Beginner perfumer · 53
Sprayer clogging: DPG or IPM? In 30 replies, most people were solving a different problem
· 25 min read
Somebody on r/DIYfragrance asked why their homemade Molecule 02 (ambroxan in alcohol, nothing else) keeps growing crystals that block the sprayer. Nearly all 30 replies said dilute it more. He wrote twice that nothing is crystallising in the bottle, only the nozzle drying out and clogging. One person asked which of the two it was, then wrote the most useful line in the thread: everybody is solving the other problem. The two have opposite fixes. Bottle crystallisation is cured by lowering the concentration; nozzle clogging is not, because once the ethanol evaporates the residue on the nozzle is 100 percent ambroxan whatever the bottle held. What you choose is a co-solvent that stays behind, and the field that decides that is vapour pressure. Lined up from the database, DPG turns out to be 3.5 times more volatile than ambroxan itself.
About an 8 minute read.
The question went like this.
"Hi, for the past couple of years I've been making my own Molecule 02 (Ambroxan only) solution with alcohol. It's great. But I always end up with lots of crystals that clog the sprayer. I would like to get rid of the crystals. ChatGPT told me to just add Dipropylene Glycol. Is this the way to go? I'm a noob."
Thirty replies came in. Most of them say the same thing.
"Dilute it more. That's...that's it. You just haven't diluted it enough." (beraelen)
"You're using too much ambroxan to the point that whenever the temperature is low enough, it will crystallize. How do you fix this? Less ambroxan in your alcohol." (kali-kid)
"Ambroxan cannot be diluted at higher than 10%." (Big-Highlight-4415)
The asker wrote the same sentence twice.
"There is no crystallization taking place in the bottle. It's only when I spray that the spray nozzle dries out and crystals form, and then it gets clogged."
One person asked the right question.
"When you say crystallize, are you meaning that the liquid on the spray nozzle dries out and clogs it, or that the bulk of the solution is crystallizing in the bottle?" (Odd_nonposter)
Having got an answer, they wrote the most useful line in the thread.
"Ok, that helps! I think everyone is trying to solve the other problem, which isn't yours!"
Two problems with opposite fixes
Crystallising in the bottle means the solution is past saturation at storage temperature. Lowering the concentration works, because it puts you back under the saturation line.
Clogging at the nozzle means the ethanol evaporates on the nozzle. The solvent leaves, the solute stays.
In the second case, the concentration in the bottle does not change what the residue is made of. Whether you mixed it at 30% or at 3%, once the ethanol is gone what sits on the nozzle is ambroxan. Ambroxan melts at 74-75 C, so at room temperature that is a solid.
"Dilute it more" does nothing for the nozzle. It lowers the amount delivered per spray, so the clog arrives later, and then it arrives.
Reid, Jones and Brown put the mechanism plainly in Int J Pharm in 2009: supersaturation can be created in situ by volatile solvent evaporation after application. They were working on beclomethasone in a metered dose aerosol, and taking the ethanol from 10% to 20% moved the onset of supersaturation from 30 minutes after application to the moment of actuation.
Whatever is left after a volatile solvent evaporates is supersaturated by construction. It is a property the spray format brings with it.
So what do you add
If the residue is going to form regardless, the thing you can change is what is in it.
You need something that stays on the nozzle and keeps the ambroxan dissolved. The field that decides whether it stays is vapour pressure.
Here are the database numbers, at 25 C except for ethanol.
| Material | Vapour pressure (mmHg) | Relative to ambroxan |
|---|---|---|
| Ethanol | 44.6 (@ 20 C) | 4,956x faster |
| DPG | 0.0319 | 3.54x faster |
| ambroxan | 0.009 (est) | 1 |
| IPM (isopropyl myristate) | 0.000329 (est) | 27x slower |
| Benzyl benzoate | 0.000250 | 36x slower |
| TEC (triethyl citrate) | 0.000175 (est) | 51x slower |
DPG has a higher vapour pressure than ambroxan itself.
That was ChatGPT's answer to the asker, and several people in the thread offered it too. In the bottle it is a useful diluent. On the nozzle it leaves before the thing it is meant to be dissolving. It does not stay.
IPM is 97 times slower than DPG, and TEC is 182 times slower.
One person's measurement lines up with that ordering.
"IPM can hold it at a greater percentage in dilution. For me 25% won't crystallize, while in DPG past 10% it does." (shackener)
What they measured is bulk saturation, which is a different property from volatility. Two independent angles still point the same way.
The second field: logP
Staying behind is only the first hurdle. What stays has to dissolve ambroxan.
Ambroxan's logP is 4.70.
| Material | logP | Distance from ambroxan |
|---|---|---|
| IPM | 7.20 | +2.50 |
| Benzyl benzoate | 4.00 | -0.70 |
| TEC | 0.10 | -4.60 |
| DPG | -0.60 | -5.30 |
Benzyl benzoate is closest. DPG is furthest.
Somebody raised it. One upvote, and the asker did not know the abbreviation.
"Isn't BB the strongest for liquefying solids? Replace half the alcohol with that." (c7b2)
"BB?" (the asker)
"Benzyl Benzoate." (c7b2)
On those two fields, that is the best-reasoned reply in the thread.
The third field turns the answer around
Benzyl benzoate melts at 18-21 C.
A bathroom cabinet, a bedroom in winter, an unheated room: all of those sit at or below that range. You would be picking a co-solvent to stop crystallisation and getting one that solidifies near room temperature by itself.
| Material | Melting point |
|---|---|
| ambroxan | 74-75 C |
| Benzyl benzoate | 18-21 C |
| IPM | 2-3 C |
| TEC | -46 C |
| DPG | field is empty in the database |
Taking all three fields together:
- TEC: least volatile, lowest melting point, but logP is 4.60 away.
