Surface layer in composites containing 4-n-octyl-4′-cyanobiphenyl.: FTIR spectroscopic characterization

Composites containing 4-n-octyl-4'-cyanobiphenyl (8CB) either confined to nanopores of molecular sieves with very large pores or coating silica nanoparticles of aerosil type at high silica-to-8CB ratios are investigated by IR spectroscopy. Band shape analysis was performed in wavenumber regions in which the peaks due to CN stretching, CH stretching and CH out-of-plane vibrations appear. Some of molecules confined to molecular sieves show spectroscopic features characteristic to a bulk-like 8CB matter located in the centre of the pores or in the inter-grain space. Other features of the IR spectra are due to 8CB molecules located in the surface layer, mostly forming hydrogen bonds between their CN groups and surface OH groups. Another part of the 8CB molecules in the surface layer may also interact by pi electrons of the aromatic rings. Hydrogen bonding is less hindered for the molecules of the surface layers onto aerosil particles than inside pores of the molecular sieves. Comparison is also made with the case of composites based on molecular sieves with small pores. (C) 2003 Elsevier Science B.V. All rights reserved.