Surface tension reduction (STR) in aqueous solutions of anionic surfactants with cobalt(III) complexes

It has been observed that, at 25.0+/-0.1 degrees C, [Co(NH(3))(6)](ClO(4))(3), [Co(en)(3)](ClO(4))(3), [Co(bpy)(3)](ClO(4))(3), and [Co(phen)(3)](ClO(4))(3) in the regions of 1.25-5.00 mM aqueous solutions cause a significant surface tension reduction (STR) of water by the surfactants, sodium dodecylsulfate (SDS) and sodium dodecylbenzenesulfonate (SBS), suggesting the formation of the 1:1 and 1:2 association complexes, {[complex](3+)(S(-))}(2+) and {[complex](3+)(S(-))(2)}(+) where [complex](3+)=[Co(NH(3))(6)](3+), [Co(en)(3)](3+), [Co(bpy)(3)](3+), or [Co(phen)(3)](3+), S(-)=DS(-) or BS(-). The effect of [Co(en)(3)](3+) on STR in SDS-water system is the largest due to a strong hydrophilic interaction between amino protons of [Co(en)(3)](3+) and sulfate oxygen atoms of DS(-). The effects of [Co(en)(3)](3+), [Co(bpy)(3)](3+), and [Co(phen)(3)](3+) on STR in SBS-water system are significant and almost same, meaning that the hydrophilic interaction between [Co(en)(3)](3+) and the sulfonate group is comparable to the hydrophobic interaction between [Co(bpy)(3)](3+) or [Co(phen)(3)](3+) and the phenyl group of BS(-). The Co(III) complexes of 1.25-5.00 mM are precipitated as {[complex](3+)(S(-))(3)} at 0.0295-0.173 mM of S(-). The precipitates, {[Co(bpy)(3)](3+)(S(-))(3)} and {[Co(phen)(3)](3+)(S(-))(3)} can be dissolved at higher molar ratio of [S(-)]/[complex(3+)] than 3.5 for SDS and 4.0 for SBS. This observation suggests that the aggregated premicelle [Co(bpy or phen)(3)](2)(DS)(-)(7) or aggregated premicelle [Co(bpy or phen)(3)](BS)(-)(4) is formed.