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

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-d35) acids, as well as binary-component films of 4:1 H:D C24:C18-d35 and 4:1 H:D C18:C18-d35 were studied. For C24:C18-d35 binary films, changes in the peak wavenumber of the νaCH2 band after heating above the main chain melting temperature (Tm) reflect a thermally induced de-mixing and phase separation of the C24 component from C18-d35. For C18:C18-d35 LB film, heating above Tm shows that C18 and C18-d35 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 Tm. Above Tm, 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-d35 LB film more easily crystallized the trans ring dimer after heating above Tm. The band splitting of the δCH2 vibration at ∼1470 cm−1 was used to monitor micro-domain phase separation in these samples. For the C24:C18-d35 binary mixture, heating the sample above Tm before cooling crystallizes the C24 chains into phase-separated micro-domains. In contrast, the C18:C18-d35 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 δCH2 band splitting in the binary C18:C18-d35 sample, and hence is less ordered and less crystalline than the C24 binary LB film.