Shape and Structure Formation of Mixed Nonionic-Anionic Surfactant Micelles.

Michael Ludwig,Regine Von Klitzing,Ramsia Geisler,Sylvain Prévost

doi:10.3390/molecules26144136

Abstract

Aqueous solutions of a nonionic surfactant (either Tween20 or BrijL23) and an anionic surfactant (sodium dodecyl sulfate, SDS) are investigated, using small-angle neutron scattering (SANS). SANS spectra are analysed by using a core-shell model to describe the form factor of self-assembled surfactant micelles; the intermicellar interactions are modelled by using a hard-sphere Percus–Yevick (HS-PY) or a rescaled mean spherical approximation (RMSA) structure factor. Choosing these specific nonionic surfactants allows for comparison of the effect of branched (Tween20) and linear (BrijL23) surfactant headgroups, both constituted of poly-ethylene oxide (PEO) groups. The nonionic–anionic surfactant mixtures are studied at various concentrations up to highly concentrated samples ( ≲ 0.45) and various mixing ratios, from pure nonionic to pure anionic surfactant solutions. The scattering data reveal the formation of mixed micelles already at concentrations below the critical micelle concentration of SDS. At higher volume fractions, excluded volume effects dominate the intermicellar structuring, even for charged micelles. In consequence, at high volume fractions, the intermicellar structuring is the same for charged and uncharged micelles. At all mixing ratios, almost spherical mixed micelles form. This offers the opportunity to create a system of colloidal particles with a variable surface charge. This excludes only roughly equimolar mixing ratios (X≈ 0.4–0.6) at which the micelles significantly increase in size and ellipticity due to specific sulfate–EO interactions.

Highlights

Surfactants are widely used in various industrial, technological or domestic applications [1]
In order to close this gap, we investigate the aggregation behaviour of mixtures of nonionic surfactants and well-known anionic sodium dodecyl sulfate (SDS) surfactants
Small-angle neutron scattering (SANS) data of pure surfactant solutions are shown in Figure 3 for the three different surfactants used in this study: Tween20 (Figure 3a), BrijL23 (Figure 3b), and SDS (Figure 3c)

Summary

Introduction

Surfactants are widely used in various industrial, technological or domestic applications [1]. Understanding the structure and properties of surfactant mixtures is important for application, and in fundamental science: alkyl ethers with headgroups constituted of poly-ethylene oxide (PEO) groups (often abbreviated as CnEm) are often used to study properties of mixed surfactant solutions [8,9]. Their chemical structure is a great model system, allowing to vary the hydrophilic/hydrophobic character of the nonionic surfactant systematically when mixed with other surfactants

Methods

Results

Discussion

Conclusion