Synthesis, Micellar and Surface Properties of Cationic Trisiloxane Surfactants with Different Siloxane Hydrophobic Groups

Three cationic trisiloxane surfactants, 1-methyl-1-[bis(trimethylsiloxy)methyl]silyl-propylpyrrolidinium chloride (Si₃pyCl), 1-methyl-1-[bis(triethylsiloxy)methyl]silyl-propylpyrrolidinium chloride (Et-Si₃pyCl), and 1-methyl-1-[bis(vinyldimethylsiloxy)methyl]silylpropylpyrrolidinium chloride (Vi-Si₃pyCl) were synthesized. The aggregation behavior of the trisiloxane surfactants with different siloxane hydrophobic groups in aqueous solution was investigated by surface tension and electrical conductivity measurements. The structures of hydrophobic groups of the trisiloxane surfactants can obviously influence their surface activities and thermodynamics. All the three cationic trisiloxane surfactants have excellent surface activity. Owing to the steric hindrance of hydrophobic groups, the CMC values increase following the order Et-Si₃PyCl < Vi-Si₃PyCl < Si₃PyCl. The $\Delta G_{\text{m}}^{\text{o}}$ values increase in the order Et-Si₃PyCl > Vi-Si₃PyCl > Si₃PyCl, attributed to the decrease in the hydrophobic effect. The micellization processes of these surfactants are entropy-driven.