Phase Boundaries for Mixed Aqueous Micellar Solutions of Dimethyldodecylamine Oxide and Sodium or Magnesium Dodecyl Sulfate with Regard to Chemical Processes in the Systems

The literature data bear witness to strong synergistic effects in mixed aqueous solutions of alkylamine oxides and alkyl sulfates. Physicochemical properties of these systems depend essentially on pH, which is due to protonation of amine oxide. Interplay of several types of intermolecular interactions including chemical ones results in complex phase diagrams. New experimental data presented in the paper refer to the solid-liquid equilibrium (the Krafft boundary), the composition of crystallizing complexes, and the critical micelle concentration (cmc) in mixed aqueous solutions containing dimethyldodecylamine oxide and sodium or magnesium dodecyl sulfate. The temperature maximum suggesting the formation of a 1:1 amine oxide-dodecyl sulfate complex is observed at the curves of dissolution temperature versus surfactant-based composition; the shape of the curves indicates that complexes of other stoichiometries can be formed in solutions enriched with amine oxide. The dissolution temperature for the 1:1 mixture (and correspondingly, the concentration of the complex in the solution) grows with growing acidity. In both systems under study, the dissolution temperature becomes 48 °C at a pH of about 5 and does not change on further pH lowering. Chemical analysis has shown that the solid phase precipitated from basic solutions contains metal ions whereas at pH ≤ 5 the complex between the protonated amine oxide and dodecyl sulfate anion is crystallized. The cmc value for 1:1 acidic mixtures does not depend on the cation nature and is 2 orders of magnitude lower than that for solutions at the natural pH (which are slightly basic). The pseudophase model taking into account the reactions of amine oxide protonation and 1:1 complex formation reproduces satisfactorily the pH effect on the solubility diagrams in the dimethyldodecylamine oxide-sodium dodecyl sulfate-water system.