What substance is responsible for allowing water and oil to mix together?

An emulsifier is a substance that allows water and oil to mix together by reducing the surface tension between the two liquids. This ability to blend immiscible substances is crucial in the formulation of various beauty and skincare products, where both water-based and oil-based ingredients are often combined. Emulsifiers contain molecules that have both hydrophilic (water-attracting) and lipophilic (oil-attracting) parts. When added to a mixture, these molecules position themselves at the interface of oil and water, stabilizing the mixture and preventing the two phases from separating.

In contrast, surfactants, while they can lower the surface tension and assist with mixing, primarily function as agents that help cleanse and emulsify dirt and oils rather than specifically stabilizing a blend of oil and water for long-term use. Solvents are typically used to dissolve substances but do not create stable mixtures of oil and water. Preservatives serve to inhibit microbial growth and prolong the shelf life of products but do not contribute to the blending of oil and water. Therefore, emulsifiers are the key components in creating stable emulsions, making them the correct answer.