Invited paperConcentration dependence of thermal isomerization process of methyl orange in ethanol

- Concentration dependent thermal isomerization (TI) rate of methyl orange.
- Enhancement of TI process by the interaction between azo dyes and cation.
- Interaction with cation causes concentration dependence of TI rate.

The thermal isomerization (TI) rates of methyl orange (MO) and 4-dimethylaminoazobenzene (DMAAB) in ethanol (EtOH) are measured. Usually TI rates of azobenzene dyes are known to be concentration independent. However, the TI rate of MO showed a concentration dependence whereas that of DMAAB did not. The TI rate of DMAAB in EtOH became larger by the addition of alkali halide. This phenomenon is caused mainly by the interaction between DMAAB and cation. MO is a derivative of DMAAB in which one end of the azobenzene is substituted by a SO3−Na+ group. The interaction with the dissociated Na+ ion is considered to be an origin of the concentration dependence of the TI rate of MO.