Effect of Steric Hindrance on the Aggregation Behavior of Cationic Silicone Surfactants in Aqueous Solutions

Abstract

To study the effect of steric hindrance on the surface activities and thermodynamics of cationic silicone surfactants, the adsorption and aggregation behavior of cationic silicone surfactants with different siloxane hydrophobic groups and headgroups, bis(trimethylsiloxy)methylsilylpropylpyridinium chloride (Si3pyrCl), N,N,N-triethyl-3-[tri(trimethylsiloxy)]silylpropylammonium chloride (Si4AEtCl), and 3-[tri(vinyldimethylsiloxy)methyl]silylpropylpyridinium chloride (Vi–Si4pyrCl), were investigated by surface tension, electrical conductivity, DLS, TEM, and 2D NOESY NMR measurements. All the three cationic silicone surfactants have excellent surface activity. Owing to the steric hindrance of hydrophobic groups and headgroups, the CMC values increase following the order Vi–Si4pyrCl < Si4AEtCl < Si3pyrCl. The micellization process of Si3pyrCl and Si4AEtCl in aqueous solution is enthalpy-driven, while Vi–Si4pyrCl is entropy-driven at low temperature and enthalpy-driven at high temperature and the degree of counterion binding of Vi–Si4pyrCl increases as the temperature increases, caused by the intermolecular interaction between the pyridinium group and vinyldimethylsiloxy hydrophobic region of the micelles. Moreover, the results obtained in this paper can be useful in understanding the structure-properties relationship of cationic silicone surfactants and the design of novel silicone surfactants.