- Benzyl benzoate: closest logP, melting point inside the room-temperature range.
- IPM: vapour pressure barely above TEC's (27x slower against 51x slower), logP gap of 2.50 which is half of TEC's, melting point of 2-3 C which is low enough.
- DPG: last on all three.
IPM wins the three-way trade-off. Which is the answer the two people who diagnosed the problem correctly gave.
"One poster here says Ambroxan stays in IPM at 25%. If you have a 3:1 ratio of IPM to Ambroxan, it's guaranteed not to crash out and clog the sprayer when the ethanol evaporates." (Odd_nonposter)
3:1 is 25%, the same figure shackener reported.
One limit to flag
The reasoning above is "leave a non-volatile layer behind with the ambroxan dissolved in it." That idea has been tried seriously in pharmaceutics and it did not go well.
Leichtnam and colleagues worked on a testosterone spray in J Pharm Sci in 2007. They screened antinucleant polymers, two looked promising by differential scanning calorimetry, and those two did improve the long-term stability of saturated solutions. Sprayed onto skin, delivery showed no improvement relative to a non-stabilised control.
Their conclusion is candid: the in situ crystallisation process is more complex and incompletely understood, cannot be predicted from DSC experiments, and the evaporation process that takes place when a spray is pulverised needs better characterisation.
What happens past the nozzle is not solved on the pharmaceutical side either. Thirty replies failing to solve it is not a failure of effort.
About "Ambroxan cannot be diluted at higher than 10%"
That is the most categorical sentence in the thread, so I checked it.
The database's ambroxan solubility field lists "alcohol; dipropylene glycol" and gives no percentage at all. The 10% has no source in the database.
Across 954 published formulas, 95 contain an amber ether, and the highest dose among them is 17%.
Those are concentrates rather than finished sprays, and the solvent systems differ. Still, 10% is not a physical line.
Who uses these four in the corpus
| Solvent | Formulas | Share | Median dose |
|---|---|---|---|
| Benzyl benzoate | 43 | 4.5% | 10.5% |
| DPG | 34 | 3.6% | 15.0% |
| TEC | 5 | 0.5% | 9.3% |
| IPM | 4 | 0.4% | 13.5% |
Benzyl benzoate and DPG dominate professional formulas; IPM barely appears.
Read that carefully. These 954 are concentrate formulas, solving dissolution and dilution rather than nozzle residue. The optimum at the concentrate stage and the optimum for a clogging sprayer are two different optimisations.
A check while I was here
Last time I used Yalkowsky's general solubility equation on maltol. Running it on ambroxan: melting point 74.5 C, logP 4.70.
log S = 0.5 - 0.01 x (74.5 - 25) - 4.70 = -4.695
That converts to 4.77 mg/L, against the database's 2.436 mg/L. A factor of 1.96.
Both are estimates, so this compares two estimators rather than validating either.
What you can actually do
- Work out which problem you have first. Does it crystallise sitting still? Or does a sprayed droplet crystallise as it dries? The fixes are opposite.
- For bottle crystallisation: lower the concentration, or switch to a solvent system that holds more.
- For nozzle clogging: lowering the concentration does not help. After the ethanol evaporates the residue is 100% solute regardless of what the bottle held.
- Pick a co-solvent by vapour pressure first. DPG is 0.0319 mmHg at 25 C, above ambroxan's 0.009. It will not stay on the nozzle.
- Then look at the logP distance. Ambroxan is 4.70, DPG is -0.60.
- Then check the co-solvent's own melting point. Benzyl benzoate is 18-21 C and will set in winter.
- Try IPM starting at 3:1. That is the same figure two independent replies arrived at. IPM will reduce projection, and that is the price.
- Heating is temporary. One reply warned not to heat too fast because it crystallises as soon as it cools, which describes supersaturation rather than insufficient heat. This is the fourth time the same thing has come up across these articles.
What this doesn't establish
- I ran no nozzle experiment. This is reasoning from database fields, two papers and forum reports. I have not sprayed anything.
- Three of the vapour pressures are estimates (ambroxan, IPM, TEC) while DPG and benzyl benzoate are measured. Mixing them is a risk, and the 3.5x gap between DPG and ambroxan happens to be one measurement against one estimate.
- Vapour pressure is not evaporation rate. A thin film on a nozzle involves airflow and an unknown area ratio. I use it for ranking, not for rate prediction.
- logP is a partition coefficient, not a solubility. "Like dissolves like" is a rule of thumb, and I have no measured solubility of ambroxan in any of these co-solvents.
- How I weighted the three fields was my choice. I put "does not evaporate" ahead of "close logP" on the grounds that something which leaves cannot help. That ordering has no experimental backing.
- Leichtnam 2007 studied testosterone, not ambroxan, and tested antinucleant polymers rather than a change of co-solvent. I cite it to show the problem is unsolved, not to predict that IPM will fail.
- Reid 2009 used a metered dose aerosol containing HFA 134a, and propellant evaporation differs from a finger pump.
- The 25% figure comes from two forum replies, one of which may be quoting the other. Whether those are two independent measurements, I do not know.
- The four-solvent corpus statistics are from the concentrate stage and do not transfer directly to a finished spray.
- I did not find measured solubilities of ambroxan in IPM, TEC or benzyl benzoate. The database's solubility field lists only alcohol and DPG, with no figures.