Effect of Pressure on the Solubilization of a Fluorescent Merocyanine Dye by a Nonionic Surfactant

The target dye, which is a derivative of Merocyanine 540 bearing a naphthoxazole headgroup, persists as a monomer in ethanol solution but dimerizes in water under ambient conditions. Analysis of the absorption spectrum indicates that the dimer has an oblique geometry with the two molecules being held at an angle of ca. 55 degrees. Applying high pressure to the system forces the two molecules into closer contact, resulting in a decreased partial molar volume of 3.1 cm(3). One molecule of the monomeric dye enters a neutral micelle formed from Triton X-100, where it is highly fluorescent and free of exciton coupling. The result of applied pressure on these latter systems depends on the concentration of surfactant. Above the critical micelle concentration (CMC), applied pressure has little effect other than to increase the viscosity inside the micelle. At very low surfactant concentration, applied pressure forces monomeric dye into the dimeric form, as observed in the absence of Triton X-100. It is notable, however, that the pressure effect on the dimerization constant is exaggerated in the presence of surfactant. At intermediate surfactant concentrations, applied pressure leads to a marked change in the CMC. In particular, applied pressure reduces the partial molar volume of the micelle by ca. 7.9 cm3 and induces micelle formation at relatively low concentration of surfactant. For example, the CMC falls from ca. 250 mu M at atmospheric pressure to only 50 mu M at 460 MPa.