Application of the spin probe method to cast films of amphiphiles

Abstract

Electron spin resonance (ESR) spectra for cast films and spin-coated films of stearic acid (STA), dioctadecyldimethylammonium chloride (DODAC) or didodecyldimethylammonium bromide (DDAB) mixed with spin probes, 5-doxylstearic acid (5-DXSTA) or 16-doxylstearic acid (16-DXSTA), were measured. The anisotropy of the ESR spectra was determined by varying the angle Θ, between the direction of the applied magnetic field and the direction normal to the sheet. The temperature dependence of the ESR spectra was measured to examine the effect of molecular motion on the ESR spectra. For the cast and spin-coated DODAC and DDAB films, a clear Θ dependence of the ESR spectra was observed, especially at higher temperature when 5-DXSTA was used as a spin probe, while the ESR spectra for these films were almost isotropic at higher temperature when 16-DXSTA was used as a spin probe. From this result, it was confirmed that the methylene chains close to the head groups in cast and spin-coated films of DODAC and DDAB were carrying out anisotropic rotational Brownian motion around the axis perpendicular to the film plane, while the tail parts of the methylene chains were doing almost isotropic rotational motion at higher temperature. The temperature dependence of the ESR spectra for cast and spin-coated films of DODAC and DDAB showed that the flexibility of methylene chains in DDAB films was larger than that in DODAC films, and this difference was ascribed to the difference in the gel-to-liquid crystal transition temperatures of DODAC and DDAB films. On the other hand, the ESR spectra for cast and spin-coated films of STA mixed with 5-DXSTA or 16-DXSTA did not show clear anisotropy anywhere in the temperature range used in this study. From this result, it was shown that amphiphiles in these films had a less ordered structure compared with those in DODAC and DDAB films.