The pH dependence of UV-vis absorption and resonance Raman spectra of 5 X 10-4 M aqueous solutions of an azobenzene-containing, single-chain amphiphile has been studied at 4 and 28 °C, temperatures which are below and above the liquid crystalline-gel phase transition temperature (Tc = 24 °C) of the neutral solution, respectively. At 4 °C, absorption spectra of the solution in the pH range between 4.0 and 11.2 give sharp bands ascribable to the gel state, as expected for the solution below T0. However, above pH 11.5 the solution exhibits spectral features typical of a mixture of liquid crystalline and molecularly dispersed states, indicating that Tc is markedly decreased at these high pH values. On the opposite side of pH, i.e., at pH-0.3, the solution gives the absorption spectrum characteristic of the coagel state. At pH-1.1, a strong absorption band attributable to protonated species appears. At 28 °C, on the other hand, spectra of the solution in the pH range between -0.7 and 11.4 give the liquid crystalline and monomer bands. Below pH-0.8, they give a strong band due to the protonated species. The observed spectral changes with pH were fully reversible. Raman spectra of the aqueous solution at extremely low pH values indicate that the quinoid form is predominant in the resonant hybrids of protonated species.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Surfaces and Interfaces