Aggregation Behavior and Intermolecular Interaction of Cationic Trisiloxane Surfactants: Effects of Unsaturation.

Abstract

Imidazolium/pyridinium-based trisiloxane surfactants containing a phenyl or vinyl group in the hydrophobic siloxane chain, bis(vinyldimethylsiloxy)methylsilylpropyl-pyridinium chloride (Vi-Si3pyrCl), bis(vinyldimethylsiloxy)methylsilylpropyl-imidazolium chloride (Vi-Si3minCl), and bis(phenyldimethylsiloxy)methylsilylpropylimidazolium chloride (Ph-Si3minCl), were synthesized and confirmed by nuclear magnetic resonance (NMR) (1H, 13C, and 29Si NMR), mass spectrometry, and Fourier transform infrared spectrometry. The effect of the phenyl/vinyl group on their micellization behavior was studied by surface tension, electric conductivity, dynamic light scattering, 2D nuclear Overhauser effect spectroscopy (NOESY) NMR, and transmission electron microscopy. Owing to the hydrophobicity of the siloxane groups and cationic head groups, the critical micelle concentration (cmc) values follow the order Ph-Si3minCl < Vi-Si3pyrCl < Vi-Si3minCl < Si3pyrCl. Ph-Si3minCl has a larger γcmc value, resulting from the introduction of the phenyldimethylsiloxy unit (π-π stacking interaction). The β values of Vi-Si3minCl and Ph-Si3minCl increase with the increase in temperature, which is attributed to the intermolecular interaction which hinders the association of Cl- with the imidazolium ring and confirmed by 2D NOESY NMR. In aqueous solutions, the investigated cationic trisiloxane surfactants can self-assemble into spherical aggregates.