Zwitterionic [(4-n-alkyl-1,4-bisazoniacyclohex-1-yl) methyl]-pentafluorosilicates: A new class of surface-active compounds with a hexacoordinate silicon atom

A series of [(4-n-alkyl-1,4-bisazoniacyclohex-1-yl)methyl]pentafluorosilica tes (alkvl = hexvl, heptyl, octyl, nonyl, decyl; compounds 14-18) were synt hesized and studied for their surface activity. The zwitterionic pentafluor osilicates with hexacoordinate Si atoms 14-18 were prepared by reaction of the respective [(4-n-alkylpiperazin-1-yl)methyl]trimethoxysilanes [obtained by treatment Of (MeO)(3)-SiCH2Cl with the respective n-alkylpiperazine in the presence of NEt3] with HF in water/ethanol. Surface tension measurement s with solutions of 14-18 in 0.01 M hydrochloric acid proved that these com pounds are surfactants, the increase of the n-alkyl chain length resulting in an increase of surface activity (14 --> 18). The equilibrium surface ten sion vs concentration isotherms for 14-17 (solutions of "surface-chemically pure" samples in 0.01 M hydrochloric acid) were analyzed quantitatively.