Click here for the post on substitutions, click for the post on emulsifiers!)
What's the difference between BTMS-25 and BTMS-50?
BTMS-50 is behentrimonium methosulfate (and) cetyl alcohol (and) butylene glycol. It has 50% behentrimonium methosulfate, the active ingredient for conditioning our hair, the cationic quaternary compound. It contains cetyl alcohol to boost the substantivity of the conditioner, and butylene glycol as a humectant.
BTMS-25 is Behentrimonium Methosulfate (and) Cetearyl Alcohol. It has 25% behentrimonium methosulfate. It contains cetearyl alcohol to boost the substantivity of the conditioner. It does not contain a humectant.
A quick aside on how conditioning agents work...
The cationic quaternary compound is a positively charged compound - the behentrimonium methosulfate - that adsorbs to our hair strand to give us all those lovely conditioning qualities like a reduction in friction and combing forces, a reduction in static electricity, an increase in moisturization, and so on. Adsorption means the molecules accumulate on the surface of your hair. It's different than absorption in that it doesn't penetrate the hair fibre - it sits on top. We call this substantivity. A material that is positively charged will be attracted to the surface of our hair, which is negatively charged. The cationic quaternary compound is hydrophobic - "scared of water" - so it will resist removal by water alone. (The more hydrophobic the quaternary compound, the less likely it is to be removed by water alone.) So the positively charged cationic quaternary compound is attracted to your negatively charged hair fibre and clings on to the surface.
We can add fatty alcohols like cetyl alcohol or cetearyl alcohol to a conditioner to increase the substantivity of the product. It's suggested to add it at 50% of the conditioner amount.
If you use 8% BTMS-50, you're getting 4% behentrimonium methosulfate in the conditioner, so you'd use 2% fatty alcohol. (Use the fatty alcohol at 1/4 the amount of the cationic compound.)
If you use 8% BTMS-25, you'd be getting 2% behentrimonium methosulfate in the product, so you'd add 1% fatty alcohol. (Use the fatty alcohol at 1/8th the amount of the cationic compound.)
But here's the thing - the conditioner already has a fatty alcohol in it, so is there a point to adding more? It depends upon your hair type. I have oily hair, and I find that adding more fatty alcohol leads to greasiness in a shorter period of time. My best friend has fine, normal hair and she finds it weighs her hair down. So adding the fatty alcohol is up to you.
How to use BTMS-25 in a conditioner recipe that calls for BTMS-50?
If your goal is to get the same amount of behentrimonium methosulfate in the product, then it seems logical to suggest that you would want to use double the amount of BTMS-25. A recipe that calls for 8% BTMS-50 would require 16% BTMS-25...but I think you'd end up with something with a balm consistency instead of a liquid conditioner, so I'm not totally comfortable recommending this. Instead, I recommend making the recipe how it is written, and seeing how you like it.
I did an experiment last year to see how much conditioner my hair really needed. You can see the results here and see my new recipe here. I encourage you to conduct the same experiment at home. It really opened my eyes as to how little I really need! And you'll see that I sometimes make recipes that total more than 100%!
The short answer that in a conditioner, I recommend substituting BTMS-25 for BTMS-50 at equal amounts and see how you like it. I know this contradicts what I said in a previous post, but I've learned more about our ingredients in the last few years and I've heard from readers who have successfully made products with BTMS-25, so I feel more comfortable suggesting a straight 1:1 substitution. As for the amount of cetyl alcohol, you can reduce it by half or leave it the same. It's up to your hair type. This really is a situation in which you'll have to do some experimenting to see what works best for you!
I found this post from a while ago - BTMS-25 and products not emulsifying. Please share your thoughts!