Palmitate films

Fig. 12. Ethyl palmitate films when the molecules are nearly independent. The ester groups are fully exposed to the water, and alkaline hydrolysis is rapid, k = 0.037 min. (Alexander and Schulman, 18).

Fig. 2. Maps (2x2 pm) of. sample topography (height variation) and pull-off adhesive force for a Langmuir-Blodgett film on mica consisting of a I I mixture of palmitic (Cl6) and lignoceric (C24) fatty acids [46].

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. 

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.

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... 

Remarkable results were obtained with pentaerythritol tetra-palmitate. This symmetrical body gave a film of exceptional rigidity, so that it withstood a force at one end of 5 6 dynes per cm. without any support at the other end. When first put on the area was 100 A, per molecule and was reducible by compression to 80 A., or about four times the area of a single closely packed chain. The four chains must therefore lie parallel, and two of them must be bent back through a large angle. 

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. 

Eight binary systems are reported. Six include cholesterol as one component of the mixed film. The second components in these six films were myristic acid, methyl palmitate, ethyl palmitate, 1,2-dimyristin, 1,2-dimyristoyl-3-lecithin, and l,2-didecanoyl-3-lecithin. In addition, the systems trilaurin-dimyristoyl lecithin and triolein-dimyristoyl lecithin... 

The calculation of surface viscosity by this method had a maximum sensitivity and precision of 3 X 10 5 and 1 X 10 4 surface poise, respectively, in the low viscosity range, and 1 X 10 3 and 1 X 10 - surface poise, respectively, in the high viscosity region. The method offers extreme simplicity in construction and operation, and is more adaptable to high viscosity films (greater than 10 2 surface poise) than the canal or slit viscometer. Recent work (31) with the canal viscometer indicates an inability to measure film viscosities of palmitic and stearic acid mono-layers over pH 9.0 to 9.5 KOH substrates. 

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... 

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

Example. The results from surface balance experiments involving surface films of palmitic (C15H31COOH), stearic (C17H35COOH), and cerotic (C25H51COOH) acids show that each exhibits an area per molecule in the monolayer of 21 A2. This is only possible for molecules with such different hydrocarbon tail lengths if they each orient vertically in their respective monolayers. 

---