ingredient compatibility and pH

natiyo123natiyo123 Member
edited August 18 in Formulating
Hi, Im new to formulating cosmetics & recently got new samples to use in deodorizing/cleaning formulations but Im having trouble understanding the suitable pH:

sodium lauroyl lactylate
zinc lactate
zinc PCA

Ive searched in ulprospector TDS and some articles to try to understand more but Im not finding enough info...

1) SLL is anionic, yet Ive seen some products that have citric acid or lactic acid in them? 10% solution in water is acidic, wouldnt it be more compatible in basic pH bc of the (-) ? also, HLB is aprox 14, so why is it not very soluble in water?

2) I understand Zn2+ is antimicrobial and works better in acidic formulas, but why is it considered a neutral salt? could it still be antimicrobial in basic solution?

PS: I still dont have a formula yet, bc I dont understand how to use them , but I would like to use them (separately) in very acidic or very basic pH in same or 2 different formulas

Comments

  • PerryPerry Administrator, Professional Chemist
    You seem to have a misunderstanding of what pH means.  
    I would suggest you first go through this video about pH.

    1.  SSL is a salt. In water its ions dissociate into Na+ and Luaroyl Lactate-. There is an equilibrium that happens and changing the pH can affect the solubility of the Lauroyl Lactate, but not much. 

    pH and solubility are independent factors. While you can adjust the solubility of some compounds with pH, nothing you do to pH will affect the solubility of a compound like mineral oil. 

    Also, HLB does not really apply to charged surfactants. HLB is most relevant to nonionic materials. When there is a charged ion, additional factors that affect solubility come into play.

    2.  See this video about dissociation of materials in water.  Basically, in a more basic solution you push the equilibrium of the dissociation of Zn (Lactate) or Zn PCA more towards the salt side which means in a more basic solution there are less Zn2+ ions available to provide the antimicrobial effect. 
  • PharmaPharma Member, Pharmacist
    natiyo123 said:
    ...
    1) SLL is anionic, yet Ive seen some products that have citric acid or lactic acid in them? 10% solution in water is acidic, wouldnt it be more compatible in basic pH bc of the (-) ? also, HLB is aprox 14, so why is it not very soluble in water?

    2) I understand Zn2+ is antimicrobial and works better in acidic formulas, but why is it considered a neutral salt? could it still be antimicrobial in basic solution?
    ...
    1) Because lactylate esters are anionic but not very strong acids/bases, their apparent HLB depends on pH.  As an examle, Evonik lists sodium stearoyl lactylate as HLB 10-14, foodgrade SSL has a HLB of 8.3 (partially neutralised), and theoretical HLB of SSL would be 17 (100% salt form). At very low pH SLL and SSL turn into lauroyl and stearoyl lactylic acid, respectively, and get a HLB of something in the range of maybe 4. Well, they would hydrolyse over time...
    Using HLB is stupid because A) using HLB is stupid per se, B) lactylates differ greatly between manufacturers and often contain mixtures of fatty acids and number of lactic acid monomers, C) apparent HLB change is most pronounced in the cosmetic pH range, and D) lactylates are often used as liquid crystal formers and that stage lays outside the rationally calculable space of commonly used theories. Furthermore, 'standard' lactylate has 2 units but pure sodium fatty acid-2-lactylates are seldom used in cosmetics, hypothetical calculations are hence compromised from the beginning.
    Lactylates are surfactants and surfactants are usually poorly soluble in water. What they do is forming micelles. As mentioned, the strength of lactylates isn't in their micelle forming ability but in forming alpha gels, lamellar structures which don't look like solutions but emulsions/creams.
    BTW just because a product contains citric or lactic acid doesn't automatically make it acidic ;) . Who knows what else is in there and which starting pH raw materials have (several detergents are fairly alkaline).

    2) It's a neutral salt if its partner also forms neutral salts. That's a rule in chemistry. Zinc ions at higher pH turn into hydroxides and oxides (and carbonates if CO2 is present). These are poorly soluble and hence inactive. Even if you put some sodium hydroxide in a solution of zinc sulfate (more 'neutral' than Zn PCA and Zn lactate which aren't quite neutral), it turns cloudy because of precipitating zinc hydroxide.
  • very thorough explanations,

    thanks for the answers
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