Effect of Hydration on the Very Slow Droplet–Lamellar Transition in Dioleylsulfosuccinate/Decane/Water System: A Small Angle X-ray Scattering Study

Abstract

Time-and temperature-dependence of the small angle X-ray scattering (SAXS) profiles for homogeneous and transparent solutions of the sodium dioleylsulfosuccinate (SDOleS)decanewater system with various molar ratios of [H 2 O]/ [SDOleS] have been examined. The time-dependence of the SAXS profiles has provided evidence that the droplet lamellar transition occurs spontaneously but very slowly at 298 K. This behavior depends upon the quantity of water solubilized into the SDOleS microemulsions. Some microstructural models have been proposed to explain this very slow transition.It is well known that dynamic surface phenomena occur in the self-assembled systems formed on the airwater or oil water interface within the time scale 1,2 ranging from 10 ¹3 to 10 3 s. Extensive studies of these phenomena have been carried out for more than seventy years.In self-assembled systems of surfactants with low molecular weight, change in surface-tension occurs rapidly (usually in less than 1 s).2 Germasheva and Panaeva 3 observed a slow surface-tension equilibration in micellar solutions of sulfosuccinates, which could not be interpreted solely on the basis of diffusion to the surface. For very pure samples of sodium diheptylsulfosuccinate (SDHpS), Lucassen and Drew 4 reported a similar slow surface-tension variation and a simultaneous increase in the surface dilational modulus. They suggested that knowledge of the detailed crystal structure should provide useful information about ordering in a membrane. van den Tempel and Lucassen-Reynders 2 suggested that slow changes in surface-tension and an increase in elastic modulus might be caused by the cooperative ordering in the surface. The effect of such surface properties on the formation of a three-dimensional (crystalline) structure, similar to that of a lamellar liquid crystal, was studied by Franses et al. 5 Their results imply that the observed variation in surface-tension is a reflection of the change in aggregational structure on the surface.The reversed micelle of sodium 1,2-bis(2-ethylhexyl)sulfosuccinate (AOT) (Scheme 1) incorporates a relatively large number of water molecules in its polar core, thereby forming a water droplet 69 and providing a water/oil interface. In a freshly prepared AOT-reversed micelle, cooperative ordering in the membrane structure at this oil/water interface should occur until the membrane reaches its final ordered state. This concept implies that a difference in the aggregational structure of the interface exists between freshly prepared and aged interfaces. Previous studies on phase behaviors of the AOToilwater systems 715 have been focused on the final ordered structures, a limitation imposed by the fast rate of attainment of the final ordered state. However, interpretation of such an ordering process at the molecular level is rare, in spite of the paramount importance of the practical applications of surfactants.In this present study, an AOT-homolog with longer hydrocarbon chains, sodium dioleylsulfosuccinate (SDOleS) (Scheme 1) has been synth...

Complex, Dynamic Behavior of Extremely Asymmetric Di-n-Alkylphosphate-Anion Aggregates, the Long-Chain Effect and the Role of a Limiting Size: Cryo-TEM, SANS, and X-Ray Diffraction Studies

“…The difference in aggregation structure between the freshly prepared and aged samples, in the sodium dioleyl-sulfosuccinate (SDOleS)–decane–water system, has been examined by the SAXS . The time-dependent SAXS profiles provided evidence that the droplet-lamellar transition occurs spontaneously, but very slowly at 298 K, and depends on the quantity of water solubilized into the microemulsion.…”

“…The very slow surface-tension equilibration in the SDHpS aqueous solution and the spontaneous droplet-lamellar transition in the SDOleS–decane–water system may be ascribed to the molecular-skeleton characteristic of these surfactants as the common structural unit of the asymmetric succinate-skeleton (O 3 S-CH­(CH 2 -COO-R 1 )-COO-R 1 , R 1 : n -alkyl group), possibly leading to the difference in the aggregation behavior between freshly prepared and aged interfaces. The effect of the spontaneous curvature of the oil–water interface − formed by the succinate-surfactant systems may play an important role.…”