NMR self-diffusion study of aqueous solutions of tetraoxyethylene n-octyl ether (C₈E₄)

The structure of solute aggregates in aqueous solutions of C₈E₄ was investigated in a temperature range below the phase separation curve. NMR self-diffusion measurements (FTPGSE technique) were performed over a composition range from 0.9 to 34 wt % of the amphiphile, i.e., from about 4 times the critical micelle concentration (cmc = 0.22 wt % at 25°C) up to 5 times the concentration of the critical composition (w_c ≈ 7 wt %). The experimental temperature range extended from 25 to 37°C (lower critical solution temperature T_c ≈ 37.9°C, with D₂O). We find that the self-diffusion coefficient of C₈E₄ decreases with increasing surfactant concentration at each experimental temperature. On the other hand, no general trend of the temperature dependence of the self-diffusion coefficient is seen. As the line width of the main ¹H signal of the alkyl chain protons is nearly constant over the entire experimental concentration and temperature range, any significant growth of individual micelles from spherical to elongated (cylindrical) shape can be ruled out. Accordingly, the observed concentration dependence of the self-diffusion coefficient is discussed in terms of spherical micelles and aggregates (clusters) of spherical micelles.