On the Formation of Water/Oil‐Microemulsions

Abstract

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.

Molecular Dynamics Simulations of the Interior of Aqueous Reverse Micelles

“…The three fundamental parameters used to define our model system are the number of surfactant molecules, n s , the ratio of water molecules to surfactant molecules, w 0 , and the micelle radius, R. For a given value of w 0 , n s has been determined by linear interpolation of Eicke and Rehak's reported values of 〈n s 〉 measured in the water/AOT/isooctane system by light scattering. 41 The total volume of the system, V, can then be calculated by assuming a fixed molecular volume for each of the three species in the micelle. The molecular volume of water was assumed to be the bulk value of 30 Å 3 (0.997 g/cm 3 ), based on the density of water at 25°C, while the molecular volumes of Na + and Z -were taken to be 6 and 57 Å 3 , respectively.…”

“…The sizes of the ions are chosen to match those of the sulfonate and sodium in AOT and the compositions of the model micelles as determined from the experimental aggregation numbers. 41 Simulations have been performed over a range of micelle sizes of 1 e w 0 e 10. The results of the simulations show that for all of the sizes studied water forms distinct molecular layers, which are easily observed in water density profiles.…”

Synthesis, Functionalization, and Design of Magnetic Nanoparticles for Theranostic Applications

“…[116] Fletcher et al also provided a detailed investigation of the temperature, oil solvents and additives on the exchange kinetics in AOT systems. [117] By the superb advantages of such interfacial film of surfactants to split the confined nanoreactors of the encompassing solution as well as its ability for exchanging material, microemulsions (typically 1-50 nm in diameter) [118] can serve as a proper asset to stabilize the entire solution [119] and offer excellent control over shape, size and phase of the produced nanoparticles. [57] Synthesis of MNPs under microemulsion process entails simple stages as follows:…”

An Advanced Approach on the Study of Emulsion Polymerization: Effect of the Initial Dispersion State of the System on the Reaction Mechanism, Polymerization Rate, and Size Distribution of Polymer-Monomer Particles

“…A significant attention in the literature was devoted to the investigation of the properties of microemulsions (ME) and conditions of their formation [81][82][83][84][85][86][87][88][89][90][91][92][93][94], however, the thermodynamic equilibrium of lyophilic systems [81,82] was mainly considered.…”