Micellization and adsorption of zwitterionic surfactants at the air/water interface

Zwitterionic surfactants are formally neutral but with headgroups containing both a positive charge center and a negative charge center separated from each other by a spacer group, with a long hydrophobic tail attached to one of the charge centers, usually but not always the positive charge center. The micellization and adsorption properties of zwitterionic surfactants depend on specifics of the surfactant structure such as the length m of the hydrophobic alkyl chain, the length n of the intercharge spacer and the nature of the headgroup charge centers. Micellization is favored by an increase in the hydrophobic tail length m, but goes through a maximum for interchange spacings of n = 3-4 methylene groups. There are additional effects from the presence of additional hydrophilic substituent groups in the spacer. Specific binding of anions and the cation valence of added electrolyte are factors that also modulate the micellization and adsorption properties of zwitterionic surfactants in the presence of added electrolyte. Anions in particular bind preferentially to zwitterionic micelles independent of the relative order of the charge centers in the headgroup. The anion binding affinities follow a Hofmeister series and impart a net negative charge to the micelles. Micellization is temperature-dependent and exhibits enthalpy-entropy compensation, with entropy dominant at lower temperatures and enthalpy more important at higher temperatures. The judicious manipulation of these factors permits control of the interfacial properties of zwitterionic surfactants, responsible for a wide range of applications in chromatography, electrophoresis, cloud point extraction, solubilization, stabilization of biomolecules and nanomaterials and catalysis. (C) 2017 Elsevier Ltd. All rights reserved.