Thermally induced micro-domain formation in fatty acid Langmuir–Blodgett films observed by attenuated total reflectance infrared spectroscopy

Abstract

We have utilized attenuated total reflectance infrared (ATR-IR) spectroscopy to study the temperature dependence of conformational transitions and micro-domain structure in single- and binary-component Langmuir–Blodgett (LB) films. The single-component films such as lignoceric (C24), stearic (C18), and perdeuterated stearic (C18-d 35 ) acids, as well as binary-component films of 4:1 H:D C24:C18-d 35 and 4:1 H:D C18:C18-d 35 were studied. For C24:C18-d 35 binary films, changes in the peak wavenumber of the ν a CH 2 band after heating above the main chain melting temperature ( T m ) reflect a thermally induced de-mixing and phase separation of the C24 component from C18-d 35 . For C18:C18-d 35 LB film, heating above T m shows that C18 and C18-d 35 components do not phase segregate and retain a degree of conformational flexibility, even at low temperatures. The carbonyl CO stretching region between 1800 and 1600 cm −1 showed that each LB film contained a mixture of cis – trans ring dimer conformations at all temperatures below T m . Above T m , the LB films converted to the higher energy cis isomer, while upon further cooling the more stable trans isomer predominated. The shorter chain length C18:C18-d 35 LB film more easily crystallized the trans ring dimer after heating above T m . The band splitting of the δ CH 2 vibration at ∼1470 cm −1 was used to monitor micro-domain phase separation in these samples. For the C24:C18-d 35 binary mixture, heating the sample above T m before cooling crystallizes the C24 chains into phase-separated micro-domains. In contrast, the C18:C18-d 35 binary mixture shows that a much smaller domain size is calculated, indicative of a higher degree of chain miscibility. The shorter C18 hydrocarbon chain length produces a smaller δ CH 2 band splitting in the binary C18:C18-d 35 sample, and hence is less ordered and less crystalline than the C24 binary LB film.