Monolayer pressure = surface tension difference

The monolayer resulting when amphiphilic molecules are introduced to the water—air interface was traditionally called a two-dimensional gas owing to what were the expected large distances between the molecules. However, it has become quite clear that amphiphiles self-organize at the air—water interface even at relatively low surface pressures (7—10). For example, x-ray diffraction data from a monolayer of heneicosanoic acid spread on a 0.5-mM CaCl2 solution at zero pressure (11) showed that once the barrier starts moving and compresses the molecules, the surface pressure, 7T, increases and the area per molecule, M, decreases. The surface pressure, ie, the force per unit length of the barrier (in N/m) is the difference between CJq, the surface tension of pure water, and O, that of the water covered with a monolayer. Where the total number of molecules and the total area that the monolayer occupies is known, the area per molecules can be calculated and a 7T-M isotherm constmcted. This isotherm (Fig. 2), which describes surface pressure as a function of the area per molecule (3,4), is rich in information on stabiUty of the monolayer at the water—air interface, the reorientation of molecules in the two-dimensional system, phase transitions, and conformational transformations. 

The detachment experiments were carried out at the different surface tension of the water surface. The surface pressure was controlled in the range of 0-10 mN m-1 with ethyl tetradecanoate monolayer that was confirmed not to be transferred onto a substrate under the experimental conditions. The detachment amounts of cadmium octadecanoate LB films at various surface pressures are shown in Figure 19. At the high surface pressure (the low surface tension) the detached amount became negligibly small, which means the surface tension of water is the driving force of the detachment of LB films at the air-water interface. 

Figure 8-8 (A) The Langmuir-Adam film balance. Tension on the moveable barrier is recorded for different areas of the surface between the barriers. This gives the surface pressure jt, which is the difference between the surface tension (y0) of a clean aqueous surface and that of a spread monolayer (y) K = ya-y. Courtesy of Jones and Chapman.81 (B) Surface pressure (7t)-area per molecule isotherm for a typical fatty acid (e.g., pentadecanoic acid C14H29C02H) at the aqueous-air interface. From Knobler.81a...

Different molecular areas of Langmuir monolayers can be determined. They can be defined in three ways Ao is the area per molecule extrapolated to zero differential surface tension, Ac is the minimum area per molecule at the collapse point, at the point in the tt - A isotherms where the pressure is the maximum reversible pressure (or collapse pressure ttc) and Am is the area at the midpoint pressure rrm = 0.5 TTC. 

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 ... 

The Gibbs adsorption isotherm shows the dependence of the extent of adsorption of an adsorbent on its bulk concentration or pressure. However, we also need to know the state of the adsorbate at the surface. These are interrelated because the extent of material adsorb-tion on a surface depends on the state of the surface. The behavior of the molecules in the surface film is expressed by a surface equation of state which relates the spreading pressure, n, which is the difference between the solvent and solution surface tensions, %= % - y to the surface concentration of the adsorbent. This equation is concerned with the lateral motions and interactions of the molecules present in an adsorbed film. In general, the surface equation of state is a two-dimensional analogue of the three-dimensional equation of state of fluids, and since this is related to monomolecular films, it will be described in Sections 5.5 and 5.6. It should be remembered that on liquid surfaces, usually monolayers form, but with adsorption on solid surfaces, usually multilayers form (see Section 8.3). 

Film Pressure The pressure, in two dimensions, exerted by an adsorbed monolayer. It is formally equal to the difference between the surface tension of pure solvent and that of the solution of adsorbed solute. It can be measured by using the Film Balance. 

All surface-active molecules, such as soaps and lipids, can be prepared as monomolecular layers at the air-water interface. Driven by the reduction of the surface free energy of water, these molecules spread when applied to the surface, for example, from a volatile solvent (Fig. 10). The physical properties of these monolayers were first investigated in the 1940s by Langmuir after his first work on molecules in insoluble monolayers [41]. A so-called Langmuir trough filled with water defines an exactly known area for the spread molecules (Fig. 10). At low lateral density, these molecules behave like a quasi-two-dimensional gas. If the area for these molecules is reduced by a movable barrier, this lateral compression will eventually lead to a measurable lateral pressure n (force F per unit length of barrier). Which can be measured by a so-called Wilhelmy balance (see Kuhn et al. [42]), It is the difference between the surface tension of the free, y, and the layer-covered, y, water surface ... 

In order to understand the different effects of lipids on gas cell stability (Figure 7.10), we need to digress momentarily to focus on the properties of monomolecular films or monolayers. Monolayers may be studied by the use of a surface film balance, also called a Langmuir trough. When a compound is spread or adsorbs at an interface, it generally lowers the interfacial tension. The change in interfacial or surface tension (y) in the case of an air-aqueous interface is called the surface pressure (II) and is defined by... 

The surface pressure of a monolayer film, n, is defined as the difference between the surface tension of the pure supporting liquid, [Pg.160]

For many years, the main experimental tool for these studies was the film balance (see Figure 4.19). Various workers, including Pockels, Langmuir, and Adam, made major contributions to its development (see Gaines, 1966). A small, known quantity of the surfactant to be studied is dissolved in a volatile solvent and deposited carefully by pipette on the surface of a pool of water. The solvent is chosen so that it spreads rapidly over the water and then evaporates, leaving the surfactant uniformly distributed as a monomolecular layer (or monolayer) in the region between the two barriers. One of the barriers is movable, so that the area occupied by the surfactant film can be varied. A torsion balance is provided to measure the surface pressure (i.e., the difference between the surface tension of pure water and that of the film-covered surface). More commonly in modem instruments, a Wilhelmy plate is used to measure surface tension in the film region. 

To form a Langmuir monolayer, the molecule of interest is dissolved in a volatile organic solvent (frequently chloroform or hexane) that will not react with or dissolve in the subphase (1,2,4). A quantity of this solution is placed on the surface of the subphase, and as the solvent evaporates, the siuTactant molecules spread and alter the surface pressure of the water surface. A barrier designed to measure this surface pressure (D), relative to that of the pin-e subphase, is the principle behind the Langmuir balance. Alternatively, the siuTace pressure is measured as the difference between the surface tension (y) of the monolayer and that of the pure subphase iyo), n = yo — K- A common method for measining surface tension involves using a Wilhelmy plate, usually a piece of platimun or paper that is wetted by the subphase, suspended from a balance. As the monolayer is compressed by using the moveable barrier to reduce the sinface area, the surface pressure increases. A plot of the siuTace pressin-e versus surface area is called a pressure versus area isotherm (or Il-A isotherm). Isotherms are normally plotted in terms of area/molecule, and the imits of surface pressure are mN/m. 

Amphiphiles, the representatives of which are soap, surfactant and lipid, have a hydrophilic polar head and lipophilic nonpolar tails. They always remain on the interface between water and oil and form monolayers of surfactants in a water/oil/amphiphile ternary system. This monolayers or interfacial film reduce the surface tension between water and oil domains. In a three-component system the surfactant film exists in various topologically different structures such as micelles, vesicles, bicontinuous microemulsions, hexagonal arrays of cylinders or lamellar structures depending upon the pressure, temperature and the concentration of the components [1,2]. Microemulsions are thermodynamically stable, isotropic and transparent mixtures of ternary amphiphilic systems. When almost equal volume fractions of water and oil are mixed with a dilute concentration of surfactants, they take... 

A commonly used alternative to the Langmuir film balance method of determining surface pressure is to measure surface tension using a Wilhelmy plate (Section 4.3.1), dipped into the monolayer at different stages of compression. 

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