Effects of isotopic substitution on the critical behavior of aqueous solutions of n-dodecylhexaoxyethylene glycol monoether (C12E 6)

J. P. Wilcoxon, D. W. Schaefer, E. W. Kaler

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Abstract

The critical behavior of a nonionic surfactant, n-dodecylhexaoxyethylene glycol monoether (C12E6) 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 H2O (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.

Original languageEnglish (US)
Pages (from-to)1909-1917
Number of pages9
JournalThe Journal of Chemical Physics
Volume90
Issue number3
DOIs
StatePublished - Jan 1 1989

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