Effect of hydrophobic chains on the aggregation behavior of cationic silicone surfactants in aqueous solution

Three cationic silicone surfactants, (2-hydroxyethyl)-N,N-dimethyl-3-tri-(trimethylsiloxy)]silylpropylammonium chloride (Si₄ACl), (2-hydroxyethyl)-N,N-dimethyl-3-bis(trimethylsiloxy)methyl]silylpropylammonium chloride (Si₃ACl), and (2-hydroxyethyl)-N,N-dimethyl-3-bis(trimethylsilylmethyl)methyl]silylpropyl-ammonium chloride (Si₃C₂ACl), with the same headgroups and different hydrophobic groups were synthesized. Their aggregation behavior in aqueous solution was investigated by surface tension and electrical conductivity. The results show that all the three cationic silicone surfactants perform admirable surface activity. Because of the effect of the hydrophobic groups, the critical micelle concentration values increase following the order Si₃C₂ACl?<?Si₄ACl?<?Si₃ACl, and Si₃C₂ACl packs more compactly at the air/water interface compared with Si₄ACl and Si₃ACl. Electrical conductivity studies show that all the three cationic silicone surfactants have low degree of counterion binding. Thermodynamic parameters ( $ \varDeltaH_m^0 $ , $ \varDelta S_m^0 $ , and $ \varDelta G_m^0 $ ) of micellization derived from electrical conductivities indicate that the micellization for both Si₃C₂AC and Si₃ACl in aqueous solution is enthalpy-driven, and that for the Si₄ACl is entropy-driven. For the heat capacities, $ \varDeltac_{m,p}^0 $ , values are positive for Si₄ACl, indicating that there is an attractive interaction between the nitrogen atom of one surfactant molecule and the oxygen atom of another surfactant molecule for Si₄ACl.