Aggregation and adsorption properties of benzyldimethylhexadecylammonium tetrachloromanganate(II) metallosurfactant in water–ethylene glycol medium

Abstract

The aggregation and adsorption properties of metallosurfactant, benzyldimethylhexadecylammonium tetrachloromanganate(II) (BDHA₂[MnCl₄], where BDHA represents benzyldimethylhexadecylammonium group) in binary aqueous mixtures of ethylene glycol (EG) were investigated by tensiometric, conductometric, dynamic light scattering (DLS) and transmission electron microscopy (TEM) methods at 298.15 K. The percentage of EG (by weight) was varied between 0 and 100%. The physicochemical properties determined and discussed include critical micelle concentration (cmc), counterion binding constant (β), Gibbs free energy of micellization ($\Delta G_m^0$), surface excess at cmc (${\mathrm{\Gamma }}_{\mathit{cmc}}$), minimum area per surfactant molecule ($A_{\min }$), surface pressure (π_cmc), minimum free energies (G_min) and size of aggregates. This study revealed that with increase in percentage of EG the cmc of BDHA₂[MnCl₄] metallosurfactant increases and the cmc of BDHA₂[MnCl₄] in EG is about 158 times higher than that in water. The counterion binding constant values increase with increase in percentage of EG from 10% to 70% and thereafter decrease. The adsorption of metallosurfactant at the air–solution interface decreases as the EG content in the bulk increases. The $\Delta G_m^0$ values were found to be negative and the negative value decreases with increase in percentage of EG. Gordon parameter and ratio of surface tension of medium to that of the metallosurfactant solution at cmc are used to represent solvophobicity of the medium. DLS measurements revealed that the hydrodynamic diameters (d_H) of the micelles increase with increase in percentage of EG (in pure EG, d_H = 28.3 nm). TEM measurements showed the presence of spherical micelles.

Graphical Abstract

Formation of spherical micelles in EG.

Keywords:

• Metallosurfactant
• critical micelle concentration
• counterion binding
• solvophobicity
• dynamic light scattering

Acknowledgments

T. A. Wagay is thankful to Council of Scientific & Industrial Research (CSIR), New Delhi (India) for the Research Associateship. The authors are also grateful to Prof. K. Ismail, Department of Chemistry, North-Eastern Hill University, Shillong, India for the valuable discussions.