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Carboxylic acid pH.
I ran out of liquid soap today for the house, So I decided to make more. Most of the lab materials are packed, but I found some AKYPO RLM-45CA, and decided to use that, along with some Stepan APB and Plantapon LGC-Sorb. (It came out fine, but that’s not the issue.)
RLM-45CA is supplied as a pure carboxylic acid. I measured the pH and it was around 2.5. The NaOH and KOH being packed, I decided to neutralise it with some TEA that I’ve been using to make sanitiser.
As supplied, RLM is a thin clear liquid with a high (70% I think) ASM. It has a pleasant fruity odour. I began titrating TEA while stirring and taking frequent pH readings. First it turns milky, then clarifies. At pH 4, clear again, the viscosity went way up. Like syrup. I continued titrating with TEA and at pH 4.5 or so, the viscosity suddenly dropped again and remained fairly low until pH 5.5 which was my target pH, so I stopped there.
I should perhaps add, that RLM is not a salt-thickening surfactant; it tends to be cationic or anionic depending on pH. With acid pH it’s anionic.
I’m unsure as to why the viscosity was so pH dependent over such a narrow range. It’s annoying because it makes the effect useless for thickening; I don’t want a soap with a pH of 4.
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