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Why does salt thicken shampoos?

When I first got into the cosmetic industry I worked on hair care products, specifically, shampoos. One of the things that I found fascinating about shampoos was that if you added salt to them they would get thicker. I later found that if you added too much they get thinner. It turns out there is a range of salt concentrations at which a shampoo formula will get thicker or thinner. We call this the Salt Curve and it is an important thing to know because it can help save many production batches.

Salt Curve and Shampoos

To understand why salt affects the viscosity of a shampoo (or body wash) you have to first realize that shampoos are mixtures of surfactants that arrange themselves in tiny structures called micelles. The viscosity of the shampoo solution depends on the size and packing structure of these micelles.

Since shampoos are typically made from anionic surfactants the outer surface has a specific charge density. This will affect the way that the micelles can pack together. A higher charge density will cause the micelles to repel and result in a thinner solution.

The sodium ions from the salt lower the charge density of the micelle surface. This makes them more able to pack closer together and creates a thicker solution. This is why salt thickening is really only affective with anionic-based or anionic/nonionic-based shampoos.

Micelles and salt

While salt can help increase the viscosity of a shampoo system, it works differently in different formulas. Some systems will be highly tolerant of salt while others get thin as water with just a small increase in salt concentration. This is because the size of the micelles is dependent on a number of factors including

  • Concentration of surfactants
  • Type of surfactants
  • Ratio of surfactants
  • Temperature
  • Charge density

The way salt reduces the charge density is that it pushes the dissociation equilibrium of the surfactant to the left. As an example, consider this dissociation equation for Sodium Lauryl Sulfate.

C12SO4Na = C12SO4- + Na+

More sodium ions push the equilibrium to the associated state.

This drops the micelle charge density and the size increases. This in turn causes more micelle agglomeration which can ultimately lead to lammellar structures which can form a gel.

Creating a salt curve

When formulating an anionic cleansing system, it is useful to create a salt curve so when your manufacturing people present you with a product that is too thin, you’ll know exactly how much salt they should add to get the right viscosity. Here’s how you do it.

1. Make a 500 g batch of shampoo (or body wash) and leave out the salt.

2. Split the batch into ten 50g samples.

3. Add salt levels in the following increments. (0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%)

4. Record viscosity & plot viscosity versus concentration.

Most systems should not tolerate much more than 2% salt levels. Based on the results you can refine your salt curve to figure out even tighter % levels.

Each formula type you make should have a salt curve like this so you can tell Production how much they’ll need to add to adjust the viscosity. You might also consider doing the same thing with the fragrance as it can affect the viscosity in a similar way.

Why?

We’ll save that answer for another time.

{ 36 comments… add one }

  • tywo 01/26/2015, 2:01 am

    thanks all for the wonderful contributions……i also experience similar situations often.
    i work with sls, cdea, nacl and the end result is fantastic but as soon as i add fragrance it becomes watery drastically. as a matter of fact, i thought its due to a particular fragrance until i experiment with 6 different fragrances but are the same, become watery..
    lastly mr perry, keep the good work and more grease to ur elbow. i include stpp in my formulation becos of water hardness what can u say to that?
    you are doing a gr8 work sir, i planned to attend one of ur seminar last yr but was denied travelling visa but hopefully i will take one of ur online course this yr.
    thanks all

  • FREDDY ROJAS 12/31/2014, 2:02 pm

    I am writing to ask if you teach how to make shampoo formulas and the method to mix every ingredient. I am really interested in this and I there is no any school to teach it. I would be very grateful if you could give me any additional information or suggest me places where I can get some lessons.

    I am looking forward to hearing from you,

    FREDDY ROJAS

  • C.S.Rajagopalan 10/28/2014, 9:59 am

    Sir

    I am using Disodium lauryl sulphosuccinate as the primary surfactant (36%) using 1.1 % PEG -150 distearate in my Face wash preparation. I have taken consecutive batches and dont find any problem in my viscosity. One fine day having doing the formulation with the same thickener with the different lot of Dosiu-Lau-sulphosuccinate I found the final viscosity dropped down drastically. I knew the problem is due to the sulphosuccinate and this has happened. If such things are happening intermittently what shall we do because we cannot alter the formulation again and again. Please advice…

    • Perry Romanowski 10/30/2014, 11:43 am

      Find a better, more reliable supply of Disodium Lauryl Sulphosuccinate.

    • sulastomo 12/31/2014, 1:46 am

      usually if using sulphosuccinate surfactant it’s difficult to thickening, my opinion you can
      using polymer (acrylate polymer or crosspolymer or cocamide (MEA, MIPA or DEA).
      But some time polymer can cause sticky or stretching feel and cocamide type can cause clean and dry so you have to consider about moisture agent.

