Gaseous monolayers

The film balance may be regarded as a two-dimensional piston, and the most commonly studied property is the surface pressure (n) versus area (A) isotherm. The analogy to a PV isotherm is so appropriate that in the gaseous monolayer regime the two-dimensional analogue of the ideal gas law pertains 114 = nRT. It is therefore reasonable to relate discontinuities in n/A isotherms as the monolayer film is compressed in two dimensions to... 

All of the experiments in pure and mixed SSME systems, as well as in the Af-stearoyltyrosine systems, have one common feature, which seems characteristic of chiral molecular recognition in enantiomeric systems and their mixtures enantiomeric discrimination as reflected by monolayer dynamic and equilibrium properties has only been detected when either the racemic or enantiomeric systems have reverted to a tightly packed, presumably quasi-crystalline surface state. Thus far it has not been possible to detect clear enantiomeric discrimination in any fluid or gaseous monolayer state. 

One end of each surface-active molecule in a monolayer is anchored firmly to the Uquid surface by the attraction of the polar head group for the aqueous subphase, while the hydrophobic portion is displaced easily from it. If the molecules are separated widely as in a gaseous monolayer, the simple two-dimensional gas law is approached, namely, irA = kT, where k is the Boltzmann constant. The hydro-phobic chains are free to assume almost any orientation above the surface and may sweep out circles with radii as long as their tails by rotating around their point of attachment at the head group. However, intermolecular translational movements are restricted to the two-dimensional interfacial plane because the hydrophilic head groups cannot leave the aqueous surface. 

The primary evidence for the conversion of gaseous monolayers at the air-water interface to other intermediate states lies in the abrupt changes found on the n-A isotherms of many film-forming compounds. So many of these isotherms have been reproduced in fine detail in a number of laboratories under a variety of conditions that they cannot possibly be rejected wholesale as artifacts. The sharp transitions from curves to plateaus, where the molecular area varies readily at constant surface pressure, may be related... 

Langmuir trough or film balance, to be described in sec. 3.3.1). With such compression the surface pressure k increases. Surface equations of state, relating n to the area A and the temperature T can be formulated, entirely analogous to the three-dimensional equivalent. For instance, for a very dilute, gaseous, monolayer the two-dimensional equation of state is... 

As the density of the molecules In the surface is gradually increased, the monolayer, starting from a very dilute, gaseous monolayer, may undergo a series of phase transitions from a gas (G) phase via a liquid expamded (LE), into a liquid condensed (LC) and, finally, into a solid (S) state. These transitions are not always distinct in other systems more than one condensed phase may occur. The G, L and... 

At very low molecular densities, i.e. at very low Interfacial pressures, the mono-layer exhibits gaseous behaviour. The molecules are far apart, but, unlike in a three-dimensional gas, they are not completely disordered. Because of their amphi-polar nature, the molecules exhibit a preferential orientation relative to the surface-normal. As stated in sec. 3.1, the interfacial pressure exerted by an ideally dilute monolayer is equivalent to the osmotic pressure of an ideal three-dimensional solution. Ideal gaseous monolayer behaviour means obe3dng relation [3.1.1]. 

The surface film acts as a two-dimensional analogue to normal matter in that it may exist in different physical states, which in some ways resemble solids, liquids and gases. In this section we shall consider the three different states of monolayers of simple amphiphiles, referred to as solid or condensed, expanded, and gaseous monolayers (see Fig. 6.7). 

Monolayers of polymers and proteins lack the characteristic features described in the previous section. Most produce smooth curves, typical of those for gaseous monolayers of amphiphiles. 

Figure 1. Upper Schematic of it-A isotherms in the transition region F-F represents region where bulk of film material is in the condensed monolayer state G-G represents the region where virtually all of the film is in the gaseous monolayer state. Area per molecule is the total area occupied by the sum of all lipid molecules in the surface.

Chemical Activities by the Surface Vapor Pressure Method. Surface pressure measurements in the transition region between the condensed and gaseous monolayer states of a single lipid component spread as a monolayer on water yield a value of ir which is independent of the surface area. This value—the surface vapor pressure, irv—is analogous to the vapor pressure of a liquid in equilibrium with its vapor. When a second lipid component is in the surface, the limits of miscibility in the condensed phase may be determined on the basis of the surface vapor pressure dependence on the mole fraction in the condensed phase (8). 

For the first type, the Gibbs monolayers are often described as two-dimensional gaseous (or gas-like) monolayers formed on the surface of dilute solutions, having a low surface excess, and the surface pressure, n, is regarded as the two-dimensional osmotic pressure of this solution. When a two-dimensional gaseous monolayer is present on a dilute solution, the surface tension decreases linearly with the increase in concentration of the added surfactant, at constant temperature... 

The stabilizing influence of the surface viscosity of a typical monolayer was found to be negligible, in contrast to that of the surface elasticity, which (even for a so-called gaseous monolayer) was sufficient to cause a dramatic stabilization. The shift in the Pearson neutral-stability curves brought about by the presence of a gaseous monolayer and also by a close-packed monolayer on a 1-mm deep water substrate is shown in Fig. 16. For this example, the gaseous monolayer having a surface pressure of 0.2 dyn/cm effected a 500-fold increase in the stability criterion, whereas, in... 

For the dodecanoic acid, in contrast to 1-dodecanol, the aggregation model yields a quite high aggregation number of n = 4, while no indications exist for the formation of larger clusters. Also, no data for spread dodecanoic acid monolayers could be found in literature which exhibit characteristic inflection points in the FI-A isotherm. The dependencies at 15-20°C usually correspond to the type of gaseous monolayers [34]. Therefore, a certain similarity exists in the behaviour of fatty acids and alcohols, i.e. for a smaller number of methylene groups the system... 

It is generally found that good stabilization of foams and emulsions requires compounds that give condensed monolayers. Compounds that give expanded or gaseous monolayers do not, as a rule, act as stabilizers in fact, they are more likely to be destabilizers. The compounds that enhance loaf volume or have little effect are those that give condensed... 

Similarly, an ideal gaseous monolayer should follow the corresponding law... 

When the molecules are far apart they are considered to lie nearly flat on the surface and distributed one by one in a two-dimensional gas phase (Gaines, 1966). An ideal gaseous monolayer should follow the two-dimensional gas equation ... 

When the gaseous monolayer is compressed, a transition into the so-called liquid-expanded phase usually takes place. This phase has been the subject of many controversies. Most of this discussion, however, took place before the structure of the lamellar liquid crystalline phase was known. It should be mentioned in this connection that Phillips et al. (1969) used such correlations in their interpretation of the monolayer structure of dipalmitoylphosphatidyl-choline which will be further discussed in Section 8.10. 

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