Assessment of the aggregation and adsorption behavior of newly synthesized tetradecylpyridinium-based metallosurfactants and their interaction with bovine serum albumin

Abstract

Tetradecylpyridinium (TP) based metallosurfactants, TP₂[MCl₄] (M = Mn, Co, Ni, Cu, Zn), were synthesized and characterized using elemental analysis, NMR (¹H, ¹³C) and FT-IR spectroscopy. Thermal gravimetric analysis showed that TP₂[MCl₄] metallosurfactants are thermally more stable than the precursor tetradecylpyridinium chloride (TPC). Their aggregation and adsorption properties have been investigated in aqueous medium by measuring the surface tension and conductance, and using dynamic light scattering (DLS) and transmission electron microscopy (TEM) methods. The critical micelle concentration (cmc) values of the TP₂[MCl₄] metallosurfactants (cmc = 1.116–1.137 mmol kg⁻¹) are about 71% lower than that of TPC (cmc = 3.983 mmol kg⁻¹). The charge neutralization of the metallomicelles due to counterion binding is only about 30 to 35%. The surface excess and area per molecule of TP₂[MCl₄] metallosurfactants at the cmc are 0.64–0.68 μmol m⁻² and 2.45–2.59 nm², respectively. DLS, TEM and zeta potential (ζ) measurements revealed the presence of small (average hydrodynamic diameter = 4–7 nm), spherical and stable (ζ > +45 mV) micelles in aqueous solution. Binding of the metallosurfactants with bovine serum albumin (BSA) was studied using the steady-state fluorescence method. The metallosurfactants readily quench the intrinsic fluorescence intensity arising from the tryptophan residues incorporated in the BSA. The binding is of 1 : 1 type and the binding constant value depends on the metal ion. The influence of hydrocarbon chain length on the cmc, surface excess and binding with BSA has been discussed.