How does a pencil and a liquid eyeliner differ?
It's helpful to understand the ingredients that go into making eyeliner:
Base - Base ingredients are essentially the backbone of the formula. Pencils are solid formulas and use a blend of waxes, silicones and/or oils (kind of like a fancy crayon!). Liquid liners use a blend of water and polymers to help form a "film" on the skin. These ingredients are usually listed as copolymers on the label
Colorants - Colorants used in eyeliners (and other cosmetics for around the eye) must be approved by the FDA. This just means that colorants you might see in your nail polish aren't necessarily safe for the eye area. Typical colorants include iron oxides, titanium dioxides, mica and ulteamarine pigments.
Control Agents - These are added to make sure the eyeliner meets specifications when it's manufactured and that it maintains a high quality once you get it home. These include ingredients that control pH (the acid/base balance of the product), keep the product free of bacteria and mold and make sure the product applies nicely. In some oil-based formulas, an antioxidant may be added to keep the waxes and oils from going rancid. Typical control agents include citric acid, methylparaben, propylparaben, xanthan gum (used as a thickener in liquid liner) and tocopherol .