Photochemically induced breakdown of rod-like micelles: light-induced viscosity changes

Abstract

The photochemical decomposition of three types of azo compound is used to change the viscosities of surfactant solutions. The azo additives are solubilized in systems containing rod-like cetyl-pyridinium salicylate micelles, and decomposed by UV irradiation. For solubilized 4-hexyl-phenyl-diazonium chloride, the viscosity reduces to 14% of its initial value after photolysis. Aryl-diazonium compounds suffer from thermal instability. Sodium 3-alkyl-phenyl-azosulphonates, which are more stable in the solid phase and in aqueous solution, exhibit a strong tendency to decompose on contact with the micellar solution. In contrast, the compound 3-methyl-phenyl-azo-phenylsulphone is stable in the micelles. On UV irradiation the viscosity reduces to 6% of its value after solubilization. The concentration of azo additive required to produce the photorheological effect corresponds to one-seventh of the surfactant concentration. A quantum yield of 0.025 ± 0.002 has been determined for the photolysis at 308 nm in toluene and ethanol. On average, approximately 11 quanta per solubilized molecule are required to produce the viscosity breakdown. Gas chromatography—mass spectrometry (GC—MS) analysis reveals that alkyl-benzenes are the products of the photochemical reaction in the micellar system: these hydrocarbons decrease the viscosity, as they convert the rod-like micelles into small spherical aggregates.