Langmuir monolayers experiments

Partly soluble triblock copolymers are also sometimes used for monolayer studies. Such investigations could provide data on desorption kinetics, and allow for comparison of the film structure, whether spread or adsorbed. However, attention should be paid to data interpretation in such cases because intricate equilibriums take place in such systems. A somewhat confusing study has been presented concerning the monolayer miscibility between PLA and PEO-PPO-PEO (also known as Pluronic) in monolayers [53]. The authors attempted to discuss interactions between the triblock copolymer and a homopolymer (PLA) on the basis of Langmuir monolayer experiments however, the results show unrealistic values for molecular areas, and therefore conclusions from those measurements cannot be quantitative. In particular, surface pressure-area isotherms for pure polymers and their mixtures reveal, in the compressed state, areas per monomer unit of the order of 3 h and below. Such low values cannot be real and most probably result either from material dissolution in the subphase or poor spreading at the air-water interface. Indeed, the isotherms do not appear smooth, which suggests low film stability and difficulties in forming a true monolayer. 

One of the other challenging biologiccil applications of Langmuir monolayers concerns lung (or pulmonary) surfactant mixtures. This issue was already mentioned on the first page of Volume 1, and briefly explained thereafter. With the experience of the present chapter we now come back to it and give it a broader background. 

By far the most common experiment performed on Langmuir monolayers is the determination of surface pressure-area isotherms. The monolayer can be prepared by depositing a solution of the amphiphile in a volatile solvent on a clean water surface the film spreads spontaneously as the solvent evaporates. The surface pressure n is defined as the difference between yo, the surface tension of pure water, and y, the surface tension of the surface covered by the monolayer ... 

Penetration systems at the air-water interface in which a dissolved amphiphile (surfactant, protein) penetrates into a Langmuir monolayer are interesting models for a better understanding of various complex processes. Most of all, penetration systems can simulate properties of biological membranes typically comprised of lipids mixed with proteins. First penetration experiments have been described by Schulman and Hughes in 1935 [110]. In the... 

In a different application of HT electrochemistry involving an alkanethiol-coated working electrode. Miller and coworkers devised a way to measure the flow rate of a Langmuir monolayer along the metal-alkanethiol/solution interface [37, 40,41). In this experiment, an alkanethiol-coated gold electrode is initially touching... 

Fig. 3 Schematic depiction of the Langmuir trough apparatus and the positioning of the electrodes used in the lateral monolayer flow experiments. The inset shows the Au-coated glass slide working electrode modified with a self-assembled monolayer (SAM) of dodecanethiol. The working electrode is touching the air/water interface. A compound that forms a Langmuir monolayer is deposited on the water surface. It immediately spreads to cover the entire interface. Subsequently, the Langmuir film flows across the triple phase boundary into the SAM/water interface forming a bilayer. The lateral flow is completed when the entire electrode/solution interface is coated with a bilayer (from Ref 37).

Zhang et al. used Langmuir monolayers to study the initial stage of nucleation and crystallization of calcium phosphate [160]. The monolayers used in their study consisted of dipalmitoylphosphatidylcholine, arachidic acid, and octadecylamine. The experiments show that the adsorption of calcium ions to the respective mono-layer is a prerequisite for subsequent nucleation. The authors also demonstrate that calcium phosphate forms through a multistage assembly process, in which first an amorphous calcium phosphate dihydrate layer forms, which then recrystallizes to form a crystalline hydroxyapatite layer (Fig. 12). This transformation of an amorphous phase to a crystalline phase provides direct evidence of a multistep crystallization process, which the authors claim is similar to the processes occurring in biomineralization. 

Abstract Detailed investigations of interfacial crystallization procedures are important to understand the basic principles of biomineralization processes. These interfacial phenomena can also be used to form new types of biomimetic composite materials. In a series of experiments we studied the influence of Langmuir-monolayers on the formation of ultra-thin calcium carbonate Aims. We systematically compared experiments performed at the water surface wi results obtained at the water/oil interface. 

KEY EXPERIMENT THE LANGMUIR MONOLAYER AS A UNIVERSAL TOOL EOR THE STUDY OF LIPID-PROTEIN INTERACTIONS ... 

Fig. 3.5 Representation of a scheme of an experiment (upper set of drawings) and the obtained experimental results presented as AFM images (middle part) and cross-sectional profiles (bottom) that provides evidence of silica nucleation and shell formation on biopolymer macromolecules. Scheme of experiment. This includes the following main steps. 1. Protection of the mica surface against silica precipitation. It was covered with a fatty (ara-chidic) acid monolayer transferred from a water substrate with the Langmuir-Blodgett technique. This made the mica surface hydrophobic because of the orientation of the acid molecules with their hydrocarbon chains pointing outwards. 2. Adsorption of carbohydrate macromolecules. Hydrophobically modified cationic hydroxyethylcellulose was adsorbed from an aqueous solution. Hydrocarbon chains of polysaccharide served as anchors to fix the biomacromolecules firmly onto the acid monolayer. 3. Surface treatment by silica precursor. The mica covered with an acid mono-...

Because the adsorbed HM-HEC molecules exhibit such slow rates of chain reorientation, the effects of molecular weight, amount of hydrophobic substitution and chain lengths of the hydrophobes on the interfacial properties of HM-HEC monolayers can be investigated by two kinds of dynamic experiments hysteresis and stress-jump, using a Langmuir trough film balance. 

An HM-HEC monolayer at the air/aqueous interface was formed by adsorption from an aqueous solution of the polymer placed in the Langmuir trough overnight. In "stress-jump" experiments, HM-HEC monolayers were placed under rapid compression to a large degree and surface pressure was measured as a function of time after compression was stopped. (The compressional "jumps" required a minute or two to complete, and in some cases were on the order of the polymer monolayer relaxation times. See later section for discussion). In hysteresis experiments, the adsorbed monolayers were subjected to continuous compression-expansion cycles at a specific speed, while surface pressure was determined as a function of surface area. 

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