Inverted micellar aggregates and the W/O-microemulsion domain of the system H20/A0T/t-C8H18 have been investigated at various temperatures, AOT and H20 concentrations, and molar ratios [H20]/[A0T] in the range 0-60 using photon-correlation spectroscopy. Experimental evidence is presented that a clear distinction is possible between micellar and swollen micellar solutions, i.e., microemulsions, according to the degree of hydration or the amount of solubilized water, respectively. The onset of the phenomena characteristic for microemulsions occurs when water becomes the major component in the colloidal aggregates. Moreover, it is shown that the conventional definition of the microemulsion domain with respect to the upper boundary is artificial since there is no discontinuity in the property of the microemulsion. Also the so-called "free" water concept is discussed in the light of the results presented.
We present a model for the electrical conductivity of a water-in-oil microemulsion. The conductivity is explained by the migration of charged aqueous droplets in the electric field. Charged droplets are formed by spontaneous number fluctuations of the ions residing on the droplets. The magnitude of these fluctuations is directly related to the Coulomb energy that is required to charge up a droplet. The present model for the conductivity contains no adjustable parameters and is entirely consistent with experimental results on AOT-water-isooctane microemulsions.
Summary. The formation of a so-called water/oil (W/O)-microemulsion (AOT/isooctane/water) was followed by light scattering and ultracentrifuge measurements. With increasing weighed-in water concentrations the microemulsion is stabilized by repeated aggregational processes of micelles containing water due to a decrease of the free intcrfacial enthalpy.This process conforms very satisfactorily to a model describing an adsorption of surfactants at the water/hydrocarbon interface resulting from dipole-image dipole interactions.While there exists already a large number of experimental results on so-called microemulsions formed by nonionic and ionic surfactants in aqueous and nonaqueous media (e.g. [l-41) relatively little is known quantitatively with regard to the mode of origin of these microemulsions. In a preceding paper details of the solubilization process and micellar aggregation phenomena have been investigated with the help of dielectric increment measurements [5]. More recently the exchange of water and aqueous electrolyte solutions between micelles in apolar media was followed by fluorescence quenching experiments. Such an exchange was to be expected as a necessary prerequisite with respect to the experimentally observed considerable inonodispersity of lipophilic micelles containing polar liquids [6]. Finally, the stabilizing effect of polar liquids on lipophilic micelles was predicted theoretically on the basis of dipole-image dipole interactions between the solubilized water 'pool' and the surfactant molecule [7].In order to check the predictions of this theoretical approach and to investigate details of the micellar aggregation process and thus of the formation of a so-called W/O-microemulsion a micellar system with a particular high solubilization ratio (water/surfactant) was selected.Finally, it is believed that the present investigation may contribute towards a clarification as to the thermodynamic stability of these systems.Materials and Methods. -Aerosol OT (= AOT = Sodium di-2-ethylhexylsulfosuccinate, Mo1.-Wt. = 444.56) was used as a suitable representative model of an anionic surfactant because of its pronounced solubilizing capacity in some nonpolar organic solvents. The compound, a commercial product (Fluka AG, Buchs/SG) of pharmaceutical quality, had been purified and dried additionally before use. The procedure is described elsewhere [ 8 ] .According to preliminary investigations isooctanc was chosen as the most suitable solvcnt. The commercial product (Fluka AG, Buchs/SG) of high grade purity was distilled over 'Siccon' followed by a continous distillation over sodium/potassium alloy. The residual water content of the isooctane was determined by the Karl-Fischer-method to bc 0.0052%. Bidistilled water was used as solubilizate.
Photon correlation studies in H2O/sodium di‐2‐ethylhexylsulfosuccinate (AOT)/i‐C8H18‐systems, pertinent IR. investigations, and vapor pressure osmometric measurements with alkylated quaternary ammonium di‐2‐ethylhexylsulfosuccinates strongly suggest water to be pre‐requistite for the micellization in apolar media.
We present an experimental study of an AOT-decane-aqueous 0.5% NaCl microemulsion in which the oil-to-water ratio can be arbitrarily varied. This allows an investigation of the structural inversion from an water-in-oil microemulsion into an oil-in-water microemulsion which happens completely continuously without any phase separation. Conductivity, viscosity, and electrooptical Kerr effect data confirm the presence of two percolation processes. This structural inversion takes place in two stages: With increasing oil content, first a water continuous microemulsion transforms into a bicontinuous structure at roughly 20% oil (oil percolation threshold) and then at roughly 80% oil (water percolation threshold) and bicontinuous microemulsion turns into an oil continuous structure. These two percolations margin three structural regimes of a microemulsion: oil-in-water regime, water-in-oil regime, and a bicontinuous or oil and water continuous regime.
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