The Effect of Degree of Protonation on the Phase Diagrams of Alkyldimethylamine Oxides

For aqueous NaCl solutions of alkyldimethylamine oxides (C_n-DAO, n = 12,14,16), the potentiometric titration curves and the phase diagrams have been drawn as a function of the degree of protonation, α, surfactant and NaCl concentrations, C, and the temperature, T. The logarithm of apparent dissociation constant changes complicatedly with an increase in the degree of protonation, being related to the variations in the content of monomeric molecules and micelles and in micelle aggregation number. The T-α-C, diagrams for aqueous NaCl solutions of C_nDAO with a surfactant concentration of 0.3 x l0^-2 g cm^-3 exhibit the liquid-liquid phase separation region above the Krafft temperature and at high NaCl concentrations. The consolute phase boundary spreads parabolically on both sides of α -0.5, slightly dependent on the temperature. The critical value of lower consolute NaCl concentration is 1.4 M for C₁₄DAO. 0.32 M for C₁₄DAO and 0.08 M for C₁₆DAO in the T-α-c diagram for 0.37 M NaCl solutions of C₁₄DAO, two liquid phase region is spheroidal in shape with ranges of T = 25-75°C. α = 0.38-0.60 and c = 0.05-1.67 × 10^-4 g cm^-3. The effect of the degree of protonation on the liquid-liquid phase separation is discussed in connection with the solute-solvent and solute-solute interactions and the micellar growth.