Stearic acid monolayer

Still another manifestation of mixed-film formation is the absorption of organic vapors by films. Stearic acid monolayers strongly absorb hexane up to a limiting ratio of 1 1 [272], and data reminiscent of adsorption isotherms for gases on solids are obtained, with the surface density of the monolayer constituting an added variable. 

There is a fair amount of work reported with films at the mercury-air interface. Rice and co-workers [107] used grazing incidence x-ray diffraction to determine that a crystalline stearic acid monolayer induces order in the Hg substrate. Quinone derivatives spread at the mercury-n-hexane interface form crystalline structures governed primarily by hydrogen bonding interactions [108]. 

Salts of fatty acids are classic objects of LB technique. Being placed at the air/water interface, these molecules arrange themselves in such a way that its hydrophilic part (COOH) penetrates water due to its electrostatic interactions with water molecnles, which can be considered electric dipoles. The hydrophobic part (aliphatic chain) orients itself to air, because it cannot penetrate water for entropy reasons. Therefore, if a few molecnles of snch type were placed at the water surface, they would form a two-dimensional system at the air/water interface. A compression isotherm of the stearic acid monolayer is presented in Figure 1. This curve shows the dependence of surface pressure upon area per molecnle, obtained at constant temperature. Usually, this dependence is called a rr-A isotherm. 

These measurements showed that in-plane lateral proton diffusion was facilitated at air-water interfaces on which stearic acid monolayers were formed, with a surface diffusion coefficient that depended critically on the physical state of the monolayer, and which was at most ca. 15% of the magnitude in bulk solution. These promising initial studies... 

FIG. 30 Schematic (not to scale) of the arrangement for SECM measurements of proton transport at a stearic acid monolayer deposited at the air-water interface. The UME typically had a diameter, 2a, in the range 10-25 pm and the tip-interface distance, d < la. 

Figure 3 shows the Tsp dependences of logK (max) for stearic acid monolayer on the water surface and the ED patterns of the monolayer transferred onto the hydrophilic substrate at the surface pressure of 20 mN-m. The homogeneous monolayer was formed on the water surface at this magnitude of surface pressure. The ED patterns were taken at the same temperature as... 

Fig. 7. The TSp dependences of the limiting area for the stearic acid monolayers.

Fig. 14. Surface area dependences of surface pressure and frequency maximum of the CH2 asymmetric band for (a) crystalline stearic acid monolayer and (b) amorphous myristic acid monolayer.

Table 3.Crystallographical distortion and continuity of stearic acid monolayer prepared by multi-step creep method and continuous compression method. 

Fig. 24. A filtered AFM image of a stearic acid monolayer on a scan area of 20x20 nm-. The stearic acid molecules were regularly arranged in a hexagonal array over 20x20 nm-.

Benzene solutions of lignoceric (CH3(CH2)->2COOH) and stearic (CH3(CH2)i6COOH) acids with concentrations of 1 X 10 3 and 3 x 10-3 mol-L1, respectively, were spread on the pure water surface atTsp of 293 K. Since Tsp is below Tm of the lignoceric acid (Tm - 347 K) and the stearic acid (Tm =317 K) monolayers [31] those monolayers are in a crystalline state. The subphase water was purified with the Milli-QII system. The lignoceric acid monolayer was prepared at a surface pressure of 5 mN-m 1 by a continuous compression at a rate of 1.7 X 10 3 nm--molecule 1 s 1. The stearic acid monolayer was prepared at 23 mN-m-1 by the continuous compression method or the multi-step creep method [39]. The multi-step creep method is a monolayer preparation method for which the monolayer is stepwisely compressed up to a... 

Figure 28(a) shows a nonfiltered AFM image of the stearic acid crystalline monolayer which was prepared by the multi-step creep method with a scan area of 5 X 5 nm2. Though scanning was done repeatedly on the stearic acid monolayer prepared at the high surface pressure of 23 mN- nr1 by the multi-step creep method, the monolayer was not damaged by the... 

Figure 1. Melting process of a stearic acid monolayer at the air/water interface. The BAM images of the domain structures (a) 41 °C, (b) 45 °C, (c) 47 °C, and (d) at 20 °C after melting.

Collapsing and nucleation processes of a stearic acid monolayer have been also studied on the different aqueous subphases by a Brewster angle microscope [15]. Both of the temperature and the pH value of the aqueous subphase can influence the nucleation process and produce very different collapsing patterns. 

No direct method exists by which monolayer film structures on water can be studied. Therefore, the LB method has been used to study molecular structures in past decades. The most useful method for investigating the detailed LB-deposited film structure is the well-known electron diffraction technique (or the scanning probe microscope [Birdi, 2002a]). The molecular arrangements of deposited mono-and multilayer films of fatty acids and their salts, using this technique, have been reported. The analyses showed that the molecules were almost perpendicular to the solid surface in the first monolayer. It was also reported that Ba-stearate molecules have a more precise normal alignment compared to stearic-acid monolayers. In some investigations, the thermal stability of these films has been found to be remarkably stable up to 90°C. 

A recent report describes the radioactive labeled chromium (iii) ion adsorption on stearic acid LB films. The adsorption of chromium (iii) on a stearic acid monolayer on the surface of CrCl3 was described. Stearic acid monomolecular films on I0 3 M CaCl2 subsolutions were deposited in paraffin-coated microscope glass slides by the LB technique (pH range 2-9). 

Figure 8. Effect of varying concentrations of ATP on adsorption of calcium to stearic acid monolayer. (O), no ATP (, ), I0"5M ATP (A), 10 4M ATP, (A), 10 3M ATP ordinate, surface concentration of Ca2+ in moles/sq. cm. abscissa, —log molar concentration of Ca2+ at pH 7.6...

The method of measuring surface pressure by a modified Wilhelmy plate and surface potential by an ionizing air electrode has been described (41). For surface potential measurements, the electrometer was calibrated with stearic acid monolayers on 0.01N HC1, for which a value of 395 to 400 mv. at 21 sq. A. per molecule is assumed to be standard (17). 

Figure 2. Surface pressure, potential, and viscosity vs. molecular area of stearic acid monolayers spread at air-water interface over substrates containing ammonium and alkali metal cations (0.5N). 30° C., rapid compression...

Figure 6. Surface pressure vs. molecular area for stearic acid monolayers spread on aqueous substrates of varying pH at 30° C. and 0.5N total cation concentration. Rapid compression...

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