Langmuir films monolayers

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. 

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 second step, Figure 32b, consists of the covering of the styli with cadmium arachidate LB films. Monolayers of arachidic acid (in principle, it is also possible to use stearic or behenic acids with practically the same results) were spread over the surface of 10 " M CdCli water subphase in a Langmuir trough. The monolayer was compressed to a surface pressure of 27 mN/m and transfered onto styli by a vertical dipping technique. Up to six monolayers were deposited. 

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. 

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. 

Fig. 1 Schematic representation of the Langmuir film balance used for the measurement of pressure-area monolayer film properties. Reprinted with permission from Arnett et al., 1989. Copyright 1989 American Chemical Society.

Until very recently, there has been little or no experimental protocol for obtaining quantitative dynamic surface tension data on monolayer films. In most cases, the experimental set-up has consisted of a simple Langmuir film balance equipped with a variable-speed motor to drive the moving barrier. Hysteresis data were then obtained at a number of compression/expansion rates and compared qualitatively. This experimental set-up was improved considerably by Johnson (Arnett et al., 1988a), who modified a special... 

The advantages of SAMs are that they are sturdily anchored at a fixed distance from the metal substrate, may be more robust than Langmuir films, and can be convenient to prepare. A disadvantage is that uniform monolayer coverage, so easily achieved kinetically for LB films, is more difficult to obtain in SAMs. This is because SAMs are created by random attack on the electrode surface, in contrast to Langmuir films, which are transferred when they are close-packed. 

However, transfer of the monolayers to solid substrates was inefficient because of the small size of the hydrophilic anchor, which imparted a hydrophobic/hydrophilic imbalance. Accordingly, increasing the size of the hydrophilic anchor facilitated the formation of stable Langmuir films that could be transferred to sohd substrates with a transfer ratio of 1 (Nierengarten et al. 2001). Bayer s research group reported a related approach in which the fullerene was modified with carboxylate-terminated dendrons and alkyl chains (Maierhofer et al. 2000). This amphiphilic fullerene derivative formed stable monolayers that could be compressed and expanded without hysteresis (Fig. 11.46d Maierhofer et al. 2000). 

Peleshanko S, Sidorenko A, Larson K, Villavicencio O, Omatska M, McGrath DV, Tsuknik VV. Langmuir-Blodgett monolayers from lower generation amphiphilic monodendrons. Thin Sohd Films 2002 406 233-240. 

Description of the different mimetic systems will be the starting point of the presentation (Sect. 2). Preparation and characterization of monolayers (Langmuir films), Langmuir-Blodgett (LB) films, self-assembled (SA) mono-layers and multilayers, aqueous micelles, reversed micelles, microemulsions, surfactant vesicles, polymerized vesicles, polymeric vesicles, tubules, rods and related SA structures, bilayer lipid membranes (BLMs), cast multibilayers, polymers, polymeric membranes, and other systems will be delineated in sufficient detail to enable the neophyte to utilize these systems. Ample references will be provided to primary and secondary sources. 

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