Condensed films with close-packed chains

Molecules with benzene rings and several hydroxyl groups in the ring, with one long chain, usually form condensed films with the benzene rings upright and closely packed side by side (k). In studying these compounds it had been... 

Careful work by Nutting and Harkins1 shows, confirming some indications in earlier work of the author s, that condensed films of fatty acids and alcohols, with close-packed heads, occupy slightly smaller areas as the chain length is considerably increased. When the chains are closely packed, the area also appears a little smaller with the longer chains, but the data are not so complete. 

Studies of monolayer films have included alcohols and carboxylic acids with hydrocarbon chains of varying length in the range C10-C26 [20, 21]. In the liquid condensed and solid states the 11- isotherms have similar shapes at high surface pressures. These data show that the monolayer in the self-assembled state occupies an area of 0.205 nm per monomer. This result is independent of chain length and provides evidence that the monolayer consists of a close-packed structure with all molecular units oriented with their hydrocarbon chains perpendicular to the interface. Under these circumstances, strong attractive van der Waals forces are present between the hydrocarbon tails. As a result, formation of the solid-state film can be irreversible, so that the film does not break up when the surface pressure is decreased. 

FIGURE 8.16. A monomolecular film of a straight chain carboxylic acid such as myristic acid on distilled water will show a sharp transition in the n-A curve as the head groups become more closely packed. That kind of transition may be viewed as something like a reversed sublimation in which the film passes from gaseous to solid condensed without passing through the liquid expanded state. With very careful experimental work, it is sometimes possible to identify an intermediate liquid expanded phase as illustrated for myristic acid on 0.1 N HCl. 

Figure 9.13. Typical forms of monolayer films at L/L and LN interfaces (a) moderately close-packed with significant chain mobility (liquid expanded) (b) close-packed with tilted orientation and reduced chain mobility (liquid condensed) (c) close-packed with essentially vertical orientation and very limited chain mobility (condensed solid).

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