Why use cornstarch at all? It seems to me that any amount of that will leave a film. Don't really understand your recipe. You said 100g of SLSa, that's a solid surfactant if I recall correctly; but then you have another 200g of surfactant, presumably liquid? And you've got 200g of baking soda (sodium bicarbonate)? This seems like you're half way between a bubble bar and a solid shampoo bar. What's the use of all that bicarb? It seems to me that it should have a counterpart to react with and make foam, say citric acid for instance. Bicarb: 84g = 1 mol citric acid: 192g = 1 mol.
So for each 100g of bicarb you would need exactly 100/84 * 192 grams of citric acid, or 228g of citric acid.
It seems to me that whoever 'designed' this recipe wants to use the corn starch as a binding agent but IMO it is not suitable. Better to use binding agents that are 100% water soluble. For instance, sorbitol (say 5%); also acts as a skin hydrating agent. And 0.2% EDTA as well. So my recipe would be more like:
100g sodium bicarbonate 250g citric acid 150g SLSa cocoamidopropyl betaine, Q/S to make the right consistency. FO 1% EDTA 1g
But it would be just a starting point, you understand... there is more citric than necessary to neutralize the bicarb, because the surfactant has a basic pH; you would need to adjust the final pH to around 5.5 using a sample dissolved in distilled water.
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