Back in July, I got my first gel manicure in yearsssss and I stumbled upon an iridescent light purple and decided this would be the color to grace my nails for the next 2 weeks. The manicurist informed me that this polish was actually color changing gel polished and I knew it was this was truly the one. The purple polish turned really dark when cold and a light lavender when warm – much like a mood ring, this nail polish reacts to body temperature. This occurrence is due to a chemical property called thermochromism.
My undergrad research was focused on a thermochromic polymer that would change color in response to light, heat, chemical environment and potentially mechanical force. Changing the conditions of the polymer’s environment could affect how the color would appear to the visible eye. My portion of the research project was to find the ideal conditions in which this polymer would attach to a metal salt, in a solvent system and change colors when exposed to UV light. So when I found a successful system the polymer would form one or two bonds with the metal salt ion in the solvent system. My hallmark research moment was discovering a system that gave a 4:1 bond, which had never been seen before!
If this is starting to sound a bit complicated, let’s put it in terms of nail polish – which I know we can all get behind. To get started, nail polish is full of polymers. The repeating chemical units allow the pigment to spread over the nail and form a hard layer. Scientists pretty much have dibs on these thermochromic polymers so cosmetic companies use leuco dyes to create color changing nail polishes.
Leuco dye is a pigment compound that can switch between a colorless and colored form due to a change in light, pH or heat. Leuco dyes switch between their two chemical forms, one of which allows a free bond to open up and accept an H+ ion. When the system is cold (dark purple) a free H+ will attach itself to a weak acid present in the system, when warm (light purple) the free H+ will attach to the dye causing the pigment to go colorless and the color dissolved in the solvent will remain.
When the environment surrounding my nails changed temperatures, the leuco dye reacts with the other present compounds in the polish, causing a chemical change in the leuco dye structure and thus a color change on my fingertips.
Check out my Instagram (@cosmetic_composition) for a video of the color changing happening on my nails!