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

Abstract

It has been observed that, at 25.0 ± 0.1 ° 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.