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Hot and Cold
By: Steve Herman
Posted: April 27, 2012, from the May 2012 issue of GCI Magazine.
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When the temperature is above 43°C, the TRPV1 receptor is activated. It activates a surface pore that causes calcium ions to flow into the cell. This causes an electrical change in the cell, which sends a message to the brain that it is hot outside. Capsaicin also binds to this receptor and thus sends a signal to the brain that says “hot.” When the temperature is in the range ~8–28°C, the TRPM8 receptor tells the brain “cold.” Menthol also binds to this receptor and sends the same cold message.
Menthol has more biological activities than just its cooling sensation though. It has analgesic properties due to its activation of some κ-opioid receptors, and this can make the skin less susceptible to the feeling of a sting or rash. Menthol also is an antipruritic, which means it can help treat an itchy scalp.
Put simply, menthol can interact with several receptor areas in the body, and scientists have decoded many of the detailed mechanisms to explain exactly what is happening in these interactions. The effects of menthol are real and well understood.
For beauty and personal care applications, menthol typically has a strong initial effect on the skin and then fades. However, manipulating the molecule to have a milder initial impact but to continue its release over time has long been a desired variant, and a typical approach to this is converting menthol to an ester, which breaks down over time. Menthyl lactate is a common example, and Symrise has commercialized a number of these esters. A recent example is Frescolat X-cool (INCI: Menthyl Ethylamido Oxalate), which the company claims is 82% stronger than its other cooling agent, menthyl lactate. It also is said to be gentle to the skin and has no unpleasant odor.
Another aspect of menthol that can be seen as an interesting trick is its use in the creation of an eutectic mixture—turning two solids into a liquid at room temperature under precise conditions. Eutectic mixtures are familiar to those working with metal alloys, but are much less familiar to those in the beauty industry.
One hundred percent of each separate component is represented on the right and left sides. As blends are created, the melting point drops, meeting at a minimum somewhere in the middle. If this temperature is below room temperature, the result is a liquid. At a molecular level, a eutectic mixture will form when the two components are close enough in structure to dissolve in each other but different enough to disrupt the crystalline structure, lowering the melting point.
One such eutectic menthol mixture has been patented by Sino Lion,2 and consists of a precise ratio of menthol and menthyl lactate. The benefit of this product includes easier handling and the ability to encapsulate the liquid for controlled release in products such as toothpaste.
More Than Menthol
Many other ingredients that provide a sensory impact are also known. For example, Wilkinson Sword did extensive work in the 1970s resulting in two carboxamides with cooling capabilities being marketed, WS-3 and WS-23.