Tuning cationic micelle properties with an antioxidant additive: a molecular perspective

Abstract

In this work, we characterize the micellization and morphology transition induced in aqueous cetyltrimethylammonium bromide (CTAB) solution by the addition of the antioxidant propyl gallate (PG) using tensiometry, rheology, and small-angle neutron scattering (SANS) techniques combined with the molecular dynamics (MD) simulation approach. The adsorption of CTAB at the air-water interface in the presence of varying [PG] revealed a progressive decrease in the critical micelle concentration (CMC), while the changes in different interfacial parameters indicated enhancement of the hydrophobicity induced by PG in the CTAB micellar system. The dynamic rheology behavior indicated an increase in the flow viscosity (n) as a function of [PG]. Moreover, the rheological components (storage modulus, G?, and loss modulus, G?) depicted the viscoelastic features. SANS measurements depicted the existence of ellipsoidal micelles with varying sizes and aggregation number (Nagg) as a function of [PG] and temperature. Computational simulation performed using density functional theory (DFT) calculations and molecular dynamics (MD) provided an insight into the atomic composition of the examined system. The molecular electrostatic potential (MEP) analysis depicted a close proximity of CTAB, i.e., emphasized favorable interactions between the quaternary nitrogen of CTAB and the hydroxyl group of the PG monomer, further validated by the two-dimensional nuclear Overhauser enhancement spectroscopy (2D-NOESY), which showed the penetration of PG inside the CTAB micelles. In addition, various dynamic properties, viz., the radial distribution function (RDF), the radius of gyration (Rg), and solvent-accessible surface area (SASA), showed a significant microstructural evolution of the ellipsoidal micelles in the examined CTAB-PG system, where the changes in the micellar morphology with a more elongated hydrophobic chain and the increased Rg and SASA values indicated the notable intercalation of PG in the CTAB micelles.