Case study 6 · Location
Micelle or droplet: why a shampoo’s fragrance did not stay on hair
A shampoo’s fruity fragrance did not linger on hair, while a commercial comparison did. The difference lies in where the fragrance sits at the moment of rinsing, and in what processing did to it beforehand.
The problem
The shampoo contained 10% sodium laureth sulfate, 1% cocamidopropyl betaine, 0.1% guar hydroxypropyltrimonium chloride, an apple fragrance oil at 0.3–1% and salt for viscosity. In processing, the guar was dispersed in the fragrance oil and the mixture was added at pH 10 under high shear. A commercial shampoo that left noticeably more fragrance on hair listed guar hydroxypropyltrimonium chloride with polyquaternium-7, cocamide MEA, lecithin, and fragrance materials including coumarin and alpha-isomethyl ionone.
What an HLB-based reading checks
An HLB-based reading asks whether there is enough surfactant, at a suitable HLB, to solubilise the fragrance into a clear product. Here there is, so the reading is satisfied. But clarity is the clue: a clear shampoo means the fragrance has been solubilised into micelles, and that is where the deposition problem starts. HLB was not designed to say where an ingredient will be when the product is diluted on hair.
What a placement (TDA Logic) reading asks
The placement question is which structure each ingredient sits in, because that decides what it does [1]. Three points follow.
How much fragrance survived processing?
Apple accords are rich in esters. Esters hydrolyse under alkaline conditions, and heat and shear speed this up. At pH 10 the hydroxide concentration is a thousand times that at pH 7, so part of the fragrance is likely to have been lost before filling.
Micelle or droplet?
At 10% sodium laureth sulfate, far above its critical micelle concentration, a fragrance at below 1% is almost entirely solubilised in micelles. On rinsing, the cationic guar and the anionic surfactant form a coacervate phase as the shampoo is diluted, and that coacervate captures dispersed oil droplets efficiently. Material dissolved in micelles is poorly placed for capture and largely leaves with the rinse water, although some exchange occurs as the surfactant is diluted.
Which fragrance materials are substantive?
Light, volatile, fruity top notes are among the least substantive materials in perfumery. Heavier, less volatile and more lipophilic materials deposit and linger. As general formulation knowledge, higher logP and lower volatility both tend to go with greater substantivity, so the two are read together when judging how long a material will stay on hair. The comparison shampoo’s fragrance included such materials.
Practical fix
- Disperse the guar in glycerin or propylene glycol rather than in the fragrance, and hydrate it in water.
- Add the fragrance at the end, at the final pH and at room temperature, not at pH 10 under high shear.
- Pre-mix the fragrance roughly 1:1 with a substantive oil carrier so that part of it stays as droplets available for coacervate capture.
- Use guar hydroxypropyltrimonium chloride at 0.2–0.3%, choosing a grade with a charge density suited to deposition.
- Set the final pH at about 5.5.
- Ask the fragrance house for a version built for substantivity from shampoo.
Takeaway
The same ingredient behaves differently in a micelle, a droplet or a vesicle. Asking which structure holds it at the moment that matters, here the rinse, is the question that explains why a clear, well-solubilised fragrance does not stay on hair.
Discussed publicly
The formula problem behind this analysis was discussed in a public forum thread. See In the field for the link and context. Other cases: case-study index.
References
- Pensive Beauty® Nanoscience Labs (2026). TDA Logic: The Tri-Domain Placement Logic Succeeding 1940s HLB Logic in Nanoscale Cosmetic Formulation (Version 1.0) [Preprint]. Zenodo. doi:10.5281/zenodo.23384911