Hey Growmies,
I've had a big breakthrough tonight, actually a couple.
First, I have finally figured out terpene evaporation temperatures, which are very difficult to find online. If you try to search for this, what comes up are
boiling points. And the boiling point information is sometimes not accurate. Evaporation temperatures are important to the process of drying and curing. In addition, terpene evaporation temperatures are interesting with respect to the research I've been doing on the role of terpenes in bud rot and leaf mold resistance.
The key to understanding (and finding info on) terpene evaporation temperatures is
vapor pressure. I'm still in the process of fully understanding this; however, vapor pressure is a measurement of the
volatility, which is the "tendency of a substance to evaporate at normal temperatures."
The below chart is from an research paper I came across, titled
Vapor Pressure, Vaping, and Corrections to Misconceptions Related to Medical Cannabis’ Active Pharmaceutical Ingredients’ Physical Properties and Compositions, Aharon M. Eyal, et al. 2022 ...
Here we can see that the monoterpenes are by far the most volatile, and start evaporating at 68°F (20 C). At 120°F (50 C) and above, they are really taking off. The primary monoterpenes in cannabis are pinene, myrcene, limonene, terpinolene, terpinene, ocimene, and terpineol (a monoterpenoid). The sesquiterpenes are much less volatile, and start evaporating at 212°F (100 C). The primary sesquiterpenes are: beta-caryophyllene, alpha-humulene, and bisabolol (a sesquiterpenoid).
In terms of bud rot resistance, and potentially leaf mold resistance as well, clearly the monoterpenes are the only ones to consider. The higher the daytime temperatures, above 68°F, the more they will off-gas. A more complete list of vapor pressures for the monoterpenes is shown in the below, Table 3.
Below is Table 3 from Aharon M. Eyal, et al., showing both boiling points and vapor pressure at sea level (1 atmosphere or 760 mm Hg) of various terpenes...
Now we can see which of the
monoterpenes is the most volatile at 68°F (20 C), looking at the vapor pressure from the above chart (Torr -or- mm Hg), with caryophyllene and humulene listed at the end for comparison...
alpha-Pinene........ 3.570
beta-Pinene......... 2.180
beta-Myrcene........ 1.690
Limonene............ 1.130
Terpinolene......... 0.799
beta-Caryophyllene.. 0.021
alpha-Humulene...... 0.010
Next, I'll fill out this monoterpene volatility chart by adding the other primary monoterpenes, and increasing the temperature to 77°F (these number are from other sources, not the above research paper)...
alpha-Pinene........ 3.489
beta-Pinene......... 2.399
beta-Myrcene........ 2.090
alpha-Ocimene....... 1.970
alpha-Terpinene..... 1.665
beta-Ocimene........ 1.559
Limonene............ 1.550
Terpinolene......... 1.126
From this we can see pinene and myrcene are the most prominent off-gassing terpenes at a normal daytime temperature of 77°F. In my research on pine and non-pine terpenes, I found correlation with sativa/indica distinction, and also with bud rot and leaf mold resistance. Now we can ask the questions, which are more volatile, pine or non-pine terpenes? Since myrcene and limonene are the only two prominent non-pine terpenes*, it's obvious that pine terpenes are the most volatile. (*Linalool clocks in a just 0.170, so it's not very volatile. bisabolol is a sesquiterpene, so barely registers any volatility at 77°F. Same for nerolidol.)
This is a cool discovery, because I theorized that terpinolene was the active terpene for bud rot resistance, in terpinolene dominant phenos. I also theorized that pinene/myrcene dominance was responsible for leaf mold resistance. When terpinolene is dominant, along with high levels of other pine terpenes, the overall effect is a lot of off-gassing of pine terpenes, and this supports my theory of bud rot resistance.
It's important to note that total off-gassing is a function of both the volatility of the terpenes present, and the percent by weight of each terpene present. If you've got both high volatility, and high percent, good things are going to happen.
~~~~~
Second, I have finally also figured out the boiling points of terpenes. If you look these up online, there's wild disagreement depending on the source. I think the variation is due to the fact that a lot of chemical databases will show boiling point temperatures at various atmospheric pressures other than 1 atmosphere (which is 760 mm Hg), i.e. the pressure at sea level.
This is my own table, based on data from two different chemical databases that were in agreement, except as noted with "(2)", and at 1 atmosphere...
Here we can see the boiling points as not a fixed number, but as a range, with the first column being the lowest recorded boiling point. The chart is ordered with the highest boiling point first (beta-caryophyllene). The far right column, "type," is the type of terpene: S=sesquiterpene, M=monoterpene, Mn=monoterpenoid.
This chart is important for understanding vape temperatures; i.e. as long as your vaporizer is operating at 368.6°F or above, you'll be getting all the main monoterpenes. You can't vape beta-caryophyllene, because the combustion temperature of cannabis is 451°F. Terpineol at 423-424°F is fairly close to the combustion temperature.
