EFFECT OF STRUCTURAL VARIATION WITHIN CATIONIC AZO-SURFACTANT UPON PHOTORESPONSIVE FUNCTION IN AQUEOUS-SOLUTION

The cationic azo-surfactants possessing different spacers and tail alkyl chain lengths have been synthesized by azocoupling of p-alkylaniline or o-, p-ethoxyaniline with phenol, followed by alkylation and quaternalization with dibromoalkane and trimethylamine, respectively. These surfactants showed a good solubility in water. A reversible trans-cis isomerization of the azo-surfactants by photoirradiation was assessed by UV-Vis absorption spectra. Due to a difference in HLB between the trans- and cis-surfactants, the observed critical micelle concentration (CMC) values and the electric conductivity of the surfactant solution at above the CMC were significantly affected by the photoinduced trans-cis isomerization. The azo-surfactants bearing moderate alkyl chain lengths such as surfactants 6 (R(2) = C2H4, R(3) = C4H9) and 9 (R(2) = C4H8, R(3) = C2H5) were found to be effective to achieve large CMC changes (3.6 mmol/L for 6 and 5.9 mmol/L for 9) by UV-light irradiation. The replacement of the tail chain species also affected the photoresponsive function. The surfactant 12, possessing p-ethoxy group as the tail chain, was found to form a stable micelle aggregation as compared with the structurally related surfactant 10 having ethyl unit as its tail group, but it exhibited a large CMC change (5.3 mmol/L) by UV-light irradiation.