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

摘要

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 (CI8-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 v(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:CI8-d35 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 delta CH2 vibration at similar to 1470 cm(-1) was used to monitor micro-domain phase separation in these samples. For the C24:CI8-d35 binary mixture, heating the sample above T-m before cooling crystallizes the C24 chains into phase-separated micro-domains. In contrast, the C18:CI8-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 delta CH2 band splitting in the binary C18:CI8-d35 sample, and hence is less ordered and less crystalline than the C24 binary LB film. (C) 2003 Elsevier Science B.V. All rights reserved. [References: 43]