Transepidermal Water Loss (TEWL), the natural process of water evaporating from the skin into the air.
By forming this barrier, petroleum jelly traps the existing moisture already present in the skin, preventing its loss. It does not add water to the skin like a humectant would, but rather locks in the skin’s natural hydration. This mechanism makes it one of the most powerful occlusive agents available, capable of reducing TEWL by up to 98%. The retention of this internal water helps hydrate the outermost layer of the skin, the stratum corneum, which improves skin softness and texture.