Preparing Narrow Size Distribution Particles from Amphiphilic Association Structures

Abstract

Small inorganic particles with a narrow size distribution have been prepared from aqueous and organic solutions under varied temperatures, pressures, and concentrations. This article reviews the preparation of such particles from microemulsions and liquid crystals and shows how these amphiphilic association structures can have a pronounced influence on the size, shape, and even chemical composition of the precipitated particles.Surfactants associate in the presence of water to form micelles and lyotropic mesophases. This is a consequence of the molecular geometrical packing requirements, superposed on the dissolution behavior of these substances. Associated structures that have shapes with the lowest free energy are preferentially formed. A balance is attained between the interactions of the hydrophobic portions of the surfactant, reducing the unfavorable contact with aqueous environs and the interactions between the headgroups. The composition regions of these different phases have been widely studied. Micelles exist as normal and reversed structures (Figure 1), while the liquid crystalline phases show a variety of structures which have been studied in great detail.Figure 1 gives a simplified example of the structural conditions in different parts of a phase diagram of the three components water/surfactant/amphiphilic substance (e.g., medium chain length alcohol). The interaction between water and the surfactant is sufficiently pronounced to permit the formation of reversed micelles. The association structures of different regions shown in Figure 1 include normal (L₁) and reversed micelles (L₂). The hexagonal phase (E) consists of a hexagonal array of cylinders with the hydrocarbon chains situated in the center of the cylinder, while the reverse hexagonal phase (F) is built of cylinders with aqueous centers. The lamellar phase (L_α) is made up of infinite sheets of bimolecularly arranged surfactant molecules separated by aqueous layers. The physical properties and reasons for the formation of such lyotropic phases have been discussed extensively.