Well, I cannot go back and edit the original post now, but I meant to explain WHY ethanol becomes more selective as temperatures decrease! Although it is still a hypothesis because I cannot easily obtain direct evidence of the facts, according to my observations, I sincerely believe ethanol selectivity at low temperatures is more physical/mechanical than chemical:
In order to make sense of both of the following systems, one must understand a bit of cellular biology and think microscopically.
At higher low temperatures, selectivity has to do with decreased solubility of undesirables in the plant; namely epicuticular wax (water-resistant protective layer on outer surfaces of leaves and plant parts, which inhibits passive evaporation of water in warm growing conditions) and the vegetable oil (e.g. food grade hemp oil and any other large-molecule fatty acid triglyceride compounds… similar to coconut LCT and MCT oils… which often coexist with free fatty acids, phosphatides, and other lipid components).
Other undesirable compounds with decreased solubility in ethanol at lower temperatures include sugars and more complex carbohydrates (sugar polymers), free amino acids and proteins (amino acid polymers), and other intracellular materials, which brings us to the concepts of the secondary effects of decreasing temperature…
At lower low temperatures, decreased uptake of undesirables by solvent has to do with the density & viscosity of the solvent, and the permeability & cellular mechanics of the plant material.
This is similar to the conceptual difference between chopping/cutting/grinding the biomass, which breaks and opens many individual cells, and shredding/hand-breaking biomass, which mostly splits it up along the cell walls, leaving cells mostly closed and intact. In the case of cannabis, the latter (unopened cells) improves selectivity in the same way super low temperatures improve selectivity, if that makes sense.
*1. Ethanol at -40°C/F:
Temperatures at or below this point render the epicuticular wax on the plant material relatively insoluble, most especially in 190 proof potable ethanol (aka ethanol:water azeotrope)… and although the wax is less soluble at this temperature, it is still much more soluble in 200 proof pure or denatured ethanol. The same rule applies to vegetable oil and other non-polar intracellular compounds that are accessible to the ethanol due to open cells. This also applies to more polar compounds like sugars, but the prevailing belief is that said intracellular sugars are slightly more soluble in 190 proof than in 200 proof and denatured… the key words here are “intracellular” and “slightly”, though. Ultimately, proper shredding minimizes access to these compounds… as does lower temperature, described in *2.
*2. Ethanol at -70°C:
Temperatures at or below this point render most undesirables almost completely insoluble AND cause the ethanol to become so dense & viscous that it is less able to penetrate stomata, other pores, and even torn openings in the cells, which are surrounded by insoluble waxy cell walls and which are now minimized in size due to thermal shrinking! In other words (as usual, most especially 190 proof) ethanol is physically less able and less likely to penetrate into (and therefore less likely to wash back out of) cell interiors, where almost all of the undesirable compounds reside!
How resin extraction occurs:
Thinking this way, it is vital to also comprehend the nature of the glandular trichome membrane and the state of the material inside those trichome “heads” when starting the extraction process. Along with an understanding of the low heat capacity of very dry biomass, this should also help explain why it is better to leave dry plant matter at room temperature and hit it with cold solvent than it is to freeze said dry plant matter…
Only a thin part of the combined outermost cell membrane (cuticle) of the secretory disc cells expands like a balloon as it fills with cannabinoid acid rich terpene resin. These disc cells are specialized to create this elastic membrane, which becomes thinner and more (molecularly) porous as it expands, just like any elastic material. When this “stretched skin” (membrane) is exposed to alcohol or other solvents, the solvent can rapidly diffuse into the resin through the pores in the membrane, causing the gland to rapidly swell and rupture, releasing the already-partially-dissolved liquid resin into the wash of bulk solvent.
Naturally, small particles and even molecules of the exploded membrane can enter the solvent when this occurs, which is one reason there is always some phosphatide and other impurities in the resin.
Also, the disc cell plasma membranes (which keep their disc cell contents from spilling out into the resin) are more vulnerable to solvent attack than the wax-covered hard cell walls on the rest of the plant, which is one reason why relatively brief solvent/plant contact time is always preferred over extended solvent residence time in cannabis extractions.