Behavior of sodium dodecyl sulfate in acidic environment

Abstract Whether sodium dodecyl sulfate (SDS) can be protonated in water under practically relevant acidities remains unclear because reported pK a values for its conjugate acid, dodecyl hydrogen sulfate (DHS), vary widely. As SDS is widely used as an electrostatic stabilizer in waterborne coating formulations, clarification of its acid stability is of practical importance. SDS protonation was therefore examined by turbidity measurements, acid titration, and 1 H NMR spectroscopy. Solutions containing 0.025–0.50 mol L −1 SDS remained fully transparent down to pH ≈ 0, with no turbidity or change upon heating to 45 °C. Titration with 1 M HCl showed a logarithmic pH decrease without buffer or equivalence points, indicating the absence of a measurable aqueous pK a . In 1 H NMR spectra (0.10 mol L −1 SDS), only ionic-strength-dependent shifts were observed, while integrals remained constant, confirming full solubility. Protonation, visible as line broadening and signal loss (~ 7%), occurred first at 1.26 mol L −1 DCl, where samples became heterogeneous. These results demonstrate that SDS remains fully deprotonated and stable under practical-relevant pH conditions, while conversion to the poorly soluble protonated form occurs only under extremely acidic, high-ionic-strength environments.