Equilibrium spreading pressure ESP

Table 1 Equilibrium spreading pressures (ESPs) of racemic and optically pure N-(a-methylbenzyl)stearamides at various temperatures and subphase acidities.0 1 ... 

The detached amounts of cadmium octadecanoate LB films at the water surface with various temperatures are shown in Figure 18. The detached amount increased linearly with increasing the subphase temperature. The detachment of LB films is concerned with equilibrium spreading pressure (ESP), which represents the equilibrium between bulk lipid crystals and a lipid monolayer on the water surface [45]. ESPs... 

The equilibrium spreading pressure (ESP) of monolayers from polar lipids has the same value as the plateau pressure defining the transition between the form with liquid chains and that with crystalline chains. Above the chain melting temperature the ESP value is the same as the collapse pressure of the monolayer phase with liquid chains. 

In contrast, the stearic acid monolayer compressed on a pH 8.0 subphase containing 10 M CaCh showed a surface pressure rising to just over 60 mM m and then rolling over into a plateau without a spike. BAM observation at 63 mN m showed chains of many small bright microcrystals. It is important to realize that monolayers at surface pressures above the equilibrium spreading pressure (ESP) are thermodynamically metastable and should ultimately transform to three-dimensional aggregates in equilibrium with a monolayer whose surface pressure equals the ESP... 

Before deposition of the material to form multilayers, the stability of the monolayer must be considered. The solid monolayer phase of most LB materials is formed at surface pressures above the equilibrium spreading pressure (ESP) of the material the ESP being defined as the pressure at which the monolayer and stable solid or liquid phases are in thermodynamic equilibrium. Consequently, the deposition of most LB materials occurs from a metastable state. This often presents no practical problems, since the approach to thermodynamic equilibrium is extremely slow. However, for some materials a loss of pressure or area is observed below the collapse pressure 11. These materials approach thermodynamic... 

Table 12 shows the equilibrium spreading pressures of each diacid. It is immediately apparent that for three of the diastereomeric pairs there are statistically significant differences. These distinctions relate stereochemical preferences in the spontaneous spreading of (+)- versus meso-monolayers in equilibrium with their respective crystalline phases. However, there appears to be no discernible trend in either the ( )- or meso-ESPs as a function of carbonyl position despite clear trends seen in their monolayer properties in the absence of any bulk crystalline phase. 

A racemic film was compressed nearly to its collapse point. It was then seeded by sprinkling crystals of pure enantiomeric amide on the surface. A rapid decrease in surface pressure was observed approaching the equilibrium spreading pressure of the enantiomer. A control experiment in which racemic crystals were sprinkled on the compressed racemic film produced a pressure drop that slowly approached, but did not reach, the ESP of the racemic film. The observed behavior was consistent with what would be expected if the enantiomer seed crystals had removed molecules of the same enantiomer from the racemic film, leaving a monolayer composed mainly of molecules of the opposite configuration. 

Enantiomeric recognition was clearly displayed in films spread from solution and films in equilibrium with their crystals, and was sharply dependent on the acidity of the subphase. Protonation of the amide group appeared to be necessary for spreading to stable monolayers. For example, the crystals of the racemate deposited on a 10n H2S04 solution at 25°C spread quickly to yield a film with an ESP of 7.7 dyn cm"1, while the single enantiomers spread only to a surface pressure of 3.9 dyn cm-1 (Table 1). Similar effects are observed at 15 and 35°C. The effect of stereochemistry on equilibrium spreading is even more pronounced at lower subphase acidities. On 6n sulfuric acid, the racemate spread to an equilibrium surface pressure of 4.9 dyn cm-1, while the enantiomeric systems spread to less than 1 dyn cm-1. 

These results for spread film and equilibrium spreading suggest that films of racemic N-(a-methylbenzy 1) stearamide may be resolved by seeding the racemic film with crystals of either pure enantiomer. Indeed, when a monolayer of racemic jV- (a-methylbenzyl) stearamide is compressed to 45 A2/molecule (27 dyn cm-1), deposition of a crystal of either R( +)- or S( — )-enantiomer results in a decay of surface pressure from the initial 28 dyn cm-1 film pressure to 3.0 dyn cm-1, the ESP of the enantiomeric systems on a pure 10n sulfuric acid subphase (Table 1). When the experiment is repeated with racemic crystals, the system reaches an equilibrium surface pressure of 11 dyn cm-1, nearly the ESP of the racemic crystal on the clean acidic interface. In either case, equilibrium pressure is reached within a two hour time period. 

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