If you’re in the US, no doubt you’ve seen the commercials. Coors Light’s parodies of NFL coaches during post-game interviews was brilliant, but I’m talking about the one’s for their cold-activated bottles:
So how does it work? The ability of something to change color with temperature is known as thermochromism, and the Coors Light bottles are printed with a thermochromic ink called a leuco dye. A leuco dye is a coloring agent which can acquire two different forms: a colorless form and a colored form. At warm temperatures, the thermochromic ink is colorless, and at cold temperatures, the thermochromic ink is (in this case) blue. Put your beer in the fridge, when the ink cools below the color changing temperature, “the Rockies turn blue,” and your beer is ready to drink.
So how do thermochromic inks work? Well, in general one aspect of a molecule that makes it colored at all is an extended conjugated pi-system. A pi-bond absorbs light. A single pi-bond absorbs light in the UV spectrum. As the pi-system extends further and further, the wavelength of light absorbed becomes longer and longer, until extended pi-systems begin to absorb light in the visible spectrum – and the molecule becomes colored (see figure). A molecule will typically absorb one wavelength of light better than others, and this wavelength of maximum absorption is designated λmax (read “lambda max”). Lycopene is red because its λmax is in the blue region. It absorbs light in the blue region and reflects light in the red region and thus appears red. If one were to disrupt an extended conjugated system in the middle of the pi-system, the extended pi-system would be severely shortened, and the molecule would (potentially) become colorless. So most widely used thermochromic inks contain an extended conjugated pi-system that is easily interrupted.
The prototypical thermochromic ink is crystal violet lactone. When the pH is high, the lactone interrupts the conjugation that would otherwise extend through all three aromatic rings. When the pH is low, the lactone becomes protonated, and the lactone opens to the carboxylic acid, leaving a tertiary, benzylic carbocation behind. This carbocation allows the conjugation to extend throughout all three aromatic rings. With the conjugation extended, λmax increases into the visible region, and the leuco dye appears, well, violet colored.
For inks, the equilibrium is controlled in a very clever way. We can’t constantly be dousing our beer bottles with acid or base depending on which color we want. For thermochromic inks, the manufacturers take the lecuo dye, some weak acid, and a high molecular weight solvent and encapsulate the components into a particle usually <50 μm in diameter. The leuco dye chosen will depend on what color is desired. Weak acids typically chosen include bisphenol A (yes, that BPA), octyl or methyl p-hydroxybenzoate, 1,2,3-triazoles, or 4-hydroxycoumarin derivatives. The solvent is typically an alcohol (laurel or cetyl alcohol), an ester (butyl stearate), a ketone, or an ether. The melting point of the solvent is important. The melting point of this solvent is the temperature at which color change will take place.
The paradox: It might seem at first glance that when the solvent is liquid, increased mixing should take place, and with more acid available in solution, the carbocation should be favored at elevated temperatures. But if that were true, the thermochromic inks should be colored at high temperatures. But almost invariably, all thermochromic inks are colored at cool temperatures and colorless at elevated temperatures. It seems that in the solid state, the leuco dye and weak acid are in contact with each other and color change takes place. In the liquid state, the two components disperse and the colorless form predominates.
Fun Facts: Note it’s not actually temperature that’s changing the form of the leuco dye. The temperature changes the equilibrium point of the acid/base reaction which changes the form of the leuco dye. So the dyes are not, technically, thermochromic… rather, they’re halochromic. The color actually changes with pH. But the temperature controls the acid/base equilibrium, which controls the color, so these inks are generally referred to as thermochromic inks. So when the color change occurs at the temperature of my refrigerator, I can use thermochromic inks to tell when my beer is cold (actually, it only tells me when my beer bottle is cold… not the actual liquid inside the bottle…) Phenolphthalein is another example of halochromicity.
On reading a few patents, it appears Coors Light utilizes thermochromic inks prepared by ChromaZone. The actual structure of the blue dye appears to be either proprietary or a trade secret or I can’t find it by browsing Google Patents or ChromaZone’s webpage. Maybe it’s just crystal violet lactone! ChromaZone’s website has a lot of neat information on thermochromic inks. Another popular manufacturer of thermochromic inks is Matsui. Their thermochromic ink page has several headers with a lot more neat information to read.
Other Uses: • Other common uses of thermochromic inks are temperature probes for microwaveable foods. Some maple syrup bottles have a black thermochromic ink which reveals the word HOT written in red when the syrup is warm enough to eat. The ink doesn’t actually change from black to red, though. The thermochromic ink changes from black to colorless… to reveal the regular ink printed underneath which has the word HOT printed in red ink.
• The fad of on-the-battery testers utilizes thermochromic and conductive inks. A three layer system is in use here. The conductive ink is printed in a strip that gradually expands in width. On top of that is printed (in regular rink) whatever design the company desires to show the battery is good. On top of that is printed the thermochromic ink. When the battery is tested, the resistance in the conductive ink causes the ink to warm. A small current will heat the narrow parts of the testing strip, and more current is needed to heat the widest parts of the testing strip. If there is enough current to heat the ink past the color-change temperature of the thermochromic ink, the ink will turn colorless and reveal the “good” indicator. As the battery drains, less of the testing strip will turn colorless and the battery will show that it is less “good.”
• A two-toned effect can be created by mixing a colored thermochromic ink with a colored regular ink. Mixing a blue thermochromic ink with a yellow regular ink will result in a layer that appears green at low temperatures and yellow at elevated temperatures.
• As we said, leuco dyes are really halochromic inks dressed up as thermochromic inks. The true thermochromic material is a thermochromic liquid crystals, with the most famous thermochromic liquid crystal being the old school mood ring. Thermochromic liquid crystals are much more sensitive to temperature change than leuco dyes, but are more expensive to manufacture. Liquid crystals appear in silly mood rings, practical LCD monitors, and more serious forehead thermometers where exact temperature is important. In a liquid crystal, the molecules are oriented in a particular direction, but that orientation direction varies periodically with depth into the crystal. The distance between repeating orientations is the pitch, and the pitch varies with temperature. The value of the pitch is typically on the order of visible light, thus as the pitch changes, the colors reflected change, and the color of the mood ring changes. See this paper for more information.
• Leaving thermochromicity for a second, transitions lenses for prescription eyeglasses also exhibit a similar effect – colorless indoors, colored (tinted) outdoors. This is photochromicity, not thermochromicity, but the concept is the same. When hit with ultraviolet light, the photochromic compound undergoes a chemical change which turns the molecule from colorless to black reversibly. This interaction with UV light, but not visible light, is important so the lenses aren’t tinted indoors. The problem with many transitions lenses is they don’t work well if there is UV tinted glass between the wearer and the sun… such as when driving a car. Many windshields block UV light, thus the UV light can’t interact with the photochromic molecules, and the wearer could experience no sunglasses effect when driving. A product called Drivewear claims to combat this problem with a combination of UV-sensitive and visible light-sensitive molecules.
So now you know! Next time you’re enjoying a Coors Light (if that’s your beer of choice), you can tell all your friends about thermochromic inks. They’ll either think you’re real cool and buy your next beer… or they’ll think you’re real nerdy and buy your next beer because you need to be more drunk. Either way, make sure you get a free beer out of it!
**Most of my information came from this awesome J. Chem. Ed. paper by Mary Anne White and Monique LeBlanc (insert joke about color chemistry paper written by White & The White here).