Why Do Glow Sticks Shine Brightest When Cold?

[Gale]

So, I know that freezing them makes them last longer, but I was just curious... why? I've also heard that freezing them makes them glow less bright, and heating makes them brighter. Since its a chemical reaction, that doesn't use or produce heat I don't see how it should matter. Then again, I don't know much chem...

 

[gravenewworld]

So, I know that freezing them makes them last longer, but I was just curious... why? I've also heard that freezing them makes them glow less bright, and heating makes them brighter. Since its a chemical reaction, that doesn't use or produce heat I don't see how it should matter. Then again, I don't know much chem...

How does temperature effect rates of reactions?

 

[Gale]

Right, I see. So the lower temperature means there's less energy for collisions between particles, thus fewer reactions? And so less light? If you increase the temperature.. more reactions at a time, so its brighter, but shorter... Is there a limit to how much you can increase the temperature?

 

[Hurkyl]

I'm sure that vaporizing the glow stick would cause some difficulties.

 

[Gale]

I'm sure that vaporizing the glow stick would cause some difficulties.

har har. I'm thinking of putting one in the oven though. I don't think it will get significantly brighter.

Also, why do they seem to glow for a much shorter time if you break them open?

 

[Hurkyl]

har har. I'm thinking of putting one in the oven though. I don't think it will get significantly brighter.

Without knowing their composition, I would worry about toxic fumes, or at least the possibility of a very-difficult-to-clean-melted-glow-stick-mess.

But if you can be sure to keep it (and anything it contacts) at a little above room temperature, it would probably be okay.

 

[Gale]

Well, the package doesn't say you shouldn't stick it in the oven. I'll just sue if there's any season damage eh.

 

[Hurkyl]

Come to think of it, a pot of hot water might be much more convenient than an oven.

 

[Gale]

True story. Probably less messy too. hmm... Good idea! I'm on it!

 

[gravenewworld]

Right, I see. So the lower temperature means there's less energy for collisions between particles, thus fewer reactions? And so less light? If you increase the temperature.. more reactions at a time, so its brighter, but shorter... Is there a limit to how much you can increase the temperature?

Right. There probably is a point of optimal temperature, it would depend on the ratio of the dye:peroxidexalate in the stick. There would be no point of heating the reaction between oxalate and the peroxide in order to make it go faster if all of the dye's electrons were already in the excited state. The optimal rate (which would depend on temperature) would probably be when the rate of the oxalate:peroxide reaction is equal to the rate at which the dye transitions between its excited and unexcited state.
heating in some boiling water i could see. sticking it in the oven? probably not. the glow stick will produce phenol which isn't the most pleasant of things and has NFPA health rating of 4. while it isn't the most deadly stuff on earth, it could produce an irritation.

 

[Gale]

I boiled them. Then broke them open. I should've let the stuff ooze into the water, but i didn't think of it til i was done. Side note: Glow sticks make nifty glowing ink pens.

Anyway, Why do they stop glowing so soon after you break them open? Does the air effect the reaction?

 

[Gokul43201]

There's two factors that affect the glowiness. First, increasing the temperature causes an increase in the reaction rate (through molecular collisions). Additionally, the relative (electronic) populations of two states (say, a ground state and an excited state) is a (Boltzmann-like) function of temperature. The higher the temperature, the more electrons get to populate the upper state.

The fractional population of the i'th state (i=1 or 2; 1 is the ground state, 2 is the excited state) is:

$$f_i=\frac{exp(-E_i/k_BT)}{exp(-E_1/k_BT)+exp(-E_2/k_BT)}$$

where E1 and E2 are the energies of the ground state and the excited state respectively.

Notice that, in the limit kT<<(E2-E1), you get f1=1, f2=0 (no glow) and in the limit kT>>(E2-E1), you have f1 = f2 = 0.5 (max integrated glowiness). Also, this tells you that once you are in the second regime (of high temperatures), any further increase in temperature gets you very little in terms of increasing integrated glowiness^*.

A real glow stick will likely have more than just two available electronic states. The above argument can be generalized to a system with n states.

* "Integrated glowiness" is something like how bright it glows times how long it glows for.

PS: For a green glow stick, E2-E1 is about 2.5eV (green light is somewhere aroung 500nm). Plug this number into the expression for f2 and you'll see that it increases by orders of magnitude between freezing temperatuers and scalding temperatures.

 

[chemisttree]

I boiled them. Then broke them open. I should've let the stuff ooze into the water, but i didn't think of it til i was done. Side note: Glow sticks make nifty glowing ink pens.

Anyway, Why do they stop glowing so soon after you break them open? Does the air effect the reaction?

Did the luminol (3-aminophthalhydrazide) solution (containing hydroxide and peroxide) dilute into the water? If so, the hydrolysis that requires both hydroxide and oxidant will slow waaay down and the light intensity will fall to scintillation counter type levels pretty quickly.

BTW, I wonder what the luminol solution would look like after the stick exploded in a microwave? Good YouTube subject!

 

[Invictious]

Just some random searching shows what happens when you put glowsticks into a microwave:



It's a tad long and a tad boring, but at least you know what happens.