STATIC LIGHT-SCATTERING AND SMALL-ANGLE NEUTRON-SCATTERING EXPERIMENTS WITH A NONIONIC SURFACTANT DEUTERIUM OXIDE SYSTEM APPROACHING THE LIQUID/LIQUID COEXISTENCE CURVE - STUDY OF THE SYSTEM C(12)E(5)/D2O/

Static light scattering experiments with C(12)E(5)/D2O mixtures are ca rried out in the ''large aggregate region'' of the phase diagram of th e system adjacent to the lower part of the binodal curve. They demonst rate the existence of concentration fluctuations with long range corre lations. For a mixture of critical composition the correlation length xi of the concentration fluctuations diverges approaching the critical temperatures T-c. The critical amplitude xi(0) has a value of xi(0) = 1.8 nm which is large compared to that found with nonaggregating bina ry liquid mixtures with a miscibility gap. The experimentally determin ed values of the critical exponents upsilon and gamma are consistent w ith the theoretical expected universal values. For mixtures of noncrit ical composition xi increases approaching the binodal curve and has a finite comparatively large value at the temperature of phase separatio n T-p. Small angle neutron scattering experiments with a C(12)E(5)/D2O mixture of critical composition show that the angular dependence of t he scattered intensity becomes only approximately temperature independ ent approaching T-c (i.e. large values of the scaling variable x = q x i (q, absolute value of the scattering vector). This is contrary to ex pectation. On the basis of the expression approximating the universal correlation scaling function for values of x<20 it is expected that th e temperature dependence should vanish. From this findings taking into account also results of dynamic light scattering obtained with the sa me system (A. Martin et al., J. Phys. Chem. 100, 13762 (1996)) it is h ypothesized that in the ''large aggregate'' region of the phase diagra m adjacent to the binodal curve concentration fluctuations with long r ange correlations and their dynamics contribute to the changes of the size and shape of the micellar structures described in the literature.