I've been formulating a shampoo, the surfactant blend is Sodium C14-16 Olefin sulfonate and cocamidopropyl betaine, viscosity is around 3000 adjusted by NaCl. It's a very simple formulation, the rest is a 0.35% fragrance and 0.01% of dye, and water.
Initially I was using 0.5% Methylchloroisothiazolinone and Methylisothiazolinone as the preservatives, but the shampoo failed the preservative challenge, so we decided to move to phenoxyethanol. However, with the exact shampoo formulation, the viscosity dropped to ~500, and no matter how we adjust the salt, the viscosity just won't come up. We also tried benzyl alcohol as the preservative, same viscosity drop.
Later, I tried to use an other surfactant blend with SLES, same thing with the viscosity drop with phenoxyethanol and benzyl alcohol. I also tried to add a thickener, a methyl cellulose, but it doesn't help much unless I add a lot of it (over 2%, budget won't allow).
I've been searching around but I couldn't find any relevant content. My guess is the benzene ring in these two preservatives somehow disturbed the surfactant aggregates. But I don't know if it's true or how to fix this problem.
I'm wondering if anyone has seen similar phenomenon or have any suggestions. Much appreciated!