The structures of Langmuir-Blodgett films of fatty acids and their salts

Recent advances in several experimental techniques have enabled detailed st ructural information to be obtained for floating (Langmuir) monolayers and Langmuir-Blodgett films. These techniques are described briefly and their a pplication to the study of films of fatty acids and their salts is discusse d. Floating monolayers on aqueous subphases have been shown to possess a co mplex polymorphism with phases whose structures may be compared to those of smectic mesophases. However, only those phases that exist at high surface pressures are normally used in Langmuir-Blodgett (LB) deposition. In single LB monolayers of fatty acids and fatty acid salts the acyl chains are in t he all-cans 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. T his structure is found irrespective of the phase of the parent floating mon olayer. The structures of multilayer LB films are similar to the structures of their bulk crystals, consisting of stacked bilayer lamellae. Each lamel la is formed from two monolayers of fatty acid molecules or ions arranged h ead 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 structur e, whereas the salts with divalent cations may have the chains normal to th e substrate or tilted. The in-plane structures are usually centred rectangu lar with the chains in the trans conformation and packed in a herringbone p attern, Multilayer films of the acids show only a single-step order-disorde r transition at the malting point, This temperature tends to rise as the nu mber of layers increases. Complex changes occur when multilayer films of th e salts are heated. Disorder of the chains begins at low temperatures but t he arrangement of the head groups does not alter until the melting temperat ure is reached, Slow heating to a temperature just below the melting temper ature gives, with some salts, a radical change in phase. The lamellar struc ture disappears and a new phase consisting of cylindrical rods lying parall el to the substrate surface and stacked in a hexagonal pattern is formed, I n each rod the cations are aligned along the central axis surrounded by the disordered acyl chains. (C) 2001 Elsevier Science B,V. All rights reserved .