Why the alcohol was there
L-Menthol dissolves in water at roughly 0.4 g/L. Ethanol at 20–40% carries ten to twenty times that, dries in seconds, and produces a cold flash by evaporation. It also stings on compromised skin, carries a flammability classification for transport, contributes to VOC limits in some jurisdictions, and reads as “drying” to a consumer who is already worried about their barrier.
An ester of menthol and citric acid changes the solubility equation: 2–3.2 g/L in water, five to eight times that of menthol, and it does not crystallise out at 4 °C. The sodium salt form goes further — a 10% solution at pH 8–9 is fully miscible with water, which is what makes a clear, alcohol-free spray practical rather than theoretical.
The salt route, in one paragraph
You can formulate with the free acid and a solubiliser, or with the 10% sodium salt solution and no solubiliser at all. The salt route is simpler and clearer; the trade-off is pH. A sodium salt solution sits at pH 8–9, so you bring the finished product back toward skin pH with citric acid and accept that the ester is partially re-protonated in the process. That is normal, and it is why the finished pH is brought down after the salt is in rather than before.
A starting formula
Percentages are w/w. This is a reference starting point from our development work, ready to be run on your base.
| Phase | Ingredient | % w/w | Function |
|---|---|---|---|
| A | Water | to 100 | Carrier |
| A | Glycerin | 3.0 | Humectant, reduces the dry-down tack |
| A | Propanediol | 2.0 | Coupling solvent, light after-feel |
| A | Panthenol | 0.5 | Soothing, claim support |
| A | PCA sodium | 1.0 | Humectant, skin-identical |
| B | Menthol citrate, 10% sodium salt solution | 5.0 | Cooling — equals 0.5% ester |
| C | Preservative system | qs | Challenge-test validated at the final pH |
| D | Citric acid, 10% solution | qs | Bring to a skin-compatible pH |
| E | Fragrance (optional) | 0.1 | Solubilise before addition |
Process: combine A, add B to the cooled phase, add C, adjust pH with D, then E. Filter before filling, and keep the bulk cool through the whole process.
Where the formulation needs attention
- pH drift. The ester hydrolyses faster away from its stability window, and a preservative system can move the pH over twelve weeks. Measure at 4, 25 and 40 °C, not just at release.
- Cloud point. Fragrance and some preservatives drop a clear system out at low temperature. Hold a sample at 4 °C for four weeks before you commit to a clear bottle.
- Preservation at near-neutral pH. Many systems are tuned for pH 5. A near-neutral spray carrying 5% salt solution is a harder preservation case than it looks; run the challenge test on the final formula.
- Packaging. Salt-containing aqueous systems are harder on aluminium than on PET or PP. Check the actuator, the dip tube and the gasket, and run a 40 °C inverted storage test.
- Spray pattern. A fine mist feels cooler than a wet stream even at the same dose. Viscosity and surface tension decide the pattern more than the pump does.
- Overdose. The instinct is to raise the sensate when the panel says “not enough”. Above roughly 1.5% in a leave-on, you are more likely to produce sting and crystallisation than more cooling.
What to test before you scale
- Stability: 4 °C, 25 °C and 40 °C for 4 and 12 weeks, upright and inverted. Check clarity, pH, odour and assay.
- Freeze–thaw: three cycles. Phase separation shows up here first.
- Preservation: ISO 11930 or your market's equivalent, on the final formula.
- Compatibility: 30 days contact with the actual bottle, actuator and gasket at 40 °C.
- Sensory: trained panel, cool/burn/tingle scored separately, with an untreated control.
- Claims: decide the wording before the panel, not after — it determines what you have to measure.
How to position it
Cooling and freshness are cosmetic claims, and they are enough to sell a spray when the sensory result is good. In the EU, “antibacterial” on a cosmetic reads as a biocidal claim; in the US, a drug claim needs a monograph or an application. We build the positioning around what the formula can measure.
Reference formulation for development discussion only. It has not been optimised for any specific market, and compatibility with your preservative, fragrance and packaging must be confirmed by you.