Case study 4 · Charge at the finished pH
Charge state decides it: a quaternary ammonium meets carbomer
A formulator reproducing a rub-off gel neutralised carbomer and then added a cationic quaternary ammonium surfactant. White clumps formed at once. A quaternary ammonium and carbomer appear together on a commercial product’s label, and the difference is the charge state of the polymer at the finished pH.
The problem
The target was a commercial rub-off gel whose label listed water, steartrimonium chloride, carbomer, alcohol, charcoal, kaolin, and lactic, citric and malic acids. The formulator neutralised the carbomer in the usual way to build viscosity, then added cetrimonium chloride. The mixture immediately formed white clumps.
What an HLB-based reading checks
There is no oil phase to match, so required-HLB arithmetic does not apply. Cationic surfactants can be given HLB values through group contributions [2], but an HLB value does not record the sign of a head group’s charge, and it says nothing about whether a cationic surfactant will associate with an anionic polymer. The interaction that decides this gel lies outside the question HLB was devised to answer.
What a placement (TDA Logic) reading asks
The placement checklist asks for the charge of every ingredient at the finished pH [1]. Here that question settles the matter.
How charged is the carbomer at each pH?
Carbomer is a cross-linked poly(acrylic acid). Its carboxyl groups have an apparent pKa of about 6; the apparent value of a polyacid shifts with its degree of ionisation, so a single figure is an approximation. Using it, the ionised fraction is 1/(1 + 10pKa − pH):
| pH | Ionised fraction | Behaviour with a cationic surfactant |
|---|---|---|
| 3.5 | ≈ 0.3% | Little binding; the two can coexist |
| 4.0 | ≈ 1% | Little binding; the two can coexist |
| 6.0 | ≈ 50% | Strong binding |
| 7.0 | ≈ 91% | Strong, cooperative binding; precipitate |
Once neutralised, carbomer is a polyanion. A cationic surfactant binds to it cooperatively, its tails associate, and the polymer–surfactant complex becomes insoluble. That is the white clumping. At pH 3–4 most carboxyl groups are protonated, so the quat barely binds and the two can share a jar.
What happens when the balance shifts?
On the skin the conditions change: rubbing concentrates the gel, the alcohol evaporates and the skin surface buffers the pH upward. The carbomer becomes more ionised and the complex forms as visible rolls. That is the rub-off effect. The rolls consist largely of the formula itself and form whether or not any skin debris is present.
Are other surfaces charged?
Yes. Kaolin carries negatively charged faces and charcoal is highly adsorptive; both can take up cationic surfactant. Under TDA Logic, cationic species facing an anionic surface are minimised or excluded. In this gel the interaction is the intended effect, so the task is to budget it: decide when, and at what pH, the complex is allowed to form.
Practical fix
- Disperse the carbomer without neutralising it.
- Add the acids before any quat, bringing the pH to about 3.5–4.0.
- Pre-dilute the quat and add it slowly, at a low level, titrating upward while watching for haze.
- Add the powders (charcoal, kaolin) at the end, and expect them to take up some of the quat.
- Unneutralised carbomer gives little viscosity at this pH, so plan the texture accordingly.
- Confirm the pH holds on stability. An upward drift in the jar would trigger the complex before use.
Takeaway
Whether a cation and an anionic polymer can coexist is set by the polymer’s charge at the finished pH. Budgeting that pairing as a planned interaction, rather than meeting it as a surprise, is the question the placement checklist asks.
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
- Davies JT (1957). A quantitative kinetic theory of emulsion type. I. Physical chemistry of the emulsifying agent. In: Gas/Liquid and Liquid/Liquid Interface, Proc. 2nd Int. Congress of Surface Activity, pp. 426–438. Butterworths.