Palmitic acid, films

Fig. 27 Equilibrium spreading pressure versus film composition for crystals of palmitic acid and racemic and enantiomeric stearoylserine methyl ester deposited on palmitic acid/SSME monolayers (a) enantiomeric crystals on enantiomeric SSME/palmitic acid films (b) racemic crystals on racemic SSME/palmitic acid films (c) palmitic acid crystals on either racemic or enantiomeric SSME/palmitic acid films.

For palmitic acid films (Figures 4 and 5), the ir-A curves for pH 5.6 to 10.3 exhibit close-packed heads and close-packed tails regions which are absent at higher pH. At pH 7.4 and 10.3, there is evidence of a phase... 

A as a Function of pH and 7r. Since the area/molecule could not be ascertained from the 7r-A isotherm of a desorbing film, the A values were calculated by extrapolating kinetic data (Equation 2) to zero t (8). Abrupt phase transitions occurred with palmitic acid films, and the 7r of the phase transition varied directly with pH (Figure 4). The anomalous decrease in A that was found immediately before film expansion (Figure 4) suggested partial ionization and the formation of condensed acid soaps (6, 8, 28). Since fatty acid monolayers are partially ionized in the condensed state (8), the apparent pKa of the fatty acid could not be esti-... 

Extrapolated surface areas were identical for palmitic acid films desorbing into bicarbonate and tris buffers (Figure 4). These data suggested that these buffers had similar effects on the ionization and expansion of the monolayers. 

A as a Function of Fatty Acid Structure. Unsaturated fatty acids have larger surface areas than saturated fatty acids (9) consequently, the unsaturated fatty acid anions form weaker fields than the saturated ones. Thus the expansion of an oleic acid film, maintained at the same tt as a palmitic acid film, was less abrupt and occurred at a lower pH than the expansion of a palmitic acid film (Figure 4). 

From the fitting, this has a value of 36 pF (slightly greater than obtained with the uncoated electrode). Rppy represents the in-plane resistance of the polypyrrole-palmitic acid film, which from the curve fitting is found to be 3030 Q. The capacitance of the polypyrrole multilayer coated interdigitated electrodes measured in vacuum was only a few pF has been ignored in the model. 

In two other studies, it was observed that C60 in LB films can quench the fluorescence of pyrene [293] and of 16-(9-anthroyloxy)palmitic acid [294] by photoinduced electron transfer. In these studies, both C60 and the electron-donating fluorophore were incorporated into a tricosanoic acid LB film in different ratios. 

The difference in the n/ A properties of these mixed chiral/achiral systems was also observed in the films dynamic properties. Figure 25 gives the surface shear viscosities of the palmitic acid/SSME systems at surface pressures of 2.5 and 5.0 dyn cm -1 at 25°C. It is clear that stereo-dependence of film flow... 

Fig. 24 Surface pressure/area isotherms for palmitic acid/stearoylserine methyl ester films at 25°C on a pure water subphase and compressed at 29.8 A2/molecules per minute. A, 16.7-33.3% B, 50% C, 66.6% D, 83.3% SSME.

Table 9 Monolayer stability limits of palmitic acid/stearoylserine methyl ester films at 25°C.

Fig. 25 Surface shear viscosity vs. film composition for the palmitic acid/stearoylserine methyl ester film system at 25°C.

Enantiomeric discrimination and its relation to film component reorganization upon compression can also be observed in dynamic surface tension hysteresis loops. Figure 26 shows the WjA isotherms generated upon five successive compression/expansion cycles (from II = 0 to lOdyncm-1) of racemic and enantiomeric films containing 17 mole percent palmitic acid. The hysteresis loops, obtained on the apparatus described in Section 2 (p. 63), show that the first compression/expansion cycle of the racemic system is repeated in each successive cycle. Upon expansion of the film from the maximum surface pressure back to Odyncm-1, the racemic film returns to its original state without detectable reorganization of the components. However, the... 

In order to test the mechanism of recognition, equilibrium spreading pressures of both racemic and enantiomeric forms of SSME were obtained in pre-spread films of palmitic acid/SSME mixtures. The films were spread from solution and then compressed to their lift-off areas. A crystal of the racemic SSME was placed on surface film mixtures of the fatty acid with racemic SSME, and the enantiomeric crystals were placed on surface film mixtures of the fatty acid and enantiomeric SSME. The results of the equilibrations are given in Fig. 27. 

As on pure water substrates, the enantiomeric crystals of SSME did not spread on the enantiomeric SSME/palmitic acid monolayer-covered surfaces, while the spreading of the racemic crystals on the racemic film-covered water was actually enhanced. The palmitic acid crystals deposited on either racemic or enantiomeric film covered substrates spread to the same surface pressure, independent of stereochemistry. 

Conversely, the racemic film system appears to be solubilized by the achiral fatty acid component. At compositions of 10-33% palmitic acid, the ESP of the racemic system varies linearly with film composition, indicating that the monolayer in equilibrium with the racemic crystal is a homogeneous mixture of racemic SSME and palmitic acid. At compositions of less than 33% palmitic acid, the ESP is constant, indicating that three phases consisting of palmitic acid monolayer domains, racemic SSME monolayer domains, and racemic SSME crystals exist in equilibrium at the surface. 

