Force-area curves

Fig. 12 Force/area curves of stearamide films on 6n H2S04 at 25°C 1, natural racemate II, enantiomeric stearamide III, mixture of solutions of enantiomers on the surface in 1 1 ratio. Reprinted with permission from Arnett and Thompson, 1981. Copyright 1981 American Chemical Society.

Fig. 13 Force/area curves of dipalmitoylphosphatidyl choline monolayers spread on pure water at 25°C (solid line) and 45°C (dashed line). The compression rate is 7.2 A2/molecule per minute. The shape of the isotherms is identical for homochiral and heterochiral films.

Fig. 15 Force/area curves for monolayers of dipalmitoylphosphatidyl choline (solid line), dimyristoylphosphatidyl choline (dashed line), and dilauroylphosphatidyl choline (dotted line) at 25°C.

Figures 40-43 compare the position of the bridging carbonyl group in each diastereomeric pair and the effects of stereochemistry on their force-area curves at 25°C. It is clear that there is a large effect of stereochemistry on the energetics of compression and expansion for a wide variety of ketodiacid surfactants all of the ketodiadds in this study showed a dependence of the shapes of their IT/A isotherms on their stereochemistry. Several facts are striking. In every case there is a sharp differentiation between the behavior of films cast from meso- and ( )-isomers. The isotherms for the...

The first irrefutable observation of a discriminating interaction between enantiomeric surfactants in monolayers was most probably made by Filippus Johannes Zeelen (84) in a detailed study concerning the synthesis, monolayer behavior, and photochemistry of a series of A -stearoylamino acid derivatives that were employed to model the conformation and photochemical decomposition of proteins. Although Zeelen was able to demonstrate significant differences in the force-area curves obtained from racemic and optically active forms of several of these derivatives, publication of this work in 1956 was... 

Zeelen found the extent of chiral discrimination to be dependent on the type of monomolecular phase that was formed. Thus, racemic and optically active samples displayed identical force-area curves (Fig. 14) when both formed liquid-expanded films, but owed considerably different curves (Fig. 15) under conditions where both samples formed a more highly condensed monolayer. 

The monolayer behavior of A-stearoyltyrosine (Fig. 16) was more complex. Under conditions (0.0liV HCl, 22 C) where the racemic material formed a condensed film having a limiting molecular area of 39 2 A, the force-area curve of L-(+)-A-stearoyltyrosine exhibited a liquid-expanded film at large areas (ca. 100-45 per molecule) followed by a transition beginning at 16.5 dynes/cm surface pressure to a condensed phase having a smaller limiting molecular area of 34 2 A . However, both these latter samples exhibited only the liquid-expanded phase on distilled water alone. 

In her initial investigation, Lundquist studied the monolayer behavior of racemic and optically active forms of both tetracosan-2-ol and its acetate derivative on 0.0 lA aqueous HCl over a considerable range of temperature (77). In each case, it was possible to demonstrate chiral discrimination between pure enantiomers versus the racemic substance. Furthermore, the extent of enantiomer discrimination was significantly temperature dependent, being enhanced at lower temperatures and frequently disappearing at higher ones. Under favorable conditions of temperature, however, the appearance of the force-area curves could be very sensitive to the optical purity... 

The force-area curves for racemic and (5)-(+)-2-tetracosanyl acetate recorded with a barrier speed of 5 cm/min are shown in Figures 17 and 18, respectively. Again, both enantiomers showed identical monolayer behavior. The film balance behavior of the racemic acetate was indistinguishable from that of the pure enantiomers at temperatures above about 27°C however, below this temperature the force-area curves differed markedly even at low surface pressures, which indicates that racemic compound formation occurs at relatively large areas per molecule. 

Encouraged by the results from static surface tension measurements, we next ran force-area curves on 3N, 6N, and ION acids. As Figure 33 indicates, there is pronounced discrimination between the racemic and enantiomeric monolayers, and those differences are sharply dependent on the subphase acidity, as had been impUed by the surface tension behavior. 

