Spread monolayers properties

Table 12 shows the equilibrium spreading pressures of each diacid. It is immediately apparent that for three of the diastereomeric pairs there are statistically significant differences. These distinctions relate stereochemical preferences in the spontaneous spreading of (+)- versus meso-monolayers in equilibrium with their respective crystalline phases. However, there appears to be no discernible trend in either the ( )- or meso-ESPs as a function of carbonyl position despite clear trends seen in their monolayer properties in the absence of any bulk crystalline phase. 

In Figure 2 the ir-A and AV-A plots for SODS on O.OIM NaCl sub-solutions having different pH values are shown. In all cases, phase transitions from liquid-expanded to liquid-condensed state are evident ( ). Acidification of the subsolution Increases the transition pressure but the transition is less pronounced at the lowest pH studied. This is also accompanied by an expansion of the condensed part of the curve. Small negative surface potentials are observed over most areas. The highest potential is obtained for film spread on the pH 2.2 subsolution. For small areas, the surface potential attains a positive value. This may be related to reorientation of the dipole moments of the molecules which occur once a threshold surface concentration is exceeded (O. Mlnglns and Pethlca (7) studied the monolayer properties of SODS on various sodium chloride solutions (0.1, 0.01 and O.OOIM) at 9.5 C, and they showed that the monolayer is only stable on the more concentrated salt solutions (0.1 and O.OIM). In this work, no noticeable... 

The surface elasticity of the solution is an especially important property because it is at this point that the air is entrapped. The phenomenon of surface transport, by which a spreading monolayer drags with it significant quantities of the underlying solution and rapidly repairs thinning spots in a lamella, was suggested to occur in quality foaming solutions. 

Now let us examine some of the properties of the spread monolayer that we have described. It was seen in the preceding chapter (e.g., Equation (6.48)) that the presence of an adsorbed layer lowers the surface tension of an interface. The phenomenon is quite general, so we redefine tt (no subscript) in the following symbols ... 

The spread monolayer just described may be discussed from two points of view. First, there are those aspects of the film that pertain explicitly to the chemical nature of the components water and the organic solute. Second, there are certain properties of the monolayer that depend on physical variables such as temperature, area of the water surface, and number of molecules of RX present. Let us briefly discuss both of these viewpoints. 

Next let us consider some of the physical properties of the spread monolayer we have described. Equation (1) states that the surface tension of the covered surface will be less than that of pure water. It is quite clear, however, that the magnitude of 7 must depend on both the amount of material adsorbed and the area over which it is distributed. The spreading technique already described enables us to control the quantity of solute added, but so far we have been vague about the area over which it spreads. Fortunately, once the material is deposited on the surface, it stays there —it has been specified as insoluble and nonvolatile for precisely this reason. This means that some sort of barrier resting on the surface of the water may be used to corral the adsorbed molecules. Furthermore, moving such a barrier permits the area accessible to the surface film to be varied systematically. In the laboratory this adjustment of area is quite easy to do in principle. As we see below, the actual experiments must be performed with great care to prevent contamination. 

By means of this apparatus, it is possible to vary the area of a spread monolayer and measure the corresponding film pressure directly. Many different variations of the film balance are available, and a number of instrumentational techniques can be combined with the Langmuir balance to obtain information on the microstructure of the films and the properties of the films. Figure 7.4b illustrates, for example, a laser optics arrangement to monitor the molecular orientation of the hydrocarbon tails of the surfactant molecules. Below in this... 

Not all solvents are equally suitable for spreading monolayers. The requirement that the solvent evaporate completely is self-evident. It has been suggested that if the organic solvent dissolves much water, the properties of the monolayer will be different from those in which no water is trapped. Verification that no artifacts are entering the observations from the solvent may be accomplished by conducting duplicate experiments with different solvents. 

A decrease in occupied area of the head group results in an increase in packing density of the molecules (45) exhibits only an expanded phase, (46) both a liquid and a solid-like phase, and (47) forms only a condensed film. Monolayer properties of many natural phospholipids and synthetic amphiphiles are described in the literature37 38. Especially the spreading behaviour of diacetylenic phospholipids at the gas-water interface was recently described by Hupfer 120). 

Techniques for spreading monolayers of polar long chain compounds on mercury in a Langmuir type film balance, and for measuring their surface area-pressure properties, have been described by one of the present authors (3). Using these techniques, it has proved possible to measure continuously the change in contact angle of a water droplet superposed on the monolayer, as the film pressure is controllably varied. This has now been done for monolayers of the normal C12-C20 fatty acids and the normal primary Ci4-Ci8 alcohols on the mercury substrate. 

Monolayer Properties. For solubility reasons it was necessary to spread poly (L-leucine) from chloroform containing 20% trifluoracetic acid. This solvent causes drops of liquid to creep up the outside of the pipet, and the observed area per residue of 16 may be slightly low. Use of a glass pipet treated with a silicone minimized the problem. Both specimens of poly(L-methionine) behaved in a similar manner. Poly(L-nor-leucine) has been investigated previously (12,16). The results presented here are in general agreement, but the work has been repeated under the same experimental conditions as for the other polymers to enable a precise comparison (Figure 3). 

Incidental to this work is more evidence that the a-helix exists at the air-water interface. While some have appeared reluctant to accept this view, no good theoretical reason exists why it should not be stable. Where the nature of the side chain might provoke other conformations [as in poly(l-valine)] or the molecular weight is low or monolayers are spread from poor solvents miscible with water, other conformations are detectable, and the monolayer properties are significantly different (5). 

The Langmuir technique is used in order to characterize monolayer properties of surface-active materials. The instrumentation consists of a shallow rectangular container (trough) in which a liquid subphase is added until a meniscus appears above the rim, whereupon the film is spread. The barrier for manipulation of the film rests across the edges of the container. For a more thorough description of the experimental setup. Petty and Barlow (8) are recom-... 

Silicone polymers and fully fluorinated surface-active materials have been found to be the best candidates for spread monolayer film studies on nonpolar liquids. Because nonpolar liquids are more difficult to manipulate in terms of their solvent properties (e.g., by changing pH, electrolyte content) it is often necessary to talk in terms of adsorbed Gibbs monolayers, rather than true insoluble monolayers. However, sometimes we must take what we can get from nature and make the most of it. 

Cholesterol exhibits interesting monolayer properties which certainly are related to its function in membranes. A characteristic II-A isotherm is shown in Fig. 8.61. The molecular cross-section area of the condensed monolayer is about 36 A, close to that in crystals. The monolayer is in a disordered state which can be observed if a powder is spread on the surface. The mobility is about the same as that of monolayers of long-chain amphiphiles... 

The association of the acidic phospholipid PI with the cationic linear polysoap PVPC6 is cooperative in both mixed spread monolayers and mixed swollen multibilayers. This association has a condensation effect on the spread mixed monolayers on one hand and affects the properties of the interbilayer structural water of swollen mixed multibilayers on the other hand. Namely the increase in the degree of... 

Monolayer systems are composed of only a monomolecnlar lipid film on water surface. The aim in these systems is to study the properties of a monomolecnlar thin lipid film spread on the surface of water, which is not possible by any other techniqne. Lately, it has been shown that such lipid films are useful membrane models for biological membranes strnctnre and function studies. The modem methods have allowed one to measure the monolayer properties in mnch detail than earlier. No other method exists through which one can obtain any direct information about the molecular packing or interactions (forces acting in two dimensions). 

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. 

---