ABSTRACT
II. Single-Chain Surfactants 456 III. Catanionic Surfactants 459
A. Symmetrical catanionic surfactants 460 B. Asymmetrical catanionic surfactants 466
IV. Double-Chain Surfactants 471 References 474
I. INTRODUCTION The typical surface-active or surfactant molecule consists of at least one polar hydrophilic part and one apolar hydrophobic part, such as a hydrocarbon or fluorocarbon chain. Although there is normally only one headgroup per surfactant molecule, there are frequently several nonpolar tails. These can be linear or branched, the most common being single and linear. Because of the coexistence of two opposite types of behavior inside the same molecule, surfactants can build different submicroscopic aggregates in water that can organize themselves into various supramolecular structures of macroscopic dimensions and different properties. The variety of supramolecules ranges from micelles to liquid crystalline
phases. In most cases, those structures can transform from one to the other as a result of sometimes subtle changes in the solution conditions (e.g., concentration, electrolyte addition, temperature changes). Literature relating to the phase spectra of surfactants in aqueous and nonaqueous solutions is plentiful. Changes in the structures are manifested by abrupt changes in physical characteristics of the solution (viscosity, conductivity, and other transport phenomena; birefringence; or the existence of characteristic X-ray diffraction patterns) [1]. With decreasing solvent content, interactions between adjacent structures increase, leading to the formation of the liquid crystalline phase. On further decreasing the solvent content, dry surfactant crystals are formed, often via solvent-containing surfactant crystals.
