Final answer:
Temporary hair color does not penetrate the cuticle layer of the hair shaft because it lacks strong chemicals like ammonia and a developer, which are present in permanent hair coloring to open up and develop the color within the hair shaft.
Step-by-step explanation:
What prevents temporary color from penetrating the cuticle layer of the hair shaft is primarily the absence of chemicals that cause the cuticle to open. Temporary hair color typically sits on the outside of the hair shaft because it lacks the necessary components that would allow it to penetrate the cuticle layer. Unlike permanent hair coloring, which requires three components: a primary intermediate (such as 1,4-diaminobenzene or 2,5-diaminotoluene), a coupling agent, and an oxidant under basic conditions, temporary hair color does not contain these strong chemicals. Specifically, it is the absence of ammonia (a common alkaline agent used to open up the cuticle) and the developer (an oxidizing agent) that prevents temporary color from penetrating deeply into the hair shaft.
Permanent hair color involves an oxidation process where the intermediary is oxidized to a quinone state, reacts with a coupler compound, and then undergoes further oxidation to form the final dye. This process produces larger dye molecules that are trapped within the hair, thereby providing a permanent color change. Temporary hair colors, on the other hand, can be washed out and do not involve the use of an oxidant or a high pH to permeate the hair's cuticle.