Gaseous Monolayer Films

The most simple type of amphiphile monolayer film or a polymer film would be a gaseous stale. This film would consist of molecules that are at a sufficient distance apart from each other such that lateral adhesion (van der Waals forces) is negligible. However, there is sufficient interaction between the polar group and the subphase that the film-forming molecules cannot be easily lost into the gas phase, and that the amphiphiles are almost insoluble in water (subphase). 

When the area available for each molecule is many times larger than molecular dimension, the gaseous-type film (state 1) would be present. As the area available per molecule is reduced, the other states, for example, liquid-expanded (L, ), liquid-condensed (L,), and finally the solid-Uke (S or solid-condensed) states, would be present. 

The molecules will have an average kinetic energy, that is, l/2k,(T, for each degree of freedom, where kg is Boltzmann constant (=1.372 x 10 ergs/T), and T is the temperature. The surface pressure measured would thus be equal to the collisions between the amphiphiles and the float from the two degrees of freedom of the translational kinetic energy in two dimensions. It is thus seen that the ideal gas film obeys the following relation  

In general, ideal gas behavior is only observed when distances between the amphiphiles are very large, and thus the value of II is very small, that is, 0.1 mN/m. It is also noticed that from such sensitive data one can estimate the molecular weight of the molecule in the monolayer. This has been extensively reported for protein monolayers (Adamson and Gast, 1997 Birdi, 1989,1999). The latter observation requires an instrnment with very high sensitivity, 0.001 mN/m. The H versus A isotherms of n-tetradecanol, pentadecanol, pentadecyclic acid and palmitic acid in the low n region showed data that agreed with the ideal film. Similar data for isotherms were reported for other lipid monolayers by other workers. The various forces that are known to stabilize the monolayers are mentioned as follows  

Heiectro is related to polar group interactions (polar group-water interaction polar group-polar group repulsion charge-charge repulsion) 

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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... 

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... 

Some amphiphilic molecules such as oleic acid and hexadecyl alcohol containing an alkyl chain and a polar head group form monolayers on the surface of water. The polar head groups of these molecules are attracted to and are in contact with water while their hydrocarbon tails protrude above it (Figure 15). The term monolayer implies the presence of a uniform mono-molecular film on the surface of water. Monolayer films can be classified as gaseous, liquid, or solid depending upon the degree of compression and the effective area per molecule. Clearly the liquid phase of a monolayer film and, more so, the solid represent constrained environments for individual molecules of amphiphiles. Monolayers, just like micelles, are dynamic species. 

The interfacial film theory is an extended interfacial tension theory, in which the adsorbed surfactant at the interface surrounds the dispersed droplets forming a coherent thin monolayer film (Figure 4.13). As the droplets approach each other, coalescence is prevented. The stability of the emulsions depends on the characteristics of the monolayer film formed at the interface. Monolayer films are classified as gaseous, condensed, and expanded films. 

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... 

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). 

Qualitatively, in a number of cases, foam stability has been correlated with viscosity of the surface film, but the relation is not really clear. There are stable foams in which the viscosity of the surface film is not particularly high and viscous monolayers that do not produce particularly stable foams. However, it appears well accepted that if the viscosity of the surface film is either very low (a gaseous monomolecular film) or very high (a solid monomolecular film), the foam produced will be unstable. In both of these cases film elasticity is low. In addition, too high a surface viscosity can slow down self-healing of thinned spots in the film by the surface transport mechanism. 

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.

Like bulk materials, monolayer films exhibit characteristics that can (sometimes with a bit of imagination) be equated to the solid, liquid, and gaseous states of matter. For films, the equivalent states are roughly defined as... 

Figure 3.18 High energy resolution He I UPS spectra of gaseous (a) (circles) and monolayer film (b) (circles) for pentacene, compared with convoluted curves of 18 Ag vibrational modes (solid curves). Energy is relative to 0-0 transition peak (dashed curve), (a) Convolution curve obtained by Voigt functions (V/c = 5 meV and Wl= 65 meV) with Sg j and hvgas.

Because of the charged nature of many Langmuir films, fairly marked effects of changing the pH of the substrate phase are often observed. An obvious case is that of the fatty-acid monolayers these will be ionized on alkaline substrates, and as a result of the repulsion between the charged polar groups, the film reverts to a gaseous or liquid expanded state at a much lower temperature than does the acid form [121]. Also, the surface potential drops since, as illustrated in Fig. XV-13, the presence of nearby counterions introduces a dipole opposite in orientation to that previously present. A similar situation is found with long-chain amines on acid substrates [122]. 

These assumptions have been expanded upon (Shah and Capps, 1968 Lucassen-Reynders, 1973 Rakshit and Zografi, 1980), especially in regard to the application of the ideal mixing relationship in gaseous films (Pagano and Gershfeld, 1972). It has been pointed out that water may contribute to the energetics of film compression if the molecular structures of the surfactants are sufficiently different (Lucassen-Reynders, 1973). It must be noted that this treatment assumes that the compression process is reversible and the monolayer is truly stable thermodynamically. It must therefore be applied with considerable reservation in view of the hysteresis that is often found for II j A isotherms. 

From these descriptions, it is seen that the films may, under given experimental conditions, show three first-order transition states, such as (i) transition from the gaseous film to the liquid-expanded (Lex), (ii) transition from the liquid-expanded (Lex) to the liquid-condensed (Lco), and (iii) from Lex or Lco to the solid state if the temperature is below the transition temperature. The temperature above which no expanded state is observed has been found to be related to the melting point of the lipid monolayer. 

As evident from the above discussion, if measurements can be made at sufficiently low pressures, all monolayers will display gaseous behavior, represented by region G in Figure 7.6. The gaseous region is characterized by an asymptotic limit as n - 0. In the limit of very low film pressures, a two-dimensional equivalent to the ideal gas law applies ... 

Two-dimensional monolayers can exist in different physical states which bear some resemblance to the solid, liquid and gaseous states in three-dimensional matter. Surface films are best classified according to the lateral adhesion between the film molecules, including end-groups. Factors such as ionisation (and, hence, the pH of the... 

The principal requirements for an ideal gaseous film are that the constituent molecules must be of negligible size with no lateral adhesion between them. Such a film would obey an ideal two-dimensional gas equation, ttA kT, i.e. the it-A curve would be a rectangular hyperbola. This ideal state of affairs is, of course, unrealisable but is approximated to by a number of insoluble films, especially at high areas and low surface pressures. Monolayers of soluble material are normally gaseous. If a surfactant solution is sufficiently dilute to allow solute-solute interactions at the surface to be neglected, the lowering of surface tension will be approximately linear with concentration - i.e. 

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