It has been shown by Harvey et al. (1989) that incorporation of palmitic acid into a monolayer spread from stearoylserine methyl ester (SSME) breaks up intermolecular SSME interactions. The palmitic acid acts as a two-dimensional diluent. Figures 52(A-C) give the Yl/A isotherms for mixtures of FE and SE C-15 6,6 -A with palmitic acid. Dilution of the monolayer cast from the second eluting isomer with 15 mol% palmitic acid separates the diacid molecules from one another on the water surface and perhaps allows for the expression of their stereochemically dependent conformations. The mixed film (15% palmitic acid/85% C-15 6,6 -A) expands at low II and behaves in much the same manner as the single-component monolayer (C-15 6,6 -A) behaves at 30°C. Addition of 15 mole% palmitic acid into a monolayer cast from the FE C-15 diacid has little effect on its energetics of compression, indicating a stronger intermolecular interaction afforded by its stereochemically dependent conformation at the air-water interface. 

Incorporation of higher mole percent palmitic acid in films spread from both isomers causes an expansion of the films beyond that obtained from simple additivity relationships. At all mole fractions of palmitic acid, the... 

The compression under which collapse of the films occurs is very variable. In the case of palmitic acid the buckling point occurred between 20 and 45 dynes per centimetre, when the experiments were carried out with fresh water. On the other hand when water with a Ph of about 6 was used, the film, after being allowed to stand for a few days either covered or open to the atmosphere, resisted compression to over 60 dynes per centimetre. 

A mixture of benzene and methanol (19 to 1) was used for spreading the alkyl phosphonates. To minimize the influence of benzene on the film properties, the concentrations of the spreading solutions were > 1.5 X 10 3 gram per ml., and the experiments were performed at tt > 4 dynes per cm. (25). Moreover, higher proportions of methanol in the spreading solution did not alter the film properties under study for selected monolayers. For the sulfates, a mixed solvent containing water-benzene-2-propanol (1 10 10) was used because with the benzene-methanol solutions the properties of the films depended on the age of solution from which the films were prepared. Stearic and palmitic acids were spread from either hexane or the benzene-methanol solvent used for the phosphonates. Identical desorption results were obtained with the two solvents. 

To determine the 7r-A isotherms of dissolving films, two extrapolation methods were employed (29) either log tt at constant AT, or log AT at constant ir was plotted as a function of /1, where t is elapsed time. Extrapolation to t = 0 yields values of A or ir to within dr 5% for rapidly desorbing films. Palmitic acid at pH 9.2 was an exception its desorption rate was so rapid that the error in A was d= 10%. All the experiments were performed at 23° d= 0.5 °C. the maximum temperature variation during a single run was < 0.2 °C. The surface potential, AV, was monitored and found not to drift by more than 10 mv. during a single run. Reproducibility was d= 10 mv. 

The results of plotting log k8 at constant it as a function of pH for sulfate, phosphonate, and carboxylate films are shown in Figure 5, and the values of A log kjApH are given in Table III. Except for palmitic acid between pH 5.8 and 7.2, A log k8/ApH values for the films were significantly less than predicted by Equation 13. Moreover, the values increasingly deviate from Equation 13 with increase in pH. 

Figure 9.7 Effect of palmitic acid (PA) on metronidazole release at 10 percent loading and 125 pm film thickness.

From simple considerations of the dimensions of the molecules, it can be seen at once that they are greatly elongated in the direction perpendicular to the surface. Palmitic acid, with sixteen carbons ii the molecule, has a molecular volume of 300 c.c., and therefore the molecule has a volume of 495 cub. A. its cross-section as measured is 20 5 sq. A., so that its length (measured perpendicular to the surface) must be some 24 2 A., if the density in the films is the same as that in bulk. It must therefore be four or five times as long as thick. 

A mixed monolayer consisting of stearic acid (9.9%), palmitic acid (36.8%), myristic acid (3.8%), oleic acid (33.1%), linoleic acid (12.5%), and palmitoleic acid (3.6%) produces an expanded area/pressure isotherm on which Azone has no apparent effect in terms of either expansion or compressibility (Schuckler and Lee, 1991). Squeeze-out of Azone from such films was not reported, but the surface pressures measured were not high enough for this to occur. The addition of cholesterol (to produce a 50 50 mixture) to this type of fatty acid monolayer results in a reduction of compressibility. However, the addition of ceramide has a much smaller condensing effect on the combined fatty acids (ratio 55 45), and the combination of all three components (free fatty acids/cholesterol/ceramide, 31 31 38) produces a liquid condensed film of moderate compressibility. The condensed nature of this film therefore results primarily from the presence of the membrane-stiffening cholesterol. In the presence of only small quantities of Azone (X = 0.025), the mixed film becomes liquid expanded in nature, and there is also evidence of Azone squeeze-out at approximately 32 mN m. ... 

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