why do you recommend blending peppermint with an oil with a higher boiling/flash point?
Because many people think the flash point temperature is related to how fast the fragrance evaporates. In other words, a scent with a lower flash point supposedly will evaporate a lot faster.
The
vapor pressure is the property that does predict a chemical's tendency to evaporate quickly or slowly. High vapor pressure = fast evaporation.
Unfortunately, vapor pressure info (and boiling point info) for fragrances is often hard to find. Flash point temperatures are easy to find, however, because sellers have to know the F.P. temps to ship these chemicals safely.
Awhile back, I compared the flash point temperatures and vapor pressures of 55 fragrance oils to see for myself whether there was a useful relationship between F.P. and vapor pressure or not. The fragrance oils had a wide range in the flash point temperatures (from about 130 F to over 200 F), but there were only small differences in their vapor pressures. In other words, a fragrance that has a high flash point temperature doesn't evaporate any much faster or slower than one with a low F.P.
Another tidbit that I thought was interesting is
all of these fragrances have vapor pressures that are at least 20 times lower than water and 35 times lower than ethyl alcohol (ethanol, grain alcohol). I (and I'm sure others) have the perception that fragrances evaporate really fast ... but they really don't.