Structure and dynamics of dense water-in-oil microemulsions below percolation threshold

Abstract

Le systeme de micro~mulsions AOT/eau/decane pr'esente une phase intgressante au voisinage de la temp'erature ambiante. Le diagramme de phase (tempgrature-fraction volumique) a un point critique bas, correspondant I une fraction volumique d'environ lo%, et est caractbiss par une ligne de percolation qui s'etend du point critique jusqu'au grandes fractions volumiques. La phase obtenue est constitu'ee de gouttelettes d'eau spheriques polydisperses entourges d'une monocouche d'AOT, et dispersges dans un continuum d'huile. Ces gouttelettes interagissent entre elles via une interaction d e type "coeur dur" 1 laquelle s'ajoute une interaction 1 courte distance qui augmente avec la temp'erature. Nous montrons que le modGle de spheres de Baxter rend compte d'une rnaniLe quantitative de la structure de la phase observb ainsi que de la ligne de percolation pourvu que le paramstre d'adh'erence des sphsres soit une fonction convenable de la tempkature. Afin de dgterminer la dependance en tempErature de ce paramstre, nous utilisons les facteurs de structure mesurEs par diffusion de neutrons aux petits angles, au-dessous de la temperature critique. Nous etudions aussi la dynamique des gouttelettes au-dessous et I I'approche de la ligne de percolation par diffusion de lumicre. En supposant l'existence de clusters fractales polydisperses, rgsultant de l'attraction des gouttelettes entre elles, nous pouvons calculer d'une manicre quantitative le premier cumulant et la fonction de corr6lation dependant du temps de la densite de gouttelette. L'analyse des domges de SANS indique que, au-dessus de la ligne de percolation, on passe graduellement d'une structure en clusters 2 une structure ordomk bicontinue.

Small-angle light scattering in dense microemulsions, transition from droplet to bicontinuous phase

“…An additional confirmation of this picture is given by the scaling behavior of the scattered intensity as a function of the wave vector k. In fact the data of fig. 2 obtained in this temperature region follow the scaling form I N k-D, with D =2.3 -+0.2 a value consistent with the fractal dimension predicted by percolation [10]. All the studied samples show a similar behavior, the obtained values for the threshold temperature are reported in fig.…”

“…This phenomenon originates some kind of microinhomogeneities in the system and is well observed by SALS in the present k range. Both the results can be rationalized with the observation of neutron scattering [10,11]: above the percolation line the clustered droplets gradually transform into an ordered bicontinuous structure. The transition from coated droplets to a bicontinuous structure is due to temperature (attractive interaction) and packing effects.…”

“…Extensive measurements of conductivity and dielectric constant have shown that a static percolation picture holds above the percolation threshold, a dynamic one holding below it [7]. The percolation line and the coexistence curve in the (T, r has been well accounted in terms of the Baxter theory for adhesive hard spheres [10]. However as it has been recently pointed out, by the analysis of neutron scattering [4] and small-angle X-ray scattering (SAXS) data[ll], the phase diagram of the system shows other interesting structural configurations: at moderately high volume fraction, above the percolation threshold and below the lamellar phase, the presence of a bicontinous structure has been observed [4], whereas near room temperature and for volume fractions higher than 0.75 extensive SAXS experiments have given evidence of a multiply connected sponge phase (L8 phase) [11].…”

Clouding behaviour in surfactant systems

“…Chen et al [75] have studied a three-component microemulsion system, AOT/water/decane, to understand the electrical percolation phenomenon observed in the microemulsion system. They proposed that the percolation line observed in the phase diagram is due to the formation of transient polydispersed fractal clusters resulting from shortrange attraction between microemulsion droplets, which increases with temperature.…”

Sphere to cylinder transition in a single phase microemulsion system: A theoretical investigation

“…The usual interpretation of the scattering data in this region focuses on a fit of ͑aggregating͒ spherical droplets characterized by an average radius, radial polydispersity, and stickiness parameter describing the de-gree of aggregation. 7,8 It was found by Ilgenfritz et al 9 and Glatter et al 10 that also cylindrical structures start to become present. Only recently could a more quantitative study be undertaken of this structural transition from spherical droplets to cylindrical structures in the one-phase microemulsion system.…”