Case study 2 · Ionic strength
Electrolytes and a lamellar cream that separated after small changes
A cream built on a lamellar gel network separated after several weeks, following a handful of small changes requested by a client. Each change looked minor on its own; together they loaded the ionic-strength and oil budgets of a fixed emulsifier system.
The problem
The cream was an oil-in-water emulsion on cetearyl olivate and sorbitan olivate, an emulsifier pair that structures the continuous phase as a lamellar gel network, with a little cetearyl glucoside. It contained 1.0% sodium PCA, 0.2% sodium phytate and citric acid, and was preserved with a system relying on dehydroacetic acid. The client then added aloe, reduced the shea butter by two percentage points and raised squalane by two points.
The revised cream separated after a few weeks. It was also difficult to emulsify at production batch sizes of 20 kg and above.
What an HLB-based reading checks
An HLB-based review would recalculate the required HLB of the revised oil phase and compare it with the emulsifier system. That is a sensible step, and the shea-for-squalane swap does move the required HLB. But required-HLB matching describes one oil phase at one moment. It has no term for ionic strength, shear history, cooling rate or batch size, so each of the client’s changes could move the system away from its original match without the number showing it.
What a placement (TDA Logic) reading asks
How much ionic strength is in the water phase?
Sodium PCA is a 1:1 salt with a molar mass of about 151 g/mol. At 1.0% it supplies roughly 10 g/L ÷ 151 g/mol ≈ 0.066 mol/L, that is about 66 mM, and an ionic strength of about the same value. For a 1:1 electrolyte at 25 °C the Debye length is approximately 0.304/√I nm, which gives 0.304/√0.066 ≈ 1.2 nm, compared with about 9.6 nm at 1 mM [2]. See colloid science for the background.
Sodium phytate adds more than its weight suggests. Ionic strength is I = ½Σcizi2, so each ion is weighted by the square of its charge: an anion carrying six charges counts 36 times as much, per ion, as a monovalent ion. Aloe preparations bring dissolved mineral salts, and citric acid with its neutraliser adds more. The placement checklist counts every one of these, humectant salts and chelators included [1].
For a lamellar network the effect is direct: electrolytes compete for the water that hydrates the emulsifier head groups and can loosen the gel network that gives the cream its structure [3].
Has the oil load changed for the same interface?
Under TDA Logic, added oil shares an interface of fixed size, so the oil allowance is a budget. The analogous question here: the emulsifier level stayed fixed while crystalline shea butter, which helps reinforce the network with fat crystals, was replaced by squalane, a nonpolar liquid oil. The liquid oil that the same amount of emulsifier must hold went up, and part of the reinforcement went away.
Does the pH still suit every ingredient?
Dehydroacetic acid acts mainly in its undissociated form (pKa ≈ 5.3). By Henderson–Hasselbalch, the undissociated fraction is 1/(1 + 10pH − pKa): about 67% at pH 5.0, 39% at pH 5.5 and 17% at pH 6.0.
What the checklist does not cover
Process history sits outside both frameworks’ arithmetic. At scale, a larger batch cools more slowly through the region of roughly 55–40 °C where the gel network forms, and shear in that window can disrupt the network as it builds. This is the third place to look.
Practical fix
- Reduce sodium PCA to 0.3–0.5% (about 20–33 mM) and move the humectancy to glycerin.
- Reduce sodium phytate to 0.1%.
- Add 1.5–2% cetearyl alcohol to rebuild the network, with 0.2–0.3% xanthan gum for support.
- Homogenise hot, then use a low-shear sweep during cooling; match the laboratory cooling rate at production scale.
- Compare laboratory and production batches by polarised-light microscopy, where Maltese crosses indicate lamellar liquid-crystalline structure.
- Hold pH at about 5.0–5.5 for the dehydroacetic acid, and run preservative-efficacy testing on the production batch rather than the laboratory batch.
Takeaway
Small changes to a fixed emulsifier system need every budget recounted, not the oil-phase arithmetic alone. Counting all salts, including humectant salts and chelators, and recounting the oil the same interface must carry, would have flagged this cream 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
- Israelachvili JN (2011). Intermolecular and Surface Forces, 3rd ed. Academic Press.
- Eccleston GM (1997). Functions of mixed emulsifiers and emulsifying waxes in dermatological lotions and creams. Colloids Surf A 123–124:169–182.