In short
In a shampoo or shower gel, fragrance dissolves in surfactant micelles, and it affects the product as much as the product affects it. It can thin or thicken a base, cloud a clear gel, and change foam, then bloom when water dilutes the system. That's why a fragrance is selected and tested in the actual base, at the actual dose. The Lab's guide to fragrance in shampoo and shower gel gives practical doses and categories, and here we explain the mechanics.
What you will learn
- Explain how surfactant systems hold and release fragrance
- Predict effects on clarity and viscosity
- Plan a controlled trial in a simple base
Before this lesson: Laundry and Fabric Care
How surfactants hold fragrance
A surfactant molecule has a water-loving head and an oil-loving tail. Above a certain concentration, the molecules cluster into micelles, with the tails on the inside. Many fragrance materials dissolve in that interior. More water-soluble materials sit nearer the surface or in the water itself. When the product is diluted in the shower, surfactant falls, micelles change or come apart, and materials are freed into the water and air.
Effects on the product
| Property | What can happen | Why |
|---|---|---|
| Viscosity | Falls, or sometimes rises | Some materials change the packing of the micelles |
| Clarity | Cloudiness, or separation above a certain dose | The micelles' capacity is exceeded |
| Foam | Less, in some cases | Oil-like materials can disturb foam |
| Color | Discoloration over time | Reaction of certain materials |
| Preservation | Possible interaction | A formulator's concern; requires testing |
A formulator may correct viscosity by adjusting salt, among other methods. That's the formulator's task. As the perfumer, you should expect to be asked about it, and you should be able to supply a compound whose behavior is known.
Designing for rinse-off
- Bloom materials. Volatile, moderately oil-loving materials, such as those in many fruity, citrus and green accords, can deliver a strong effect in a warm, wet shower.
- Bottle odor. The first impression at the shelf should be pleasant and strong enough, and it may differ from bloom.
- After-rinse. Hair and skin retain only a little. Substantive, high-logP materials help. Hair can hold fragrance better than skin, depending on the product.
- Base masking. Unperfumed bases have their own odor. Smell the base alone first, then decide what must be covered.
- Allergen labeling. Rinse-off thresholds apply, 0.01% in the EU, so a low dose might still need declarations. See the allergens list.
Worked dose example, with invented figures
A 250 g bottle of shower gel at an illustrative 0.8% fragrance load holds 2.0 g of fragrance. If a citrus oil is 25% of the fragrance, and it contains an invented 3% of a listed allergen, then that allergen is 0.008 × 0.25 × 0.03 = 0.00006, or 0.006% of the product. That's below the 0.01% EU rinse-off threshold on this line alone, but other lines may contribute, so you need to add them up. The figures are for practice. Replace them with your supplier's data and your actual load.
Testing
A small trial can show you a lot, even though it's not a stability study.
- Start with an unfragranced base you can buy in small quantities, or from a formulator.
- Note the base's clarity and flow first. A simple time-to-flow test in a measured vessel can provide a number if you lack a viscometer.
- Add fragrance gradually, at set doses, and record each change.
- Smell the bottle, then a measured dilution in warm water for bloom.
- Store samples and look again after a time. Early results can change.
A real product requires a qualified formulator, a preservative challenge test and a safety assessment. A trial like this is only useful for choosing a direction.
Practical exercise: A clarity and viscosity ladder
Use a commercial unfragranced base, in small quantities. These trial samples aren't for use on skin or hair.
- Make a small fragrance, or choose a formula from your earlier work.
- Ladder it at 0.2%, 0.5% and 1.0% of the base, plus an unfragranced control. Before you start, check that those doses are permitted for your materials and category.
- Record clarity and a timed flow measurement at each dose, on the day and after a week.
- Assess headspace above each jar, then a measured 1-in-10 dilution in warm water in a covered jar. Note how the bloom changes.
- Write the dose you'd explore further, and what you'd ask a formulator.
You have done this well if you have a control, recorded numbers for at least one property, and treat the result as direction, not as validation.
Common mistakes
- Adding fragrance at the end and hoping. It can change the base.
- Judging only bottle odor. Test diluted.
- Calling a small trial a stability test. It's a screen.
Key terms
- Critical micelle concentration
- The surfactant level above which micelles form. Below it, the surfactant cannot hold fragrance in micelles, which helps explain release on dilution.
- Solubilization
- Holding an oil-like material in water, often by way of a surfactant, so that the mixture stays clear.
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.