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Why this formula become cloudy after 100x dilution?
This is a liquid laundry detergent formula
4% SLS
4% SLE2S
4% STPP
2% sodium hydroxide
Water
It is a clear liquid with good viscosity.
When i dilute it with 100x filtered soft water, it is still clear.
When i dilute it with 100x very hard water,(TDS 1200) it becomes hazy with some precipitation after one day.
pH after dilution is 10.
Question:
1. What is the problem? Salting or deactivation of SLS by hard water?
2. What can i do to improve it?
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11 Replies
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It’s not the dilution per se, it’s the hard water - poorly soluble Calcium phosphate.
Why is phosphate in your formula?
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It is the blooming of the STPP. When diluted in hard water it forms a white precipitate.
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Can you make a sample without sodium hydroxide? Sometimes STPP can hydrolyze in the presense of strong bases. Also, 4% of STPP is quite high, even for a very high water solution. I’d start with a lower amount to see if there’s precipitation (of course, only after the original precipitation issue has been solved).
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Thanks. I will make a sample without sodium hydroxide and see what happens.
I was adding sodium hydroxide to keep the pH of wash water @~10 when it is diluted during use. Without sodium hydroxide pH goes down to 6-7 when diluted during use.
An expert here said STPP works best at higher pH and is almost useless at pH 6-7. What is your opinion on this?
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@ketchito i made a sample without sodium hydroxide.
Product was clear. pH was 10. After 100x dilution pH was 6 and precipitation happened again.
For buffer, is citric acid+sodium hydroxide an good buffer?
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Buffers are formed by an acid and its conjugate base (in this case, it should be citric acid-sodium citrate, but it dependes on the pH you want to keep).
Nevertheless, if removing NaOH didn’t fix the issue, then the problem is different. Can you make a sample with only 0.5% STPP to see if the amount of precipitate is less? Also, usually you mix SLS with SLES at a ratio of 1:2 or 1:3 to reduce water hardness sensitivity of SLS.
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@ketchito i made two batches. One with 0.5% STPP and one without STPP. Both were clear but after 100x dilution with hard water became cloudy.
Then i added 0.01% citric acid to this dilution and they became clear.
What can be happening?
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STPP for sure has a better performance at pH higher than 7. Now, to keep the pH after dilution, you could instead use a buffer. But first check if it’s the higly basic NaOH the source of your problems.
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@abdullah Did you have the same turbidity with 4% of STPP than with 0.5% of STPP?
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@ketchito yes. Actual product is clear. But after 100x dilution in hard water they all became turbid.
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