Factors that influence the charge of surfactants for emulsion stability

AbdullahAbdullah Member
in Formulating
If we use ionic surfactant in emulsion for stability, what factors do influence the charge?

1. Amount of ionic surfactant
2. Ratio of ionic to total emulsifier
3. pH of emulsion
4. Type of surfactant (for example at certain pH SLS would have more charge than SSL or not)
5. Or any other factor

And for anionic emulsion, which anionic emulsifiers work well at pH 4?



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Comments

  • PharmaPharma Member, Pharmacist
    You're asking the wrong question ;) .
    A. Surfactant charge can increase emulsion stability but usually, it's the charge of the interface (called zeta potential) which has a tremendous effect on emulsion stability.
    B. Zeta potential can be influenced by many factors. Read a book.
    C. Influencing a charge of an anionic emulsifier/surfactant is basic chemistry. It boils down to pH and pKa. What does affect pH? Acids and bases. Does that directly translate to emulsion stability? No, it does not but it can.
    D: The efficacy of an anionic emulsifier doesn't solely depend on pH. Some anionic emulsifiers like SLL may work differently at a pH of 4 than they would at a pH of 6. Whether or not this is affecting emulsion stability depends on the whole system.
  • AbdullahAbdullah Member
    Pharma said:
    You're asking the wrong question ;) .
    Hopefully i am getting the right answers anyway 😉.

    A. Surfactant charge can increase emulsion stability but usually, it's the charge of the interface (called zeta potential) which has a tremendous effect on emulsion stability.
    B. Zeta potential can be influenced by many factors. Read a book.
    any book suggestion that i should read?

    C. Influencing a charge of an anionic emulsifier/surfactant is basic chemistry. It boils down to pH and pKa. What does affect pH? Acids and bases. Does that directly translate to emulsion stability? No, it does not but it can.
    does the one with lower PKa has higher charge?

    pka of SLS is -1.5 but i could find PKa of SLES, SSL and GMS citrate. Do you know how much they may be?

    D: The efficacy of an anionic emulsifier doesn't solely depend on pH. Some anionic emulsifiers like SLL may work differently at a pH of 4 than they would at a pH of 6. 
    i once did reduce the pH of SSL emulsion to 4 and it became grainy. I was using stick blender.  I don't know if it will be different with homogenizer because i only have homogenizer for large batches.
     Whether or not this is affecting emulsion stability depends on the whole system.

     
  • PharmaPharma Member, Pharmacist
    I don't think that you would buy the books I have because most of what they cover is overkill unless you study pharmacy... there are many books explaining zeta potential. Even Wikipedia might help ;) .

    Yes, lower pKa means more anionic emulsifier is actually anionic (= dissociated) at a given pH. Sulfates like SLS are kind of an exception because of the very low pKa which renders them 100% anionic under all 'normal' conditions. SLES has a similarly low pKa. SSL and GMS citrate are not chemically well defined. It is safe to assume that SSL has a similar pKa than lactic acid and GMS citrate the lower two pKa values of citric acid. However, GMS citrate is not used as salt but in undissocitated form and might not deprotonate as much as would be expected. Some suppliers even list their GMS citrate - fatty alcohol blends as nonionic emulsifiers.
  • AbdullahAbdullah Member
    Pharma said:
    I don't think that you would buy the books I have because most of what they cover is overkill unless you study pharmacy... there are many books explaining zeta potential. Even Wikipedia might help ;) .

    Yes, lower pKa means more anionic emulsifier is actually anionic (= dissociated) at a given pH. Sulfates like SLS are kind of an exception because of the very low pKa which renders them 100% anionic under all 'normal' conditions. SLES has a similarly low pKa. SSL and GMS citrate are not chemically well defined. It is safe to assume that SSL has a similar pKa than lactic acid and GMS citrate the lower two pKa values of citric acid. However, GMS citrate is not used as salt but in undissocitated form and might not deprotonate as much as would be expected. Some suppliers even list their GMS citrate - fatty alcohol blends as nonionic emulsifiers.
    Thanks a lot
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