  • zahra 09/02/2014, 10:28 am

    I already made my shampoo, just when I add fragrance , viscosity drop too much. I try with different range, different types, and Solvent?

    • Perry Romanowski 09/02/2014, 12:53 pm

      Well, you need to use a different fragrance or less of it.

    • Azeem 11/28/2014, 2:24 am

      Please check the pH of Fragrance and the solution you added for. Also check the inoic nature of surfactant you use with the ionic nature of Fragrance.

    • sulastomo 12/31/2014, 1:56 am

      some time fragrance contain solvent to soluble fragrance on surfactant like PEG-40 hydrogenated castrol oil or polysorbate-20 (example and many more). i prefer to add polymer or co-surfactant to reduce effect of fragrance.

  • Eric Karanja 06/20/2014, 2:18 am

    Hallo

    Kindly advise me on the eco-friendly ingridient to use when making a toilet bowl cleaner liquid,

    Inform me on the acid to use, essential oil, enzymes, antispetic, stabilzers n sufracant to use.

    Regards

  • Nihal 05/26/2014, 12:31 pm

    Hello Perry!

    Your book ‘conditioning agents’ is amazing! I’m a student studying Cosmetic Science and am doing a little research on viscosity and stability. My project is on hair shampoos in which we used sodium chloride for a thickener due to the anionic sodium laureth sulfate. We take measurements every week on viscosity in which it varies (decreases or increases?) I am trying to understand on why does this happen? :(

    Thank you for your time!

    Nihal

    • Perry Romanowski 05/29/2014, 6:40 pm

      Thanks for the kind words. There shouldn’t be much change in the viscosity of the system you talk about unless there is some water evaporation. See our post on salt curve analysis.

  • sandeep 04/25/2014, 9:56 am

    Tetrasodium pyrophospate in what ph thick to liquied detergent.

    • Perry Romanowski 04/28/2014, 7:01 am

      I don’t know. You have to test it in your specific formula.

  • Docblob 01/04/2014, 6:32 pm

    Does it make a difference f I use sodium chloride or magnesium chloride as a thickener (in a SLES/LAO-System)?

    • Perry Romanowski 01/05/2014, 8:12 pm

      Sodium chloride would be better.

      • nasrin 02/27/2014, 4:25 am

        why more electrolyte drops the viscosity drastically ?

        • Perry Romanowski 03/04/2014, 6:58 pm

          That’s just how nature works. If you put in too much electrolyte you reduce the ability of micelles to closely pack.

  • ashish1234 09/20/2013, 2:41 am

    Hello to all,
    I have the following problem in shampoo!!
    I am making a shampoo formulation with following ingredients:

    1. Carbopol Aqua SF-1:6-7%
    2. SLES: 30% as supplied
    3. NaoH (for neutralization)
    4. CMEA:2%
    5. Cetyl alcohol:0.5%
    6. Laureth-4:0.5%
    7. Brij 35:2.5%
    8. CAPB:5%
    9.Amodimethicone:2%
    10. Sensomer: 0.1%
    11. Propylene glycol:3%
    12. Panthenol:0.6%
    13. Parfume:q.s
    14.Preservative:q.s
    15.Water: q.s

    The viscosity of the final formulation before addition of citric acid comes out to be 3000 cp at 5o/sec shear rate. The pH of the product is 7.5 before citric acid addition.The formulation is homogeneous without any air entrapment of foam. However when I add the citric (or even lactic acid) to the formulation to bring the pH down to 5-6, there occurs drastic change in viscosity and goes down to 800 cp. Moreover milk like curdling occurs during addition of citric acid and we don’t know how foaming occurs in the whole shampoo formulation even at low rpm’s. It is said that Carbopol Aqua SF-1 gives back acid thickening but in our case we are finding thinning of the system, sudden appearance of foam, milk like curdling and entrapment of air.!!

    Please suggest me what can i do to adjust pH in a way that original consistency and characteristic of shampoo are maintained and no air or foam entrapment occurs.What can be the possible reasons for this effect of pH on Carbopol Aqua Sf-1.!!

    Thanking you all and waiting for reply..!!

    • nasrin 02/20/2014, 6:20 am

      whats brij and capb?

