Effects of isotopic substitution on the critical behavior of aqueous solutions of n-dodecylhexaoxyethylene glycol monoether (C12E6)

Abstract

The critical behavior of a nonionic surfactant, n-dodecylhexaoxyethylene glycol monoether (C12 E6) in D2O, is examined using static and dynamic light scattering and small angle neutron scattering (SANS). Modification of the short-range attractive interactions responsible for phase separation by substitution of D2O for H2O has no effect on the measured universal critical exponents describing the divergence of the correlation length (ν=0.6±0.05) and the isothermal compressibility (γ=1.2±0.1) although such substitution results in a reduction of both the nonuniversal amplitudes and critical temperatures. The measured exponents are the same as those of binary molecular fluids. However, in contrast to binary fluids, significant deviations from the Ornstein–Zernike form are observed at intermediate length scales in SANS measurements near the critical point. Isotopic substitution also preserves the dynamic scaling of the Kawasaki form for the measured Rayleigh linewidth, as found previously for H2 O (Ref. 1). However, this scaling is restricted to a more limited range of distance from the critical point than the static scaling. A maximum in compressibility at the critical isochore is reflected in the concentration dependence of both the compressibility and diffusion coefficient even large distances (∼25 °C) from the coexistence curve. Estimates of micelle size, shape, and polydispersity are obtained.