Case study 7 · Charge
Cationic and anionic in the same cream: a curl-defining hair product
A curl-defining hair cream combined an anionic emulsifying wax with a cationic conditioning surfactant, and its companion gel combined anionic gums with a cationic polymer. Counting charge on every line flags both pairings before anything is mixed.
The problem
The hair cream used 3% of an anionic emulsifying wax (cetearyl alcohol with sodium cetearyl sulfate) together with 0.5% cetrimonium chloride, a cationic surfactant, in the same emulsion. The companion styling gel was thickened with anionic xanthan gum and flaxseed mucilage and also contained a cationic polyquaternium. The formulator was looking for advice on the polymer system and on how the two products would perform.
What an HLB-based reading checks
An HLB-based reading checks that the emulsifying wax suits the oil phase, and cetrimonium chloride, which sits high on the HLB scale, would read as a compatible co-emulsifier. An HLB value does not record whether a head group is positive or negative, so the arithmetic gives no signal that the two surfactants carry opposite charges.
What a placement (TDA Logic) reading asks
In TDA Logic, the charge of every line at the finished pH is budgeted [1]. Here it helps to count charge in moles.
How much charge of each sign is present?
Cetrimonium chloride has a molar mass of about 320 g/mol, so 0.5% supplies about 5 g/L ÷ 320 g/mol ≈ 16 mM of cationic head groups. In emulsifying waxes of this type the anionic sulfate is a minor part by weight; a 9:1 fatty-alcohol-to-sulfate ratio is typical, but the supplier’s specification should be checked. On that assumption, 3% wax carries about 0.3% sodium cetearyl sulfate, which at a molar mass near 358 g/mol (between the cetyl and stearyl salts) is about 3 g/L ÷ 358 g/mol ≈ 8 mM of anionic head groups.
On these estimates the cation is in roughly twofold excess over the anion in the cream.
What do opposite charges do at an interface?
Cationic and anionic surfactants form ion pairs that pack differently from either surfactant alone. Depending on the ratio, the result can be graininess, loss of viscosity or a slow drift towards instability. With the cation in excess, the droplet surface charge may be reduced or even reversed, which makes the outcome hard to predict from either surfactant’s datasheet.
What about the gel?
A cationic polymer meeting anionic gums can form polyelectrolyte complexes, seen as haze, stringiness or separated clumps, depending on concentration and order of addition.
Practical fix
- Build the hair cream on a cationic emulsifier system, such as behentrimonium methosulfate or behentrimonium chloride with a fatty alcohol. Cetrimonium chloride is then compatible with the rest of the interface.
- For the gel, choose one charge family. Either keep the polyquaternium and replace the anionic gums with a nonionic thickener such as hydroxyethylcellulose, or keep the anionic gums and drop the cationic polymer.
- Where a cationic polymer is used, pre-dilute it and add it under stirring.
- Judge performance with a curl-retention test at high humidity, comparing curl length over time against a control.
Takeaway
Opposite charges in one product are a design decision to be counted, not discovered. Reading every line for charge at the finished pH, and converting percentages to moles, flags this pairing on paper.
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