Incompatibility of salicylic acid & ethyl lauroyl arginat HCl

Tagged: ethyl-lauroyl-arginate-hcl, incompatibility, salicylic acid

Formulating

Incompatibility of salicylic acid & ethyl lauroyl arginat HCl

Posted by Abdullah on June 1, 2022 at 10:05 am

I am trying to make a solution of salicylic acid in salt from+ ethyl lauroyl arginat HCl which is cationic.

Formula
Salicylic acid 2%
Caustic soda 0.58%
Ethyl lauroyl arginat HCl 0.2%
Phenoxyethanol 0.4%
EDTA 0.008%
Water

I make water, salicylic acid, caustic soda, EDTA & phenoxyethanol until salicylic acid is completely dissolved. Solution is clear.
When i add ethyl lauroyl arginat HCl it becomes cloudy and something very sticky is formed that is very difficult to remove even with dishwashing liquid.

I have used other methods too. Mixing ethyl lauroyl arginat HCl in water first, mixing it in glycerin then in water, using sodium salicylate instead of salicylic acid but in all of them this reaction happens between salicylic acid and ethyl lauroyl arginat HCl.

My question is: are these two ingredients incompatible?
If yes then why?

Or am i doing something wrong?

Abdullah replied 2 years, 6 months ago 2 Members · 9 Replies
9 Replies

ketchito

Member
June 1, 2022 at 11:38 am

@Abdullah You’re mixing an anionic molecule (sacicylic acid) with a cationic surfactant (arginate). I think you’d have more success if you switch the percentages of those two molecules, but I believe you have thay level of SA for a reason. Why not using an anionic surfactant? Some have a very synergistic effect with SA.
Abdullah

Member
June 1, 2022 at 1:55 pm

Thanks @ketchito

I didn’t know salicylic acid was anionic.

These two ingredients are both effective for acne so i wanted to use both of them in a single product to make it supper effect .

The goal is to sell the product as a simple leave on liquid solution to minimize the cost.

Are you suggesting an anionic surfactant to make the product a cleanser or there is another reason for using that?
Abdullah

Member
June 1, 2022 at 1:57 pm

And is there any general rule to know which ingredient is anionic, cationic or non-ionic?

I did search a lot about salicylic acid charge in Google but couldn’t find any information about it.
ketchito

Member
June 2, 2022 at 11:55 am

@Abdullah Since salicylic acid is an acid, you need to consider its pKa. If your pH is below SA’s pKa, then SA will be present as an acid (protonated), but if the pH is above SA’s pKa, then it will exist in its unprotonated form, that is, hydronium ions (positive) and salicylate ions (anionic), both swimming around the solution. That works for all acids.
Abdullah

Member
June 2, 2022 at 12:10 pm

ketchito said:

@Abdullah Since salicylic acid is an acid, you need to consider its pKa. If your pH is below SA’s pKa, then SA will be present as an acid (protonated), but if the pH is above SA’s pKa, then it will exist in its unprotonated form, that is, hydronium ions (positive) and salicylate ions (anionic), both swimming around the solution. That works for all acids.

Thanks

Is this rule only for salicylic acid or for everything?

For example: lactic acid and glycolic acid at pH 5, will it also be mostly in anionic form?
ketchito

Member
June 3, 2022 at 11:57 am

@Abdullah It’ll depend on each acid’s pKa.
Abdullah

Member
June 3, 2022 at 1:30 pm

ketchito said:

@Abdullah It’ll depend on each acid’s pKa.

I mean is this conclusion correct? “whatever amount of an acid that is above the pka will have negative charge?
ketchito

Member
June 6, 2022 at 12:02 pm

@Abdullah It depends. If you’re slightly above the pKa, you might find part of the acid protonated, and part unprotonated. The higher the pH (above pKa), the higher the amount of unprotonated acid.
Abdullah

Member
June 6, 2022 at 2:40 pm

ketchito said:

@Abdullah It depends. If you’re slightly above the pKa, you might find part of the acid protonated, and part unprotonated. The higher the pH (above pKa), the higher the amount of unprotonated acid.

Thanks