Lateral cohesion

As remarked previously, the physical state of a monolayer depends on the lateral cohesive forces between the constituent molecules. By a suitable choice of chain length and temperature, straight-chain fatty acids and alcohols, etc., can be made to exhibit the various monolayer states, one CH2 group being equivalent to a temperature change of c. 5-8 K. 

Lateral cohesion also depends on the geometry and orientation of the film molecules, so that the following factors will favour the formation of an expanded film ... 

Figure 1 shows the behaviour of the diesters of aw dibasic esters. These molecules he supposed would lie flat at the air-water interface, and, therefore, there would be little lateral cohesion between molecules. Thus, he argued that the molecules should obey the gas equation ... 

However, there are conceptual differences between the surface equation of state and the adsorption isotherm, so that the surface equation of state is only concerned with the lateral motions of the monolayer molecules and their lateral cohesive and adhesive interactions with the solvent molecules present in the monolayer, whereas an adsorption isotherm is also concerned with the interactions normal to the surface, between the monolayer molecules (as adsorbate) and solvent molecules (as adsorbent). 

The uniform oxidation of the cold-drawn filament results in a high overall degree of oxidation and associated backbone scission before surface deterioration reaches the level at which spontaneous restructuring and crack formation can occur. Lateral cohesion of these fibers was much greater than for the highly oriented filaments. For example, the latter could be easily peeled, whereas the cold-drawn fiber stretched rather than fibrillated after notching. 

If the tail is branched, the larger cross-sectional area of the molecule precludes the close packing and lateral cohesion required for the formation of condensed films and expanded films result. Similarly, molecules having two (or more) hydrocarbon groups such as esters of polyhydric alcohols, are limited in their lateral interaction and generally produce expanded films, although this depends somewhat on the length of the chains and the temperature. 

Oberlin et al [137] assume the increase in strength as the lateral cohesion increases is associated with tight bonding between defective BSU sheets (Figure 5.32). 

Considerable insight into the lateral cohesion of the floating monolayer can be obtained by examining the hysteresis of the isotherm. The greater the cohesion, the greater the hysteresis. To date, this is a rather nnderexploited approach to characterize partially compressed monolayers (i.e., at a pressure lower than jTc collapse is nsually an irreversible phenomenon). However, monolayer viscoelasticity (compression and shear moduli) has recently been determined by such an approach. ... 

The permeability of lipid bilayers to water and other bilayer-soluble components depends strongly on the state of lateral cohesion that the bilayer exhibits and the presence of defects [5], Permeability data have been obtained using micropipet... 

The tape yarn and film method (initially called the split-film method or fibrillation of plastic films) consists of extruding or casting thin films or tapes of thermoplastic crystalline POs, followed by their stretching to obtain a substantially parallel orientation of the fiber-forming crystallites embedded in an amorphous matrix. Because the interchain bonds are missing, lateral cohesion is low and the films or tapes can be spliced very easily into a network of interconnected fibers (fibrillation). 

The observed fact that chains break even after (20 minutes of) stress relaxation not only demands the integrity of the crystal blocks but also an intimate and persistent lateral cohesion between microfibrils within a fibril and between fibrils within a filament. In the same way as studied in detail in Chapter 5 for single chains the shear displacement of the ends of microfibrils against the interfibrillar cohesion permits the transfer onto the microfibrils of those forces which accumulate within the stress-transfer lengths to the axial stress a. This stress relaxes at constant filament elongation. The continued rupture of chains indicates that the axial strains of the microfibrils are maintained during such stress relaxation. Those strains, however, can only be maintained if large scale slip of microfibrils or fibrils does not occur. 

The previously discussed intimate lateral cohesion would favor the more homogeneous decrease of all relaxation moduli. The same conclusion was drawn by Geza-low et al. [8] from the uniformity of stress induced long period changes. 

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