Home Cosmetic Science Talk Formulating Precipitation in BHA Toner

  • Precipitation in BHA Toner

    Posted by doreen on August 31, 2017 at 2:38 pm

    My formula

    Water Phase
    q.s. Deionized water
    1% Salicylic Acid
    1% Sodium Citrate
    20% Propylene Glycol
    0,5% Allantoin
    0,3% Phenoxyethanol (should be water soluble to 0,5%)
    0,15% Potassium Sorbate
    0,2% Disodium EDTA

    Oil Phase
    3% PEG 40 Hydrogenated Castor Oil
    0,1% α-Tocopherol
    0,1% Tocopherol Acetate
    0,3% 
    α-Bisabolol (natural)

    - I’ve made it once with all the ingredients above: precipitation starts in about 5 days;
    - I’ve let out the allantoin: again within 5 days;
    - I’ve let out the whole oil phase (because I can’t get the liquid transparant enough) ánd the allantoin: same story…

    The formula has a pH of about 3.9-4.2

    * What could be the problem here?
    * pH 4.2 is a wee bit too high for my liking (I want it between 3.5-4.0), how do I adjust it with the buffer present?

    em88 replied 6 years, 5 months ago 7 Members · 19 Replies
  • 19 Replies
  • belassi

    Member
    August 31, 2017 at 6:00 pm

    Possibly there is a replacement reaction taking place. I would obtain a sample of the precipitate and analyse it. Obviously, the precipitate must be a compound that’s fairly insoluble in water. If you wash and dry the precipitate then you could first do a flame test to see if it’s a sodium or potassium salt.

  • bill_toge

    Member
    August 31, 2017 at 6:24 pm

    my money’s on salicylic acid

  • doreen

    Member
    August 31, 2017 at 8:17 pm

    Thanks @Belassi and @Bill_Toge
    The problem is almost certainly the salicylic acid…
    Someone sent me this link, if I interpret the graph of table 2, to dissolve 1% salicylic acid I would need at least 40% of PG in a binary solvent system with PG and water! 
    (f1= 1.0 = 100% water, f1 = 0.0 = 100% PG)

  • bill_toge

    Member
    September 1, 2017 at 1:09 pm

    @Doreen81 and those figures were obtained at 25°C: if you want it to pass stability tests at low temperatures, you’ll need more PG than that

  • doreen

    Member
    September 1, 2017 at 6:28 pm

    @Bill_Toge
    Even more?! I really need another solvent… Wish I could get methyl gluceth-20 here.

  • beautynerd

    Member
    September 1, 2017 at 11:54 pm

    @Doreen81

    Can you get isopentyldiol from the Soap Kitchen?

    Solubility of salicylic acid with that @25°C is 20%. 

  • doreen

    Member
    September 3, 2017 at 4:17 pm

    @EliseCortes
    Thanks for you suggestion, but can’t get it where I live. Just like octyldodecanol etc.

  • doreen

    Member
    September 3, 2017 at 4:22 pm

    As long as I can’t get better solvents, i’m thinking of adding 2% ethanol to the formula.

  • DRBOB@VERDIENT.BIZ

    Member
    September 3, 2017 at 5:16 pm

    Try propanediol or DMI should work.

  • doreen

    Member
    September 4, 2017 at 8:26 am

    @DRBOB@VERDIENT.BIZ
    There is only one supplier here that sells propanediol, 108 euro’s (about 130 dollars) for only half a litre, it’s just absurd… 

  • markbroussard

    Member
    September 4, 2017 at 1:15 pm

    @Doreen81:

    Are you pre-dissolving the SA first in Propylene Glycol (heated to 70C) and then adding that slowly to a heated solution containing water and your other ingredients?  Or, are you just trying to add this all in one pot?

    Also, you might try adding in 2% Polysorbate 80.

  • doreen

    Member
    September 4, 2017 at 2:43 pm

    Hi @MarkBroussard ,

    I measure the tare weight of the beaker for water loss during heating, put the water in, heat it to 70C and add the ingredients that aren’t heat sensitive individually. Adding the buffer to the mix really speeds it up. 

    I will definitely try your suggestion, thanks.

    Can you tell me how I should adjust the pH with a buffer present?
    I would like to lower it a bit, but it seems useless because of the buffer resisting pH changes. Don’t want to add an awful lot to lower it.

    p.s. Maybe this is also important, forgot to mention it: I keep it below 7C, in the fridge. 
    Crystals start ‘growing’ on the bottom of the bottle in some sort of sticky residue. The particles are tiny shards, shaped evenly.

  • doreen

    Member
    September 4, 2017 at 3:47 pm

    btw the sticky residue is also there when I leave out the oil phase.

  • markbroussard

    Member
    September 4, 2017 at 6:01 pm

    @Doreen81:

    Try the way I suggested.  Why are you keeping it in the fridge? … that may be your problem with the SA crystalizing out.

    Add a little Citric Acid if you need to lower the pH

  • belassi

    Member
    September 4, 2017 at 7:39 pm

    The particles are tiny shards, shaped evenly.
    Examine the shape with a x100 eyepiece and compare against the crystal shape of salycilic acid.

  • doreen

    Member
    September 5, 2017 at 10:44 am

    @MarkBroussard
    I keep most of my skincare products in the fridge, to inhibit microbial growth. I’ll try your suggestion and keep it at room temperature!

    @Belassi
    Good idea! Glad I still have a microscope. :) 

    Thanks!

  • doreen

    Member
    September 25, 2017 at 11:02 am

    @MarkBroussard
    It’s almost a month ago since I made the last batch and… still no precipitation!
    Obviously it was very important to predisperse the SA in propylene glycol  (instead of dissolving it in water at >70C) because I also left one in the fridge for comparison. Both solutions are still crystal clear. :-) Thank you so much for your help.

    p.s. Luckily I can get my hands on better (less irritable) solvents: butylene glycol and maybe (1,3-)propanediol. What % would you advise if I use both or only butylene glycol (in water)?

  • markbroussard

    Member
    September 25, 2017 at 9:04 pm

    My pleasure! … Glad to hear it worked for you @Doreen81

  • em88

    Member
    September 26, 2017 at 7:23 am

    Salicylic acid at 1% level should be easy to deal with.
    Dissolve 1% SA in 2.5% ethanol and add 17.5% PG No heat needed. 

Log in to reply.