Free vs Bound Water

Cafe33Cafe33 Member
edited February 12 in Formulating
I am making Syndet bars using ground SCI as a main ingredient. My formula also contains 2% Sodium Lactate 60% solution. That means 1.2 % actual Sodium Lactate and 0.8% of Water. 

1st Question - Am I right to assume that this 0.8% water is bound by the Sodium Lactate even if it is added to an anhydrous mixture of dry powder surfactants? Does it essentially stay bound? 

Aside from this 0.8% water, my formula also contains 5.60 % free water from CAPB. 

2nd Question - When water is used as a binder, is it considered bound or is that simply a term for free ions dissolved in water?

I have on hand Sodium Chloride, Sodium Citrate and Silicon Dioxide (Colloidal and regular)

3rd Question - How do I measure binding power of the different salts? 

I know that 1g of water dissolves 0.35g of NaCl from a simple google search. 

4th Question - Does this mean that 0.35 g of NaCl binds 1.00 g of water or is there more to it?

5th Question - Are there any potential complications to adding any of these salts directly to CAPB? Would using high amounts of Sodium Citrate lower the pH too much?  

I know these are a lot of questions but I would really appreciate some answers. I have been reading many books and running many tests, but sometimes answers to simple questions are difficult to find on my own.  


  • I have been reading many books and running many tests, but sometimes answers to simple questions are difficult to find on my own.  
    I agree with you on that! Sometimes one head is not better than two or more than two. Also, whatever we read can be very biased.

    I think, what you are trying to achieve is to harden SCI bar.

    I do not know whether or not the word 'binding' applies here because it looks like it is about ions, to me. Maybe is ionic bond.

    Binding usually is used on thickeners. Such as all kinds of 'cellulose', 'gums', gelatin, natural or synthetic. You can find these in tablets and capsules.

    The said 0.8% water in Sodium Lactate tends to evaporate from (some surfactants). This is the reason why most surfactant systems have humectants such as Glycerol to prevent pump nozzle from bring obstructed as the solution dries. You can see some shampoo/wash can end up like snot and 'dries' there.

    Sodium Citrate is basically pH neutral. No matter how you add it in anything, that anything stays at the same pH.

    I never make syndet bar but I used SCI, however I did not notice the effect of adding NaCl directly to CAPB. (Because I did not bother :D )

    Only yesterday did I notice that SLES and CAPB combination is enough to thicken the mixture without NaCl. Very awesome! This phenomenon is vanished as soon as more water is added (to make the final product up to 100% or 100g).

    I do not understand what you meant by "my formula also contains 5.60 % free water from CAPB". Does that mean the CAPB you have is actually 94.4% purity? Is this high purity even possible?  :open_mouth:

    It could be similar to products that claim to have 20%, 60%, 80%, or more Sodium Hyaluronate/Hyaluronic Acid. Absolute nonsense. Fair enough they say the "nano" one has the lowest molecular weight, can penetrate the skin deeeeeeeep into the dermis. None of them dares to come up and prove how much SA they actually can mix with water, not even with the nano one. Oh sure, now they say SA is too expensive to merely prove. Excuses upon excuses.  :|

  • Thanks for your response. I am using 30% Active CAPB. I have access to 37% and around 47% if Im not mistaken. I have to buy 200kg and 800kg of each respectively. I also have access through Evoniks to dry CAPB which is around 80-82% Active. 

    I am using 8% CAPB which corresponds to 5.60% water. The book I am reading named Soap Manufacturing Technology, water is referred to as a binder. It is in fact a binder. 

    My goal is to increase dish stability, I dont want the bar to get runny when exposed to water. The hardness has been good, just looking for the best formulation.

  • chemicalmattchemicalmatt Member, Professional Chemist
    The term free water may be misleading, since Aw (water activity) is a factor of all constituents involved with or interacting with the water present. I always use the term "bound water" to mean the hydrate - or water molecules - actually inherent in the material itself.  E.G. Borax decahydrate has 10 moles bound water.
  • Bill_TogeBill_Toge Member, Professional Chemist
    edited February 16
    what you call the 'binding power' of a salt is its solubility in a given solvent - there are many methods to determine this, simple and sophisticated, and solubilities of relatively common substances are widely published
    when the term 'binder' is applied to cosmetics and other formulated mixtures, it is typically used to mean a substance which acts to keep the mixture physically homogeneous; it is an entirely separate meaning which has nothing to do with the substance's power to solvate
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • Cafe33Cafe33 Member
    edited February 16
    Thank you chemicalmatt. 

    Aw (water activity) is a ratio of water vapor pressure of the sample over the vapor pressure of pure water itself.  So it is slightly more complex than what I had previously thought. Can a lab used for challenge testing measure Aw of a sample? I am about to send multiple samples for challenge testing.

    So NaCl lowers vapor pressure of water and so do other salts such as Sodium Sulfate, Sodium Lactate and Sodium Citrate.

    What about something like a powdered surfactant such as sodium coco sulfate? Would that also lower the vapor pressure of free water and in what way?   

  • Can a lab used for challenge testing measure Aw of a sample?
    As far as I'm aware, testing Aw requires specialised equipment, you'll have to ask your lab if they have it or not.
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