In short
A fragrance behaves differently in each product because its materials distribute between phases such as water, oil, micelles and air. Water-loving materials and oil-loving materials go to different places, and only what reaches the air can be smelled. That is partitioning. It explains why the same oil can seem flat in a lotion, burst open in a shower, or vanish in a detergent. You met carrier effects in Level 8, and this track goes further into the mechanisms.
What you will learn
- Explain partitioning with logP and phase volumes
- Predict which materials a base will hold back
- Plan a test comparing headspace impression in two bases
Before this lesson: Adapting to a Product Base: Level 8 Project
Scope
This track covers concepts and small-scale evaluation. Making finished functional products involves preservative systems, surfactant handling, pH, electrical safety for candles, and product laws you can't address from this page. Exercises use small, simple test bases, and they aren't for use on skin or for sale. Commercial products need formal testing and qualified formulators.
LogP
LogP is the logarithm of a molecule's partition ratio between octanol and water. Higher means more oil-loving.
| Material | Approximate logP | Tendency |
|---|---|---|
| Phenylethyl alcohol | 1.4 | Relatively water-friendly |
| Linalool | 3 | Mid-range |
| Limonene | 4.5 | Oil-loving and volatile |
| Hexyl salicylate | Above 5 | Very oil-loving, low volatility |
A simple calculation
For a two-phase system, the fraction f of a material in the oil phase is approximately:
f = K × Voil ÷ (K × Voil + Vwater)
K is the partition ratio. Worked example, using a simplified model. A product is 10% oil and 90% water by volume.
- K = 1,000 (logP 3): f = 100 ÷ (100 + 0.9) = 99.1%.
- K = 10 (logP 1): f = 1 ÷ (1 + 0.9) = 52.6%.
So even at 10% oil, a mid-logP material sits almost entirely in the oil, while a water-friendly one is split. Real products have micelles, emulsifiers and other components, and octanol isn't your oil, so treat this as a way of thinking. It doesn't predict the odor.
What a nose gets
Vapor above a product depends on a material's volatility and on how strongly the base holds it. So partitioning changes the balance of what you smell:
- In water-rich bases, oil-loving materials are pushed toward the air, or toward any oil or micelle available.
- In oil-rich bases, oil-loving top notes are held back, so citrus can seem muted, and more water-friendly materials become relatively noticeable.
- In surfactant bases, micelles hold materials. Dilution in use can then release them. This is the "bloom" a shower gel is designed for.
- On wet skin, hair or fabric, deposition favors the oil-loving and the less soluble materials.
Designing with it
| Goal | Materials to consider |
|---|---|
| Bloom in rinse-off use | Volatile, moderately oil-loving materials |
| Remains on skin, hair or fabric after rinsing | High logP, low vapor pressure |
| Presence in an oily lotion | Higher dose, or lower logP materials for a lift |
| Clear in a water-rich product | Consider a solubilizer. See solubilizing fragrance |
Remember that materials also affect the base. A fragrance can thin a shampoo, cloud a solution or destabilize an emulsion, so a formula must be tested in the product itself. See the Lab's guide to adapting a fragrance to a base.
Practical exercise: Predict, then smell, in two bases
For strips and small jars only. Don't apply these test bases to skin.
- Choose ten materials from your organ. Look up logP for each in PubChem, and note the method.
- Predict which ones would stand out, and which would drop, in an oil base and in a water and surfactant base.
- Make a small fragrance from four or five materials, spanning the logP range.
- Dose it at the same level into a neutral oil and a simple unfragranced shampoo base. Keep a sample in alcohol too.
- Close the jars, leave them for a day, and smell the headspace, coded if possible. Then test a small amount diluted with warm water for the shampoo.
- Write where your prediction held and where it failed.
You have done this well if your predictions were written first, you tested with a real dilution step, and you can name one material whose behavior surprised you.
Common mistakes
- Treating logP as a final answer. It's one simplified measure.
- Judging by the bottle alone. Test in use as well.
- Ignoring what the fragrance does to the base. Check viscosity and clarity.
Key terms
- Partition coefficient
- The ratio of a substance's concentration in two phases at equilibrium. LogP refers to the logarithm of its octanol-water value.
- Micelle
- A cluster of surfactant molecules in water, with a water-repelling center in which fragrance materials can dissolve.
- Bloom in use
- The burst of fragrance released when a product is diluted or used, such as in a shower.
Knowledge check
Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.