      • Perry Romanowski 02/20/2014, 11:17 am

        These are types of surfactants

    • sulastomo 12/31/2014, 2:18 am

      my suggestion there no effect of citric acid on carbopol aqua SF-1, it caused by Amodimethicone, because amodimethicone is very difficult to thicken. if you are still want using amodimethicone you should consider about pH, thickening agent (prefer polymer) if you are does not want using polymer you can emulsified amodimethicone with emulsifier and hydrotrop first then added into shampoo base.Process should be like this :
      1, 2, 3 soluble and melting at 70 oC
      1. SLES: 30% as supplied
      2. CMEA:2%
      3. Cetyl alcohol:0.5%
      4. Water

      added
      5. CAPB:5%
      6. Carbopol Aqua SF-1:6-7% + Water 5%
      7. Propylene glycol:0.5%

      Emulsifier
      8. Laureth-4:0.5%
      9. Brij 35:2.5%
      10.Amodimethicone:2%

      11. Sensomer: 0.1%
      12. Panthenol:0.6%
      13. Parfume:q.s
      14.Preservative:q.s

      15. NaOH (for neutralization) it’s better using Triethanol amine sensory feel better than NaOH.
      16.Water: q.s

      pH optimum at 4.5 – 6.5 for conditioner.

  • Jerry 07/18/2013, 5:53 am

    Thanks for a very informative article.

    We are making an SLS and paraben free shampoo using a base (decyl glucoside, coco betaine). It is perfectly thick and works great EXCEPT when we add the fragrance it turns completely into water. Salt doesn’t seem to work. We’ve been using Xanthan Gum to thicken but in order to get a good viscosity we have to put in so much that the shampoo gets slimy.

    Any advice on thickening an SLS free shampoo like ours?

    (I’ve read that we can try and mix the fragrance with DPG (dipropylene glycol). Do you think this will help? Or is there something else we can do that may be easier/simpler? (I’ve even bought thickeners from various vendors to no avail).

    Thanks!

    • Perry Romanowski 07/18/2013, 9:15 am

      You might try less fragrance or a different fragrance.

  • Ghassan 05/25/2013, 7:04 pm

    How do we raise the viscosity of shampoo during manufacturing?I have used the increase of salt and excessive amount of Coconut di ethanol It has got a temporary wife then disappear to become a new shampoo thin

    • Perry Romanowski 05/28/2013, 9:58 am

      This really depends on what else is in your formula.

      • Ghassan 05/28/2013, 7:54 pm

        The combination shampoo normal adopted by simple they consist of: Lauryl sulfonic acid (solution 70%) + water + salt + dissolution within 24 h (with stirring violent) = output + Tri ethanol Amin + additions: glycerin + Conditioner + propylene glycol + materialportfolio + perfume + a few of methyl alcohol + iso propyl Merestat + silicon oil emulsion vitamin + Other Option depending on the nature of poetry.And finally the pH calibration.

    • Ghassan 05/28/2013, 5:46 pm

      Thank you
      But I have an additional question, please, can I replace the salt sodium chloride whith ammonium chloride salt in the same proportion?
      Can I use magnesium sulphate with Lauryl sulfonic acid to get the shampoo does not tear the eyes? And if so; does add salt (sodium or ammonium) or rest on it and realized viscosity too?
      Finally Can I use tylose the viscosity to raise? When can I hold added without that happening during the conglomerate added?
      Then what is the damage to the hair of this added?

  • Frank 02/14/2013, 11:10 am

    Pls perry, i have made moisturizing body wash for more than 3 times now, but keep getting a thin result. This is my formula: water 72%, sles 15%, betaine 3%, HEC .5%, edta .2%, glycerine 2%, pearlizer 1%, sheabutter 2%, stearic acid 2%, castor oil 1%, citric acid, fo, preservative . But i find it hard thickening it. Pls how do i thicken. Or which thickener should i use?

    • Perry 02/14/2013, 11:14 am

      You could increase the level of HEC (up to 1%) or reduce the amount of oil in the formula (less sheabutter, less castor oil). I would encourage you to do a knockout experiment to find out what is going on in your system.

  • Jerome 08/29/2011, 2:57 pm

    if you have a shampoo base that immediately looses viscosity upon adding salt or perfume, where do you go next?

    • Perry 08/29/2011, 3:03 pm

      That really depends on how much salt you’re talking about. There are two things you can do..

      1. Lower the amount of salt (or perfume) added
      2. Increase the level of surfactant & secondary surfactant

      It’s difficult to give exact answers to this question because it is very dependent on the type of surfactants used.

  • Jerome 08/15/2011, 2:53 pm

    Thanks for the Blog. Can you talk more about what you mean by the ratio of surfactants. Also, is it necessary to add something to stabilize the Micelles?

    • Lourdes 06/21/2013, 4:57 am

      hi,
      i used excess amount of salt and it made my car shampoo thinner. to avoid loss in my production, how do i save this batch? i used sles, cdea as main ingredients. do i need to add some more sles or just water. pls help.

      • Perry Romanowski 06/23/2013, 3:06 pm

        See our article on saving batches. Basically, you make another batch with no salt and mix the two together in the proper ratio to get the thickness you want.

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