Effect of siloxane spacer length on organosilicon bi-quaternary ammonium amphiphiles

Abstract A series of organosilicon Bola-form bi-quaternary ammonium amphiphiles, [OH 5 C 3 (C 2 H 5 ) 2 N + -(CH 2 ) 3 -Si(CH 2 ) 3 -O-(Si(CH 3 ) 2 O) n -Si(CH 2 ) 3 ⋯(CH 2 ) 3 -N + (C 2 H 5 ) 2 C 3 H 5 O]Cl 2− (Si n N 2 Cl 2 , n  = 0, 4, 6, 8), with the same headgroups and different length of hydrophobic linkage has been synthesized. The critical micelle concentration (CMC) of each amphiphiles was determined by equilibrium surface tension. With the increased length of hydrophobic siloxane spacer, the CMC values follow the order of Si 8 N 2 Cl 2 6 N 2 Cl 2 4 N 2 Cl 2 0 N 2 Cl 2 . At the same time, thermodynamic parameters ( Δ G m 0 , Δ H m 0 , Δ S m 0 ) of micellization were calculated by Gibbs equation under the mass action model. It indicates that the micellization of the amphiphiles in aqueous solution is entropy-driven. Especially, the surface area ( A min ) of Si 4 N 2 Cl 2 is the largest attributing to the interaction of free-rotated siloxane spacer and its internal oxygen atom, which make the molecular stretch free. The antimicrobial property of Si 4 N 2 Cl 2 is more effective than others below the concentration of CMC, while the antimicrobial property of Si 8 N 2 Cl 2 is more effective above the concentration of CMC, which indicated that both the adsorbability in formation of micellar and the hydrophobicity arising from the different length of siloxane spacer are important to inhibit microbe. Moreover, their wetting ability has been characterized by contact angles on various material surfaces. It shows that the higher weight of lipophilic siloxane spacer leads to lower contact angels.