A cinnamic acid-type photo-cleavable surfactant.

We have developed a novel cinnamic acid-type photo-cleavable surfactant. This surfactant experiences photo-cleavage through UV-induced cyclization in aqueous solutions. The photo-cleavage not only reduces its capabilities as a surfactant but also yields two functional materials including a coumarin derivative and an aminated polyoxyethylene compound. This means that the photo-cleavable surfactant synthesized in this study is a photo-responsive function-exchangeable material. In our current study, we have characterized the photo-cleavable behavior that occurs in aqueous solutions and a resulting change in interfacial properties. The photo-cleavage induces an increased interfacial tension of a squalane/water interface and a decreased solubilization capability of the surfactant micelles.

Stable surfactant-free toluene-polyethylene-in-water emulsion prepared by ultrasonication at high temperature.

A toluene-polyethylene (PE) mixture, only partially miscible at room temperature (RT), was ultrasonically dispersed in hot water, followed by immediate cooling to give a highly stable surfactant-free oil-in-water (O/W) emulsion. This temperature effect was correlated with physical gelation of the bulk mixture. Prolonged stabilization was achieved only through dispersion at a temperature (T(d)) above the gelation temperature (T(gel)) of the toluene-low-density PE (LDPE) mixture and subsequent rapid cooling. These stabilized emulsions exhibited characteristics such as a small droplet size with a narrow size distribution, low ζ-potential, and round-shaped droplets, which were not observed for the emulsions prepared at T(d) < T(gel) or those at T(d) > T(gel) that had been subjected to slow cooling. From these results, physical gelation through crystallization and modification of the droplet surface by PE were concluded to be essential for the prolonged stability of a surfactant-free toluene emulsion.

Photoinduced increase in surfactant solution viscosity using azobenzene dicarboxylate for molecular switching.

We provide further insight into the photochemical control of viscoelasticity through the use of azobenzene sodium dicarboxylate for molecular switching. As a photoresponsive molecule, Sodium 3,3'-azobenzene dicarboxylate (3,3'-Azo2Na) was added to a solution of cetyltrimethylammonium bromide (CTAB)/sodium salicylate (NaSal), which is known for inducing the formation of wormlike micelles. This solution maintained a wormlike micellar structure, although a reduction in zero-shear viscosity was observed. When this mixed aqueous solution of CTAB/NaSal/3,3'-Azo2Na (16.7 mM each) was irradiated by ultraviolet light, the 3,3'-Azo2Na exhibited molecular trans-cis photoisomerization. We measured the dynamic viscoelasticity of the sample in the photostationary state and found that the zero-shear viscosity increased approximately sevenfold compared to the preirradiation state. This phenomenon is the opposite of the system wherein viscosity reduced by irradiation, as reported by us. We discuss the mechanism of this viscosity change.

Photochemical control of molecular assembly formation in a catanionic surfactant system.

Photochemical control of vesicle disintegration and reformation in aqueous solution was examined using a mixture of 4-butylazobenzene-4'-(oxyethyl)trimethylammonium bromide (AZTMA) as the photoresponsive cationic surfactant and sodium dodecylbenzenesulfonate (SDBS) as the anionic surfactant. Spontaneous vesicle formation was found in a wide-ranging composition of the trans-AZTMA/SDBS system. AZTMA molecules constituting vesicles underwent reversible trans-cis photoisomerization when irradiated with ultraviolet and visible light. Transmission electron microscopy observations using the freeze-fracture technique (FF-TEM) showed that UV light irradiation caused the vesicles to disintegrate into coarse aggregates and visible light irradiation stimulated the reformation of vesicles (normal control). A detailed investigation of the phase state and the effects of UV and visible light irradiation on the AZTMA/SDBS system with the use of electroconductivity, dynamic/static light scattering, and surface tension measurements and FF-TEM observations revealed that in the AZTMA-rich composition (AZTMA/SDBS 9:1) a micellar solution before light irradiation became a vesicular solution after UV light irradiation and visible light irradiation allowed the return to a micellar solution (reverse control). Thus, we could photochemically control the disintegration (normal control) and reformation (reverse control) of vesicles in the same system.

Photoinduced reversible change of fluid viscosity.

We report a reversible photoinduced fluid viscosity change. A small amount of a "photoswitchable" azobenzene-modified cationic surfactant (4-butylazobenzene-4'-(oxyethyl)trimethylammonium bromide, AZTMA) was added to a wormlike micellar solution of cetyltrimethylammonium bromide (CTAB) containing sodium salicylate (NaSal). The trans-AZTMA solution had a remarkably high viscosity as a result of the entangled network of wormlike micelles. UV light irradiation on the trans-AZTMA solution remarkably decreased the viscosity of the solution because the bulky structure of cis-AZTMA is likely to disrupt the network structure of wormlike micelles. This photoinduced viscosity change is perfectly reversible between the trans- and cis-AZTMA solutions.