ABSTRACT
A new azobenzene derivative, bis[4-(3-methylbutoxy)phenyl]diazene (DC-azo), was synthesized as yellow powder through a three-step reaction. DC-azo was crystallized from a mixture of acetone/water in the form of highly insulating gold-colored crystals with a specular reflectance of 21% over the wavelength range of 520-800 nm. The CIELAB color space of the crystals indicated that the crystals were slightly more reddish and less yellowish than metallic Au. The crystals retained their gold coloration under dry conditions and were stable up to 383 K. The X-ray diffraction (XRD) measurements estimated the long-range periodical thickness in the crystal to be 1.12 nm, thereby indicating that the DC-azo molecules were oriented such that the azobenzene moieties showed a tilt angle of 32.2° against the crystal surface. The gold-colored crystals of DC-azo could be a potential substitute for the conventional metallic paints.
ABSTRACT
This Article reports on the influence of light irradiation on the stability of emulsions prepared using a photoresponsive gemini surfactant (C7-azo-C7) having an azobenzene skeleton as a spacer. When mixtures of trans C7-azo-C7 aqueous solution and n-octane are homogenized, stable emulsions are obtained in a specific region of weight fraction and surfactant concentration. Fluorescence microscopy observations using a small amount of fluorescent probes show that the stable emulsions are oil-in-water (O/W)-type. UV irradiation of stable O/W emulsions promotes the cis isomerization of trans C7-azo-C7 and leads to the coalescence of the oil (octane) droplets in the emulsions, that is, demulsification. While the equilibrated interfacial tension (IFT) between aqueous trans C7-azo-C7 solution and octane is almost the same as that between aqueous cis C7-azo-C7 and octane, the occupied area per molecule for C7-azo-C7 at octane/water interface decreases with the cis photoisomerization of trans isomer. Dynamic IFT measurement shows that UV irradiation to the interface between aqueous trans C7-azo-C7 solution and octane brings about an increase in the interfacial tension, indicating that the Gibbs free energy at the interface increases. From these results, the cis isomerization of trans C7-azo-C7 molecules at the O/W interface due to UV irradiation leads to direct contact between the water and octane phases, because of the reduction of molecular area at the interface, and subsequently makes the emulsions demulsified.
