Isotope effects on the phase behaviour of cetyltrimethylammonium bromide in H₂O and D₂O studied by time resolved X-ray diffraction

The liquid-crystalline phases of C₁₆TAB/H₂O and C₁₆TAB/D₂O lyotropic systems, at high concentrations (60.0-90.0 wt% surfactant) have been investigated in the temperature range of 30-120°C. Time resolved X-ray diffraction (TRXRD) with synchrotron radiation was applied to record the diffraction spectra at different temperatures over this range. We have obtained highly resolved phase diagrams of the two systems and identified hexagonal, cubic, lamellar and 2D monoclinic (deformed hexagonal) phases. Isotope effects appear when H₂O is substituted by D ₂O at comparable molar concentrations of surfactant. Particularly where: (1) about 2°C higher (hexagonal-monoclinic) phase transition temperature, and (2) about 25% larger size hexagonal phase at 31°C. The isotope effects were reduced at higher concentrations and at higher temperatures. The swollen of hexagonal phase in DS, i.e. the secondary isotope effects, is induced by the fact that D₂O is a more polar solvent than H₂O.