The force-area curves for racemic and (5 )-(+>2-tetracosanyl acetate were shown in Figures 17 and 18, respectively, while those of methyl esters of racemic and (5 )-(+)-2-methylhexacosanoic acid are found in Figs. 21 and 22, respectively. All these curves were obtained under identical experimental conditions at thevarious temperatures indicated in the figures. Simple inspection shows that the force-area curves of the two racemic samples are very similar, as are those for both optically pure samples. Lundquist suggested that this is merely a result of the very similar shapes and molecular structures of these chiral surfactants. Apart from the chain length, the only structural difference is limited to a reversal of the positions of the carbonyl group and ester oxygen. 

Figure 20 shows the force-area curves obtained from an equimolar mixture of (S)-(+)-2-tetracosanyl acetate and the methyl ester of (5)-(+)-2-methylhexacosanoic acid. Comparison with either Figure 18 or Figure 22 demonstrates that the force-area curve of the mixed mono-layer has the same general appearance as those of the individual pure enantiomers. 

Thus, chiral discrimination may be observed that differentiates the force-area curves of the enantiomers of some surfactants from their racemic modifications. Apparent phase changes in the monolayer can be related to parallel behavior in the crystalline state through X-ray diffraction and differential scanning calorimetry. Formation of racemic compounds and quasi-racemates can be observed in some cases. 

The states produced by compression of monolayers are not necessarily at thermodynamic equilibrium, but may be metastable. In some cases, this is manifested clearly by different force-area curves being produced at different rates of compression. Slow reorganization of monolayer molecules is also apparent as hysteresis when films are repeatedly compressed and expanded. In chiral monolayers, the rates of molecular reorganization may be stereospecific as well as the thermodynamic behavior. 

The variation of surface pressure with the area available to the spread material is represented by a tt-A (force-area) curve. With a little imagination, tt A curves can be regarded as the two-dimensional equivalent of the p-V curves for three dimensional matter, (n.b. For a 1 nm thick film, a surface pressure of 1 mN m l is equivalent to a bulk pressure of 106 N m-2, or 10 atm.)... 

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). 

Determination of the force-area curves at the oil-water interface is usually carried out in a vessel of constant surface area. The area available to each molecule is reduced by adding more molecules to the interface. A method similar to that for air-water surfaces has also been described (7,8). In general, the oil-water interface reduces greatly the cohesion between the chains, and the pressure measured is due only to kinetic agitation and electrical repulsion. The lack of cohesion causes the pressure to be higher than for the same film at the air-water interface (Fig. 6). 

This resembles the results obtained for the oxidation of oleic acid. Further, the force-area curves of the oxidized ergosterol, lumisterol, and supra-sterol were nearly identical, with limiting areas of 60 A. per molecule, compared with 37.5 A. for ergosterol itself. 

On the other hand, despite the information about long chain sulfates, sulfonates, phosphates, and carboxylates that indicates stronger interaction with Ca2+ than with Mg2+ (i.e., in apparent harmony with the sequence of the Hofmeister (44) series), several difficulties remain. For example, while Miyamoto s data for DS (10) indicate the interaction sequence Mg < Ca < Sr < Ba from solubility measurements (as well as from temperature/CMC measurements if one accepts the Mg—Ca sequence of the present paper), this sequence, with the exception of the position of Mg and Ca, is the opposite of that found by Deamer et al. (33) from condensation effects on the force/area curves of ionized fatty acids. At the same time, the ion sequence obtained by these authors from phase transition temperatures of spread fatty acids (33) differs from that deduced from the above-mentioned condensation effects, and the latter depended strongly on pH. Lastly, definite differences in ion sequence effects exist for the alkaline earth metals in their interaction with long... 

FIGURE 26. Force/area curves for PDMS on water. Reproduced from Reference 247 by permission of John Wiley Sons, Inc... 

There are several ways of classifying the force/area curves of different organic surfactants. A convenient one for the present purposes is shown in Fig. 1, where four types of idealised monolayer behaviour are depicted. 

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