Change in the Layered Structure of Merocyanine Dye Langmuir-Blodgett Films due to N-Octadecane Addition

Abstract

The layered structure in Langmuir-Blodgett (LB) films of the merocyanine dye (MS)-arachidic acid (C20) binary system and the MS–C20-n-octadecane (AL18) ternary system with low and high C20 contents ([MS]: [C20]:[AL18]=1: 2: x and 1: 5: x, 0≦x≦5.0) has been studied by means of surface pressure (π)–area (A) isotherm and X-ray diffraction measurements. The π–A isotherm measurement indicates that AL18 tends to fill the empty space of MS; however, a certain amount of AL18 is found outside of the space of MS even for smaller mixing ratios of AL18. A phase transition is observed in the π–A isotherms of the ternary monolayers with low and high C20 contents for x≧3.0. Two different regimes can be recognized in the X-ray diffraction patterns of the LB films with low and high C20 contents: 2θ< 9° and 9° < 2θ. The bilayer unit-cell species governing the X-ray diffraction patterns for 2θ<9° changes with increasing molar mixing ratio of C20. X-ray diffraction and π–A isotherm measurements indicate that the increase in AL18 in the empty space of MS causes the expansion of the spacing estimated for 2θ< 9° in the LB films.