If we're talking about natural, there really isn't a natural surfactant out there. We can find green or something certified, like ECOcert, Cosmos, or Natrue, but there's nothing that hasn't gone through quite a lot of processing in a factory.
Decyl glucoside, a non-ionic or neutrally charged liquid surfactant, is an ECOcert ingredient, but I don't think it's an option for a bath bomb as it's a liquid, and not a very good bubbler. It's a foamer, but if you're looking for a bubbling bath bomb, this won't give you the big, quick bubbles like something like SLSa will offer. It can act as a solubilizer as it's a good one. The down side of this ingredient is that it's a liquid, which could be too much in some climates, especially humid ones like mine. You could try adding a bit of it - maybe 5% - to the bath bomb to see what you think, but I worry it'll be too wet and sticky.
You could use something like Bioterge AS90, powdered C14-16 olefin sulfonate, as it's a great bubbler with flash foam, but it isn't considered green.
caprylyl/capryl glucoside (CCG). It's a substitute for polysorbates. There is a huge down side to it; It's incredibly sticky. I mean super sticky. A drip dropped down the side of my bottle, and it took some of the wooden surface of my workshop table with it. I've been working with it in micellar waters, which is going well, but I wouldn't have it in my workshop otherwise as it's just so sticky in just about every product in which I've tried it.
So the short answer is that you could try a little decyl glucoside, but I don't think it's the best choice.
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