Why is salt (sometimes) added at the beginning?

Hello everyone,

This is a question I have been curious about for a while and have not been able to find concrete answer to: Why in formulae is salt addition occasionally broken into multiple stages of addition (typically before the primary surfactant)?

For ex (not an actual formula just threw values out for the relevant raw materials):

A) Water                   q.s
b) SLES (70%)            15.0%
C) CAPB (30%)            5.0%
D) Salt                       2.0%

If I was making this formula I would make it as above. A -> B -> C -> D. However I often find the following variation for a similar formula:

A) Water                          q.s
b) Salt                             1.5%
C [or B.5]) SLES (70%)      15.0%
D) CAPB (30%)                 5.0%
E) Salt                             0.5%

A majority of the intended salt is added before or with SLES. Why? The initial salt is typically not enough to begin any real viscosity I have found. I have heard from another that salt can help SLES dissolve; though I have never seen any evidence of this myself trying it at lab or production scale sizes. Is it just a time management strategy? SLES can occasionally take a while to dissolve so might as well dissolve the bulk of the salt while your at it? (Though salt doesn't take that long to dissolve so is that much time being saved)? Is this just an artefact of an older manufacturing system being passed down?

I have omitted many raw materials that may be added (chelators, colours, preservatives, marketing materials) so let me know if the answer involves them.

Thanks,
RDchemist15

Comments

  • Curiosity bump
  • chemicalmattchemicalmatt Member, Professional Chemist
    I cannot think of a good reason for adding NaCl in two stages @RDchemist15. Must be a stream of consciousness thing. Regarding adding salt and other solids first: outcome will be largely the same in a simple shampoo/body wash system such as the one cited. I place salt first into water with industrial formulas where I have data accrued knowing the final salt vs. viscosity and pH outcome. Likewise for  citric acid, sodium metabisulfite, other solid-state additives. Makes the compounder's job easier and saves QC the task of making batch adjustments to release.
  • @chemicalmatt thanks for the reply. Maybe someone was trying a version of your compounding procedure and found that they still required viscosity adjustments at the end and that unpredictability made the formula "variation" permanent? Though that doesn't explain why the end adjustment would be so large or why multiple sources formulate this way. Glad I'm not missing a fundamental trick/knowledge.
  • Might be to facilitate SLES dissolving in water.  
  • @em88 Ok so you're the second person to say that then. I have never read or heard any rationale behind this though. As I said I've never noticed a difference between the time it takes SLES to dissolve between salt addition before or not. Have you?
  • MarkBroussardMarkBroussard Member, Professional Chemist
    @RDchemist15

    I suspect this is simply a formulating "style" issue ... perhaps it has to do with the dissolution of the NaCl itself as opposed to having anything to do with the SLES. 

    Regardless, there should be no difference in the end result from adding all of the NaCl in one aliquot or in two aliquots nor whether you add the NaCl at the beginning of the process or the end of the process provided you know the salt curve for the formula.
    Chemist/Microbiologist formulating in the Organic & Naturals arena under ECOCert/Natural Products Assn/Whole Foods/National Organic Program guidelines focused skincare & haircare products. 

    See website for details www.desertinbloomcosmeticslab.com
  • @MarkBroussard You are correct, I have never seen an end difference between the same formula produced the two different ways.

    Is there any way this could be a part of a plant's hurdle strategy? Adding salt early to inhibit microbial growth, particularly if it could be expected that there may be a longer "delay" between metering the water and adding the preservative in production?
  • Hardly, my guess is to prevent agglomeration when pouring SLES in water, specially if it's cold process. Done in two stages to keep the viscosity low to prevent aeration. 
  • em88em88 Member
    @em88 Ok so you're the second person to say that then. I have never read or heard any rationale behind this though. As I said I've never noticed a difference between the time it takes SLES to dissolve between salt addition before or not. Have you?
    It depends on the quantity of salt. I haven't tried it. I don't think the time reduction would be much significant.
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