Langmuir film

A newer and perhaps more useful application of ellipsometry to Langmuir films is their lateral characterization via ellipsometric microscopy [146], A simple modification of the nuU ellipsometer allows one to image features down to 10-/im resolution. Working with a fixed polarizer and analyzer, some domains are at extinction while others are not and appear bright. This approach requires no fluorescent label and can be applied to systems on reflective supports. 

Electron diffraction studies are usually limited to transferred films (see Chapter XV), One study on Langmuir films of fatty acids has used cryoelectron microscopy to fix the structures on vitrified water [179], Electron diffraction from these layers showed highly twinned structures in the form of faceted crystals. 

Transmission electron microscopy (TEM) can resolve features down to about 1 nm and allows the use of electron diffraction to characterize the structure. Since electrons must pass through the sample however, the technique is limited to thin films. One cryoelectron microscopic study of fatty-acid Langmuir films on vitrified water [13] showed faceted crystals. The application of TEM to Langmuir-Blodgett films is discussed in Chapter XV. 

Films spread at liquid-liquid interfaces or on liquids other than water are discussed followed by the important effects of charged monolayers on water. Finally, the most technologically important application of Langmuir films, the Langmuir-Blodgett film deposited on a solid substrate, is reviewed. 

Most of the Langmuir films we have discussed are made up of charged amphiphiles such as the fatty acids in Chapter IV and the lipids in Sections XV-4 and 5. Depending on the pH and ionic strength of the subphase, electrostatic effects can become quite important. Here we develop the theoretical foundation for charged films with the Donnan relationship. Then we mention the influence of subphase pH on film behavior. 

Because of the charged nature of many Langmuir films, fairly marked effects of changing the pH of the substrate phase are often observed. An obvious case is that of the fatty-acid monolayers these will be ionized on alkaline substrates, and as a result of the repulsion between the charged polar groups, the film reverts to a gaseous or liquid expanded state at a much lower temperature than does the acid form [121]. Also, the surface potential drops since, as illustrated in Fig. XV-13, the presence of nearby counterions introduces a dipole opposite in orientation to that previously present. A similar situation is found with long-chain amines on acid substrates [122]. 

Mixing fatty acids with fatty bases can dissolve films as the resulting complexes become water-soluble however, in some cases the mixed Langmuir film is stabilized [128]. The application of an electric field to a mixed lipid monolayer can drive phase separation [129]. 

The transfer of Langmuir films from the air-liquid surface to a solid substrate has come to be known as Langmuir-Blodgett deposition, after its developers [130, 131]. The solid substrates are usually hydrophilic surfaces such as... 

Castillo R, Ramos S and Ruiz-Garcia J 1996 Direct observation of Langmuir films of Cgg and C- g using Brewster angle microscopy J. Phys. Chem. 100 15 235-41... 

Nierengarten J-F, Schall C, Nicoud J-F, Fleinrich B and Guillen D 1998 Amphiphilic cyclic fullerene bisadducts synthesis and Langmuir films at the air-water interface Tetrahedron Lett. 39 5747-50... 

Figure C2.4.2. Schematic sideview of tlie trough. The movable barrier is used to push tire molecules on tire subphase togetlier in tire Langmuir film which is subsequently transferred to a solid substrate.

Riohard J, Barruad A, Vandevyver M and Ruaudel-Teixier A 1988 A 2-step transfer of oonduoting Langmuir films from a glyoerol subphase Thin Solid Films 159 207-14... 

Roberts G G, MoGinnity M, Barlow W A and Vinoett P S 1979 Eleotroluminesoenoe, photoluminesoenoe and eleotroabsorption of a highly substituted anthraoene Langmuir film Solid State Commun. 32 683-6... 

Flardy R M and Soala L C 1966 Eleotrioal and struotural properties of Langmuir films J. Electrochem. Soc. 113 109-16... 

Procarione W L and Kauffman J W 1974 The electrical properties of phospholipid bilayer Langmuir films Chem. Phys. Lipids 12 251-60... 

Organized Langmuir films and poly layers of mesomorph cyclolinear polysilox-anes 98MI35. 

II. SUPRAMOLECULAR LAYER ENGINEERING A. Langmuir Film Engineering... 

Formation of Langmuir films of spherical particles at the air-liquid interface [21,22,31,43,65-84]... 

LANGMUIR FILMS OF SPHERICAL POLYMER PARTICLES A. Film Preparation... 

FIG. 1 Schematic showing the preparation of Langmuir films of latex particles at the air-water interface (a) Spreading of the latex and formation of an expanded monolayer (b) formation of the compressed monolayer. 

In the same year, Fulda and Tieke [75] reported on Langmuir films of monodisperse, 0.5-pm spherical polymer particles with hydrophobic polystyrene cores and hydrophilic shells containing polyacrylic acid or polyacrylamide. Measurement of ir-A curves and scanning electron microscopy (SEM) were used to determine the structure of the monolayers. In subsequent work, Fulda et al. [76] studied a variety of particles with different hydrophilic shells for their ability to form Langmuir films. Fulda and Tieke [77] investigated the influence of subphase conditions (pH, ionic strength) on monolayer formation of cationic and anionic particles as well as the structure of films made from bidisperse mixtures of anionic latex particles. 

C. Langmuir Films of Core-Shell Latex Particles... 

For the characterization of Langmuir films, Fulda and coworkers [75-77] used anionic and cationic core-shell particles prepared by emulsifier-free emulsion polymerization. These particles have several advantages over those used in early publications First, the particles do not contain any stabihzer or emulsifier, which is eventually desorbed upon spreading and disturbs the formation of a particle monolayer at the air-water interface. Second, the preparation is a one-step process leading directly to monodisperse particles 0.2-0.5 jim in diameter. Third, the nature of the shell can be easily varied by using different hydrophilic comonomers. In Table 1, the particles and their characteristic properties are hsted. Most of the studies were carried out using anionic particles with polystyrene as core material and polyacrylic acid in the shell. 

In order to study the structure of Langmuir films of polymers spheres, most researchers deposited the films on solid substrates using the LB technique [158-162] and analyzed the structure using a microscope. A modified version of the LB method allowing the transfer of particle monolayers is outlined in Figure 8a. 

Langmuir films have been generated not only from phospholipids but also from tetraether lipids (Fig. 14b). Tetraether glycerophospho- and glycoUpids are typical for ar-chaea, where they may constitute the only polar lipids of the cell envelope [154,155]. Tetraether lipids are membrane-spanning lipids, a single monolayer has almost the same thickness as a phospholipid bilayer. 

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