Stearic acid, limiting film area

Figure 4 shows three typical force-area curves obtained at the air-water surface. For stearic acid the curve characterizes a condensed film. It may be seen that the force remains very small till the area available to each molecule, written A, becomes very close to 20 A., the so-called limiting area. When, however, the area is reduced yet a little further, the pressure rises steeply, and the film becomes quite solid. The low compressibility indicates that there is strong repulsion between the molecules. This is now known to be due to the repulsion between the clouds of electrons associated with each hydrocarbon chain. Study of crystals of stearic acid with x-rays leads to a closely similar value for the cross-sectional area of a saturated hydrocarbon chain. The cohesion between the chains is also very high, so high indeed that if the area available is somewhat greater than 20 A. per molecule, there are islands, of the order of millimeters in diameter, floating on the surface, with the chains in each still nearly vertical (Fig. 5). 

Similar films are formed by long-chain fatty acids such as stearic and palmitic acid, for which a limiting surface area of about 0.20 nm is found. This value is very close to the cross-sectional area of the compounds in the bulk crystal as determined by X-ray diffraction. 

A typical ti-A curve for stearic acid on water at 20°C, plotted as area per molecule, is shown in Figure 8.16. The curve is found to become very steep at an area per molecule of 0.205 nm. If the area is decreased further, the pressure suddenly falls, indicating a buckling or collapse of the film. That point is commonly referred to as the yield point. The critical molecular area of 0.205 nm found for stearic acid is the same as that for palmitic, myristic, and the other members of the series with more than 12 carbons in the chain. X-ray diffraction data indicate that the cross-sectional area of the stearic acid molecule is 0.185 nm, suggesting that the critical limiting area represents the point at which the molecules become more efficiently packed, approximating the packing in the solid crystal. 

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