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I have always been told that the 'heat' from chillies came from capsaicin which is oil based. Recently I began wondering how environmentally friendly chilli based insecticide/repellent sprays could work if they are water based. A chemist friend confirmed for me that capsaicin is not soluble in water.

I minced a chilli and added this to a coffee mug of boiling water, left overnight then strained through a coffee filter. The resulting chilli water is definitely spicy, but why? What has dissolved into the water (or is the mixture some sort of emulsion of tiny oil droplets?).

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    $\begingroup$ It just triggers the nerve that senses heat. $\endgroup$ – DHMO Jan 15 '17 at 2:39
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    $\begingroup$ Hi DHMO, what trigger's the nerves? What is chemically active in the water? Thanks! $\endgroup$ – Adrian Jan 15 '17 at 2:45
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    $\begingroup$ I think what you've shown is that our olfactory senses can detect capsaicin at levels of 10's of mg/L (solubility according to Wikipedia). Most every organic liquid has some solubility in water, its just by degrees that we decide whether to call it soluble or not. Your emulsion idea is also interesting. Also, there are other oils in chili peppers that could increase the solubility of capsaicin. Did you by chance see any oily film on top of the water? If so, maybe try drinking your spicy tea through a straw! $\endgroup$ – airhuff Jan 15 '17 at 4:33
  • $\begingroup$ No oily film visible even outside in good daylight, the mixture doesnt appear to separate at all so far. I am actually hoping it doesnt separate so I can spray it on my garden plants to keep caterpillars away. I thought the coffee filter would likely soak up any obvious globules of oil from the first steep. $\endgroup$ – Adrian Jan 15 '17 at 4:42
  • $\begingroup$ The slight solubility in pure water probably explains a lot (see MaxW's answer). But don't forget the other ways to increases the amount of the capsaicin: it is more soluble in alcohols, it can be dispersed in an oily phase in a relatively stable colloid... $\endgroup$ – matt_black Jul 4 '18 at 15:23
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You got me curious, so I poked around a bit on this.

First there is insoluble and then there is insoluble. The Wikipedia article on capsaicin lists its solubility as $\pu{0.0013 g}/\pu{100 mL}$ which is $13$ parts per million. So capsaicin is "relatively insoluble", but not wholly so. Second the Wikipedia article also points out that capsaicin itself is just one of the various capsaicinoid chemicals which give peppers their "hot" taste.

At the end of the (old) Wikipedia article on the Scoville scale, a taste test for hotness, it states

[...] A measurement of one part capsaicin per million corresponds to about 16 Scoville units. [...]

So, 13 pmm multiplied by 16 Scoville units per ppm is about 200 Scoville units.

A table on hot sauces and peppers lists the Original TABASCO® brand Pepper Sauce from McIlhenny Company as being 2,500 to 5,000 Scoville units. So 200 Scoville units would be about one tenth strength sauce. That should still be enough to tingle your tongue.

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  • $\begingroup$ Interestingly if this is hot enough to keep caterpillars off my plants then I'm not going to need a lot of chilli. On the other hand from your info above I take it that it would be impossible to get more 'heat' dissolved into the water. $\endgroup$ – Adrian Jan 15 '17 at 4:59
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    $\begingroup$ There are multiple capsaicinoids in a pepper, each with their own solubility. Adding alcohol and vinegar to the water should increase the solubility of the capsaicinoids. $\endgroup$ – MaxW Jan 15 '17 at 5:12
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Lets make it simple with simple quantum mechanics. water is consists of Hydrogen and Oxygen. Oxygen have bigger nucleus so it pull electron in hydrogen but hydrogen won't let it go so Hydrogen trying to pull it in but unfortunately, Oxygen is heavier and have stronger nuclear force than hydrogen. Oxygen even able to pull 2 Hydrogen atom.

Okay, we know Oxygen is such a muscular macho savage guy with a mustache and every atoms are such a thug compton gangster but what exactly happen ?. Those make water polar. because the hydrogen electron is pulled towards oxygen. The unequal distribution of force inside hydrogen atom make it slightly positive due to the induced dipole during the bodning process. That's why we classify water as polar covalent compound. And of course oxygen have slightly negative polarity because it must maintain the stability of its own atom and bonding.

What is capsaicin ?. Capsaicin is huge hydrocarbon. based on the nucleus force analysis it is nonpolar. They have polarity but not much. they have negative polarity in C=O double bond. THey also have O-H in one of its end. And a little bit positive but in really-really tiny amount of polarity of N-H binding.

So, what will happen if I put capsaicin in the water. well, water is friendly Rey Mysterio and Capsaicin is unfriendly Big Show. Water attach to some of capsaicin polar ends and try to pull it. Unfortunately to pull one capsaicin molecule, Water need more energy because it is heavy and repulsive in much ends. But, C=O ends is seems attracted to water so, it would basically able break apart and bonding with water. C=O Double bond have negative polarity, Hydrogen in water have positive polarity so, it able to pull out Oxygen from capsaicin. Yes, without any energy given to the reaction it's can be (roughly) 95-99 % unsoluble in water.

But we know, of course, reaction sometimes requires energy. Say we need to shake it, Blend it, Boil it, Electrolysis, or even burn it. Just give it energy. The solubility able to increase. But, How far it would likely to increase ?. As you notice, in repulsion of homopolarity there's energy needed to destroy this capsaicin. It's the weakness, just put up an effort to have certain amount energy which is exert the repulsion energy between capsaicin and water.

So, what happen to you is natural. When you heating up water. The orbitals of water is changing due to the electron vibrations. There's some amount of kinetic energy absorbed and force electron to move. So, the shielding between electron in orbitals can be forced and the orbital shape is slightly changing.

That's why some of capsaicins are able to dissolved in water.

But, in your case you have coffee. Coffee is complex mixture of chemical compounds. Coffee is not only consists of caffeine it's also have a bit protein and a bit minerals. Such as potassium. So, the chemical reaction between capsaicin in coffee mixture is really complex.

But let's see why it's more dissolveable. Let's check caffeine. Caffeine is have double negative polarity C=O bond. It's enough to attract water and makes caffeine soluble in water. So, you try to dissolve capsaicin in strengthen water molecule. It's a handicap match between coffee + water againts the big show. So, who wins ? I predict the tag team win. The solubility will increase but still I predict, it will not be completely dissolve in this mixture unless you put on some milk.

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    $\begingroup$ Not quite the normal way to write about chemistry, but interesting. $\endgroup$ – Nuclear Chemist Jul 4 '18 at 15:15
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    $\begingroup$ Interesting, but a bad way to explain a simple concept. $\endgroup$ – matt_black Jul 4 '18 at 15:20
  • $\begingroup$ Nuclear Chemist, I'm not normal. matt_black, yeah I'm bad chemist, I'm not a chemist at all. I'm a physicist. And I love to do the cheap trick. Atomic mass trick $\endgroup$ – Willy satrio nugroho Jul 7 '18 at 7:13

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