Isododecane

Isododecane is a solvent whose whole purpose is to leave. It is a small branched alkane, twelve carbons arranged so that the molecules cannot pack closely against one another, and that branching is what makes it volatile enough to evaporate from skin at ordinary temperatures.

The Cosmetic Ingredient Review's assessment of the isoparaffins describes the group as functioning mostly as solvents and also as emollients, and records isododecane at the top of the reported range of use, at concentrations up to ninety per cent.^[1] The upper end of that range is the matte liquid lipstick.

Straight-chain dodecane is an oily, slow-evaporating liquid. Isododecane has the same formula, twelve carbons and twenty-six hydrogens, but the carbons are branched rather than strung in a line, which lowers the boiling point and raises the vapour pressure. The commercial material is an isoparaffin cut rather than a single isomer, and the name 2,2,4,6,6-pentamethylheptane describes its principal component - typically about eighty-five per cent of the mixture - rather than the whole.^[2]

Being a hydrocarbon, it dissolves waxes, oils and hydrocarbon resins that a silicone solvent will not touch, and it fails to dissolve some silicone materials that cyclopentasiloxane handles easily. That complementarity is the reason the two so often appear in the same formula.

Isododecane's place in cosmetics is documented mainly through patents. Revlon's cosmetic compositions with improved transfer resistance, priority 15 December 1992, claim a volatile solvent chosen from volatile silicones, C8 to C20 isoparaffins or mixtures of them, and set out a preferred blend of cyclic silicone with isoparaffin. The patent names isododecane itself, identified by supplier and by the trade name Permethyl 99A, and its first example is a lipstick containing ten per cent of it.^[3]

Procter and Gamble's transfer-resistant compositions, priority 7 November 1995, name the same material - isododecane is the most preferred volatile carrier, again as Permethyl 99A - and add the other half of the architecture, pairing the hydrocarbon with a silicone MQ resin as the film former at a preferred ratio of M to Q units of about 0.7.^[4] That pairing, a hydrocarbon that evaporates and a resin that stays, is the structure of the modern liquid lipstick. No published account dates isododecane's introduction into cosmetics more precisely than the patent record does.

A long-wear colour product has to be fluid while it is being applied and immovable a minute later. Isododecane supplies the first state. It dissolves the film former, disperses the pigment, and reduces the viscosity of the whole enough for a brush or a doe-foot applicator to lay down an even layer.^[5]

Then it goes. What is left is a concentrated deposit of pigment held in a resin film, at a solids content that could never have been spread directly. The characteristic drying-down of a matte liquid lipstick, the short interval in which it turns tacky and then stops moving altogether, is that transition happening.

The Cosmetic Ingredient Review Expert Panel published its safety assessment of the isoparaffins in 2012 and concluded that the group is safe in the present practices of use and the concentrations described in that report, when formulated to be non-irritating.^[6] That is a conclusion about a documented pattern of use at a given date, subject to a formulation condition, not a general clearance of the substance.

Isododecane is a volatile organic compound and a flammable liquid, which constrains how it is packaged, stored and shipped more than how it is formulated. European regulatory attention in this part of the ingredient list has been directed at the cyclic silicones rather than at the volatile hydrocarbons: entry 70 of Annex XVII to the REACH Regulation restricts D4, D5 and D6,^[7] and no comparable restriction applies to isododecane. Reformulation away from the cyclic silicones has widened the use of volatile hydrocarbons in their place.