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.
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