Advances in Colloid and Interface Science Ž . 91 2001 163219 The structures of Langmuir  Blodgett films of fatty acids and their salts J.B. Peng, G.T. Barnes, I.R. Gentle  Department of Chemistry, The Uni  ersity of Queensland, Brisbane, Queensland 4072, Australia Abstract Recent advances in several experimental techniques have enabled detailed structural Ž . information to be obtained for floating Langmuir monolayers and Langmuir  Blodgett films. These techniques are described briefly and their application to the study of films of fatty acids and their salts is discussed. Floating monolayers on aqueous subphases have been shown to possess a complex polymorphism with phases whose structures may be compared to those of smectic mesophases. However, only those phases that exist at high surface Ž . pressures are normally used in Langmuir  Blodgett LB deposition. In single LB monolay- ers of fatty acids and fatty acid salts the acyl chains are in the all-trans conformation with their long axes normal to the substrate. The in-plane molecular packing is hexagonal with long-range bond orientational order and short-range positional order: known as the hexatic-B structure. This structure is found irrespective of the phase of the parent floating monolayer. The structures of multilayer LB films are similar to the structures of their bulk crystals, consisting of stacked bilayer lamellae. Each lamella is formed from two monolayers of fatty acid molecules or ions arranged head to head and held together by hydrogen bonding between pairs of acids or ionic bonding through the divalent cations. With acids the acyl chains are tilted with respect to the substrate normal and have a monoclinic structure, whereas the salts with divalent cations may have the chains normal to the substrate or tilted. The in-plane structures are usually centred rectangular with the chains in the trans Abbre  iations : LB, Langmuir Blodgett; SPM, scanning probe microscopy; AFM, atomic force microscopy; TEM, transmission electron microscopy; BAM, Brewster angle microscopy; GIXD, grazing incidence X-ray diffraction; SAXS, small angle X-ray scattering; FTIR, Fourier transform infrared; ATR, attenuated total reflection; RAS, reflection absorption spectroscopy; QCM, quartz crystal mi- crobalance  Corresponding author. Tel. 61-7-3365-4800; fax: 61-7-3365-4299. Ž . E-mail address: gentle@chemistry.uq.edu.au I.R. Gentle . 0001-868601$ - see front matter  2001 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 0 1 - 8 6 8 6 99 00031-7