Role of Acids in Stabilizing Reverse Micelles: Insights from Dodecyl Sulfate

The anionic surfactant sodium dodecyl sulfate (SDS) forms reverse micelles (RMs) in two nonmiscible components above the critical micelle concentration (CMC). Although the RMs in salt or alkali solution have been investigated in previous studies, less is known on the working mechanism of acids in SDS RMs. Here, we employ all-atom molecular dynamics simulations with the Generalized Amber Force Field to elucidate how fluoroboric (HBF₄), chloroauric (HAuCl₄), phosphoric (H₃PO₄), and sulfuric (H₂SO₄) acids modulate the interfacial organization, electrostatics, and morphology of SDS-based RMs in toluene. Acid addition alters interfacial dynamics and flexibility by reducing SDS headgroup mobility and tail ordering through persistent hydrogen-bond networks. In SDS bilayer models, acids affect the interfacial tension (γ) in the order γ(HBF₄) > γ(H₂SO₄) > γ(HAuCl₄) > γ(H₃PO₄). These changes reduce micellar wall flexibility and interfacial tension, potentially promoting the formation of larger and more stable reverse micelles. These insights offer molecular guidelines for tuning RM-mediated nanoparticle synthesis, where subtle interfacial effects dictate nucleation, growth, and structural control.