The study of monolayers at the air-liquid interface offers unusual opportunities to investigate the effects of stereochemical factors in organized systems. As a first step toward developing this field, we have examined some surface properties of 7V-(a-methylbenzyl)stearamide, comparing the behavior of monolayer films of its racemic modification with its two enantiomeric forms and also with the achiral position isomer TV-phenethylstearamide. X-ray powder diffraction patterns showed that the racemic material was, in fact, a true racemate. Also, the transition points and heat of fusion of the racemate were markedly different from those of the enantiomers, as shown by differential scanning calorimetry. Although the stearamides did not spread as monolayers on water as a subphase, aqueous sulfuric acid media above 6 N evoked good surfactant behavior as manifested by reduction of the subphase surface tension. Chiral discrimination was clearly apparent in the monolayer behavior and was sharply acid dependent. Thus, the racemic A-(a-methylbenzyl)stearamide began to show surface activity on 6 N H2S04 while the enantiomers required 8 N acid to give the same response. Equilibrium spreading pressures (ESP's) for the films in equilibrium with the pure crystals were measured on a Langmuir film balance as a function of subphase acidity and temperature. Thermodynamic properties for the surface films were evaluated from these ESP values and also from static surface tension measurements made at different temperatures. By all criteria, the molecules of the optically pure samples are more strongly and completely associated, both in the crystalline and in the monolayer phase, than are those of the racemate. We propose that the acid dependence of chiral discrimination between enantiomeric and racemic films is caused by protonation of the amide head group by the aqueous acid subphase. This produces a driving force for expansion of the film which is opposed by the aggregation forces in the neutral amide molecules. These forces are stronger for the enantiomers than for the racemate.As a test of this model, a series of liquid-liquid distribution experiments was performed for the transfer of A-(a-methylbenzyl)butyramide from hexane to the series of aqueous acid solutions used for monolayer subphases. A close correlation was found between the acid dependence of the free energy of spreading for the stearamide and the free energy of transfer of the butyramide. The pXBH+ of the butyramide was calculated to be -1.5 ± 0.5 from the distribution experiments and -1.5 ± 0.2 from an independent series of indicator measurements. The free energy correlation suggests that the pXBH+ of the protonated stearamide at the air-acid interface is close to that of the butyramide in homogeneous solution. The force-area behavior and dynamic properties of the monolayers will be reported subsequently. This paper is limited to a description of the thermodynamic properties under bona fide reversible conditions.

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