Co-solvent Effect on Spontaneous Formation of Large Nanoscale Structures in Catanionic Mixtures in the Anionic-Rich Region

Farhad Golmohammadi,Ali Yousefi,Meysam Safari,Hussein Gharibi,Masoud Amiri

doi:10.1007/s10953-019-00935-6

Abstract

The aggregation behavior was investigated in mixtures of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) (anionic-rich catanionic) solutions. The study was conducted in solutions of water–ethylene glycol (EG) by means of surface tension, conductometry, cyclic voltammetry, zeta potential measurements, transmission electron microscopy (TEM) and dynamic light scattering (DLS) techniques. The degree of counterion dissociation (α), critical micelle concentration, aggregation numbers, interfacial properties, interparticle interaction parameters, and morphology of aggregates were determined. Based on regular solution theory, the cosolvent effects between SDS and CTAB as surfactants were also analyzed for both mixed monolayers at mixed micelles (βM) and the air/liquid interface (βσ). It was shown that the formation of large aggregates occurred in the presence of an excess of anionic surfactant. A phase transition from cylindrical micelles to spherical micelles in the anionic-rich regime was observed with an increase in the EG volume fraction. The inter particle interactions were assessed in terms of cosolvent effects on the micellar surface charge density and the cylindrical-to-spherical morphology change. Zeta potential and size of the aggregates were determined using dynamic light scattering and confirmed the models suggested for the processes taking place in each system.

Highlights

Surfactants have important roles in most areas, including cosmetics, detergents, electrochemistry, floatation agents, drug delivery, etc. [1], as well as diverse industrial applications
In the sodium dodecyl sulfate (SDS)/cetyltrimethylammonium bromide (CTAB) mixed system, at a total concentration exceeding C1, the conductivity remained almost constant as the concentration increased
Depending on the Ethylene glycol (EG) content of the solvent, the SDS/CTAB self-assembled into configurations ranging from cylindrical to spherical

Summary

Introduction

Surfactants have important roles in most areas, including cosmetics, detergents, electrochemistry, floatation agents, drug delivery, etc. [1], as well as diverse industrial applications. The understanding of the interactions among surfactants in the mixtures is the key to taking advantage of such beneficial effects. Colloidal behaviors such as micellization and surface adsorption are markedly different from those in single surfactant systems. In a system of mixed surfactants with non-ideal mixing effects in their aggregates, various micellar properties exhibit synergism or cooperative interaction [5, 6]. This cooperative interaction or synergism is usually attributed to non-ideal mixing effects in their aggregates and modeled in the literature by employing regular solution theory (RST) with a negative interaction parameter [7,8,9]. The electrostatic interactions (in aqueous media) between positive and negative groups, in mixtures of anionic and cationic surfactants, give these systems many unique properties

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Conclusion