Experimental evidence for a continuous variation of effective critical exponents in a microemulsion system

Abstract

2014 Nous présentons des résultats obtenus par diffusion de la lumière en différents points d'une ligne critique d'un système microémulsion quaternaire. Cette ligne débute par un point critique extreme. Les points critiques ont été approchés, soit en faisant varier la température à composition constante (chemin I), soit en faisant varier le rapport eau sur savon (X) à température constante (chemin II). Pour chaque point critique examiné, les valeurs des exposants critiques 03BD et 03B3 mesurées le long des deux chemins sont proches. Elles varient continûment des valeurs d'Ising à des valeurs très petites quand le point critique considéré s'approche du point critique extreme. Ce comportement critique complexe est analysé à l'aide des résultats du diagramme de phase. Abstract 2014 We present light scattering results obtained for a four-component microemulsion system in several distinct positions of a critical line. This line starts at a critical end point The critical points were approached either by raising the temperature at fixed composition (path I) or by increasing the water over surfactant ratio X at constant temperature (path II). For each critical point investigated, the critical exponents 03BD and 03B3 associated with the X variable (path II) are close to those obtained with the temperature (path I). They vary continuously from the Ising values to largely smaller ones as the critical end point is approached. This complex critical behaviour is discussed in relation with the phase diagram findings.

Nonuniversal Critical Behavior at the Uniaxial-Biaxial Nematic Phase Transition in a Lyotropic Mixture

“…In contrast to this view, Corti, Degiorgio and co-workers [2] reported measurements in binary lyotropic mixtures of water and non-ionic surfactants, showing continuously variable critical exponents g (susceptibility) and n (correlation length), depending partly on the size of the micelles, and more essentially on the solvent-micelles interactions. Experimental evidence of nonuniversal values for g and n was also obtained by Bellocq et al [3] for a four-component microemulsion. The measurements of Corti et al [2] on C 12 E 8 (dodecyloctaoxyethilene glycol monoether) in H 2 O and D 2 O, were repeated by Dietler and Cannell [4] who found 3D Ising exponent values, consistent with current theory and universality.…”

“…However, at variance with the conclusion of Corti et al [2], our results show a strong dependence of b on the form and volume of the micelles. Furthermore, while the preceding authors, as well as Bellocq et al [3], found critical values of g and n below the mean-field behavior, we find that b takes values below or above the mean-field prediction depending on the location of the critical point on the transition line. This crossover is explained in the framework of a phenomenological model recently proposed for describing the phase diagram of lyotropic nematics [6,7].…”

Critical behaviour of { water +n-nonane+ sodium di(2-ethyl-1-hexyl)sulphosuccinate } microemulsions

“…(9,10) Most of the experiments involved measurements of critical exponents ν, γ , and α which characterize the divergence of correlation length, the osmotic compressibility, and the specific heat at constant pressure and the critical concentration. (4)(5)(6)(11)(12)(13) Some of experimental results showed that the micellar systems behaved as three-dimensional Ising systems. A few experimental studies (9,12) have been carried out to determine the critical exponent β, which characterizes the shape of the coexistence curve.…”

Origin of nonuniversality in micellar solutions

“…An even more pronounced continuous variation of the critical indices has very recently been observed near critical solution points in microemulsions as well. 9 The origin of this anomalous behavior has not as yet been understood.…”

“…An even more pronounced continuous variation of the critical indices has very recently been observed near critical solution points in microemulsions as well. 9 The origin of this anomalous behavior has not as yet been understood.In this paper arguments are presented according to which it is genuine nonuniversality, in the renor-malization-group (RG) sense, that has been observed. To understand how nonuniversality may arise, let us first examine the current theories of micellar size and shape, 10 "" 12 which have already been used to construct mean-field theories for phase separation in micellar solutions.…”