Spread monolayers applications

Under natural conditions, Ri may be reduced by wind, so that spread monolayers of hexadecanol or similar materials may significantly retard evaporation. In the last few years they have found commercial application in reducing evaporation from lakes and reservoirs in hot, arid regions where the amount of water lost by evaporation may be so great as to exceed the amount usefully used. Another application is to reduce the evaporation from heated swimming-pools here it is important to save the latent heat of evaporation rather than the water. 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

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. 

Anotlier metliod applicable to interfaces is tlie detennination of tlie partial molecular area (7 of a biopolynier partitioning into a lipid monolayer at tlie water-air interface using tlie Langmuir trough [28]. The first step is to record a series of pressure 71-area (A) isotlienns witli different amounts of an amphiphilic biopolynier spread at tlie interface. 

The practical importance of monolayer formation is generally because of its relationship to reduction of surface tension. Air—water surface tension can affect such important phenomena as contact angle with a solid surface (affecting flotation), rate of wetting of a solid, or foaming (with applications in enhanced oil recovery or fire extinguishers), just to name a few. Reduction of air—water surface tension could, for example, cause a liquid to spread on a solid instead of beading up on it. 

Next let us consider those difficulties associated with the determination of the amount of material deposited on the surface. We have already noted that the method of depositing insoluble monolayers by spreading permits the accurate determination of n. Since the spreading technique requires solvent volatility, care must be exercised to prevent the stock solutions from changing concentration due to evaporation prior to their application to the surface. Also, precise microvolumetric methods must be used to dispense the solution on the aqueous surface since the quantity used is small. The solvent (as well as the solute) must be free from contaminants. There is also the possibility that the solvent will extract spreadable contaminants from the waxed surfaces of the float, barriers, and tray. Some workers advocate addition and evaporation of one drop at a time to minimize this. Oily contaminants may also reach the water surface from the fingers and from the atmosphere. These last sources are particularly hard to control Tests for reproducibility and blank compressions (i.e., moving the barrier toward the float on a clean surface) are the best evidence of their absence. 

The spreading of an insoluble monolayer is a process analogous to adsorption with a number of specialized applications. Thus, cetyl alcohol is spread as a monoloaycr on reservoirs to retard the evaporation of water. Some antifoaming agents act by spreading as monolaveis. 

If one pictures the adsorbed monolayer as a two-dimensional imperfect gas, it seems reasonable to assume the applicability of a two-dimensional form of the van der Waals equation in which the gas pressure is replaced by the spreading pressure and the volume by the surface area. By combining this with the Gibbs adsorption equation - Equation (2.34) - de Boer (1968) obtained the equation... 

When the monolayer material is spread from its pure liquid or pure solid (crystal), the amount deposited at the interface is so small that it may be difScult to measure accurately. For Instance, covering a surface of, say 500 cm with a spread amount of 0.5 mg m" requires the accurate weighing and application of 25 pg of material. 

In this chapter we will see how the surface activity of a molecule is related to its molecular structure and look at the properties of some surfactants which are commonly used in pharmacy. We will examine the nature and properties of films formed when water-soluble surfactants accumulate spontaneously at liquid/air interfaces and when insoluble surfactants are spread over the surface of a liquid to form a monolayer. We will look at some of the factors that influence adsorption onto solid surfaces and how experimental data from adsorption experiments may be analysed to gain information on the process of adsorption. An interesting and useful property of surfactants is that they may form aggregates or micelles in aqueous solutions when their concentration exceeds a critical concentration. We will examine why this should be so and some of the factors that influence micelle formation. The ability of micelles to solubilise water-insoluble drugs has obvious pharmaceutical importance and the process of solubilisation and its applications will be examined in some detail. 

Monolayers at the air-water interface are usually formed by spreading a solution of the molecules in a volatile solvent on the water surface (3). The solvent evaporates and the remaining surface active molecules are packed by application of an appropriate external surface pressure. In the case of complex monolayers, e.g. 

There are many applications of ellipsometry in the measurement of mono-layer and sub-monolayer films. The theory of the optical signal to be expected from an adsorbed layer less than one monolayer thick has been placed on a firm footing by Smith [15] in some elegant experiments on adsorption in a Langmuir trough. Simultaneous ellipsometric and surface potential measurements were made on various molecules spread in thin layers on mercury as the surface pressure was varied. One conclusion was the simple result that the effective thickness divided by the thickness of the island molecules in the adsorbed islands was equal to the fractional coverage of the surface area. 

Self assembled monolayers (SAMs) have aroused wide spread interest as they provide an opportunity to define the chemical functionality of surfaces with molecular precision, thus creating potential applications related to the control of wettability, biocompatibility and the corrosion resistance of surfaces of a wide range of materials II), Two families of SAMs have received the most attention, SAMs of alkanethiols on gold and alkylsilanes on silicon. 

The study of surface films of the Langmuir type covers an extremely diverse group of phenomena. Measurements of film viscosity, diffusion on the surface, diffusion through the surface film, surface potentials, the spreading of monolayers, and chemical reactions in monolayers are just a few of the topics that have been studied. One interesting application is the use of long-chain alcohols to retard evaporation from reservoirs and thus conserve water. The phrase to pour oil on the troubled waters reflects the ability of a mono-molecular film to damp out ripples, apparently by distributing the force of the wind more evenly. There are also several different types of surface films only the simplest was discussed in this section. 

Normal spreading with delayed retraction to droplets (type 2 spreading) is characteristic of nonvolatile additives which do not lower the surface tension of the solution significantly and whose monolayers at the solid interface have y only slightly smaller than Vlv ype 2 spreading is understandable if we recall that all the oils studied with nonspreading additives spread normally on metals in the absence of the adsorbed monolayer. It must be assumed that at the instant of application a "foot or meniscus of oils turns outward at the base of the drop to form a zero contact angle with the solid surface. If the adsorption of additive from this thin film is insufficient to make the critical surface... 

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