Surfactant molecules in a solvent self-associate into various kinds of supramolecular assemblies such as micelles, vesicles, and liquid crystals and their mixture, especially those of nonionic and ionic surfactants are used in many practical applications, such as detergents, cosmetics, oil recovery, drug delivery systems, emulsified polymerization, coating technology, and mesostructured nanofilms (1). For these applications, the structural and solution properties of the mixed surfactant systems should be controlled effectively. Therefore, it is useful to understand how the molecular structures of surfactants in mixtures affect the solution properties, such as the size, shape, and surface charge density of the mixed micelles. For these reasons, structural properties of nonionic-ionic surfactant mixed micellar solutions have been investigated theoretically and experimentally (2). In the mixture of two or more different surfactants (nonionic and ionic), the complex aggregation behavior of the mixture of surfactants in solution is a result of a delicate balance of opposing forces, i.e., the steric hindrance among the polar head groups of the surfactant molecules and electrostatic repulsion energy between charges on the polar head of the ionic surfactant molecules (3). Therefore, the structural properties of the nonionic-ionic mixed micellar solutions should be studied as a function of the molar ratio to determine the effect of molecular interaction between the surfactants in a mixed micelle on its formation. Practically, this understanding can help in choosing relevant surfactant structures that will result in the desired properties. NMR spectroscopy is one of the most convenient methods for simultaneous monitoring of changes in aggregate morphologies of interaction between components. In this study, we investigated the formation in water of mixed micelle using zwitterionic and anionic surfactants employing multinuclear NMR to study the influence of intramicellar interaction and surfactant molecular shape on the properties of mixed micelles. In our experiments, we kept the surfactant concentration well above their cmc values, so the observed chemical shifts are those of aggregated assemblies formed upon mixing of the surfactants. Interestingly enough, NMR experiments suggest that under the chosen experimental conditions upon mixing of pure surfactants two different families of mixed aggregates are formed both larger than the original single component micelles. The fact that the different mixed micelles coexist unchanged many days after solution preparation, suggest that the system is under thermodynamic control.
|Titolo:||NMR study of mixed micelles: zwitterionic – cationic surfactant systems|
|Data di pubblicazione:||2017|
|Appare nelle tipologie:||4.2 Abstract in Atti di convegno|