Surface active agents monolayer

Of special interest in liquid dispersions are the surface-active agents that tend to accumulate at air/ liquid, liquid/liquid, and/or solid/liquid interfaces. Surfactants can arrange themselves to form a coherent film surrounding the dispersed droplets (in emulsions) or suspended particles (in suspensions). This process is an oriented physical adsorption. Adsorption at the interface tends to increase with increasing thermodynamic activity of the surfactant in solution until a complete monolayer is formed at the interface or until the active sites are saturated with surfactant molecules. Also, a multilayer of adsorbed surfactant molecules may occur, resulting in more complex adsorption isotherms. 

Since Langmuir reported monolayer studies, a great many instruments have been designed about this method. The clean surface of water shows no change in surface tension if a barrier is moved across it. However, if a surface-active agent is present, then the latter molecules will be compressed, and this will give rise to a decrease in surface tension. 

In the present study, the monolayer technique is used to investigate the surface properties of octadecyldimethylamine oxide and sodium octadecyl sulfate, as single component films and in combination. The interpretation of the results provides a direct understanding of the mechanism of interaction between these two surface active agents. 

In addition to lowering surface tension, surface-active agents contribute to emulsion stability by oriented adsorption at the interface and by formation of a protective film around the droplets. Apparently, the first molecules of a surfactant introduced into a two-phase system act to form a monolayer additional surfactant molecules tend to associate with each other, forming micelles, which stabilize the system by hydrophilic-lipophilic arrangements. This behavior has been depicted by Stutz et al. ( ) and is shown in Figures 1-5. 

The reaction between insoluble protein monolayer and injected soluble surface-active agent was time-dependent. Injection of SDS under an unbuffered (ca. pH 6.0) liquid condensed BSA monolayer increased... 

Monolayer techniques have been used with much success to study the interaction of proteins with surfactants (11, 12, 13), ions and lipids (14), and other proteins (15). This paper investigates, through well-established procedures, the surface chemistry of monolayers of a major component of heterogeneous wheat gluten protein, gliadin, and explores these interactions with various surface-active agents. 

Surfactants are characterized by solubility in water and by their ability to lower the surface tension of water. (The term surfactant was coined in 1950 (23) as an acronym of surface active agent. ) Surfactant molecules are amphiphilic, i.e., they consist of hydrophobic and hydrophilic parts. They form monolayers on surfaces and aggregates of widely different shapes, i.e. micelles, in solutions. Surfactants are classified as... 

In most systems, stable bubbles can only be formed in water or an aqueous solution if a surface active agent is present at the interface. When two bubbles are brought into contact, the resistance to coalescence will be determined by the nature of the surfactant film or monolayer. The result of coalescence is always a decrease of interfacial area. However, before coalescence can occur, the surface films around the bubbles are compressed. This gives an increase in O and provides an elastic restoring force tending to oppose the compression. 

This threshold speed 14i plays an important role in Langmuir-Blodgett deposits, in the study of molecular electronics or for the control of wettability. A monolayer of insoluble surface active agents floating on the surface of water is transferred to a plate drawn at speed V from the bath. We must have V [Pg.27]

It was already explained that when a surface-active agent (such as surfactant or a soap) is dissolved in water, it adsorbs preferentially at the surface (surface excess, F). This means that the concentration of a surface-active agent at the surface may be as high as 1000 times more than in the bulk. The decrease in surface tension indicates this and also suggests that only a monolayer is present at the surface. For example, in a solution of SDS of concentration 0.008 mol/L, the surface is completely covered with SDS molecules. Let us consider systems where the lipid (almost insoluble in water) is present as a monolayer on the surface of water. In these systems, almost all the substance applied to the surface (in the range of few micrograms) is supposed to be present at the interface. This means that one knows (quantitatively) the magnitude of surface concentration (same as the surface excess, F). 

The study of surface tension is really a branch of surface chemistry, and its development has been exceedingly rapid in the last decade. Thus, adhesion can be considered to be partially an exercise in wetting and spreading of liquids on solid surfaces. The flotation of ores is accomplished by gravity differences as well as by the adhesion of the solid particle to an air bubble, and it involves solid-liquid-gas interfaces. It is possible to reduce the vaporization of water from bodies of water with large surfaces such as reservoirs and lakes, by adding a monolayer of a substance such as hexadecanol or other surface-active agents. The action of soaps produces a decrease in surface tension on water. Many other applications in our modem environment can be readily identified. 

A surfactant, or surface-active agent, is a molecule that will localize at the interface between two immiscible fluids. This behavior gives rise to a huge diversity of interesting phases when the molecules are combined with different fluids. Surfactants can act to stabilize droplet dispersions of one fluid in another, they can form their own complex phases as a function of concentration in a solvent, and they can even form incredibly thin stable monolayers at a fluid interface. 

Whether surface treatment of the filler will improve impact resistance depends on the extent to which adhesion to the polymer matrix is improved. For example, treatment of submicron calcium carbonate with atitanate surface-active agent (KenReact 238S, Kenrich) at a 1 percent level provides improved impact strength, presumably as a monolayer is formed on the filler surface. Use instead of 3 percent, forming a surface multilayer, an adhesive weak point, lowers impact strength as compared with no treatment. 

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