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 (Si(4)ACl), (2-hydroxyethyl)-N,N-dimethyl-3-[bis(trimethylsiloxy)methyl]silylpropylammonium chloride (Si(3)ACl), and (2-hydroxyethyl)-N,N-dimethyl-3-[bis(trimethylsilylmethyl)methyl]silylpropyl-ammonium chloride (Si(3)C(2)ACl), 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 Si(3)C(2)ACl < Si(4)ACl < Si(3)ACl, and Si(3)C(2)ACl packs more compactly at the air/water interface compared with Si(4)ACl and Si(3)ACl. Electrical conductivity studies show that all the three cationic silicone surfactants have low degree of counterion binding. Thermodynamic parameters (, , and ) of micellization derived from electrical conductivities indicate that the micellization for both Si(3)C(2)AC and Si(3)ACl in aqueous solution is enthalpy-driven, and that for the Si(4)ACl is entropy-driven. For the heat capacities, , values are positive for Si(4)ACl, indicating that there is an attractive interaction between the nitrogen atom of one surfactant molecule and the oxygen atom of another surfactant molecule for Si(4)ACl.