Stearyl alcohol monolayers

Fig. IV-13. Example of a p-polarized reflection spectrum from Ref. [154] for a stearyl alcohol monolayer on water. The dashed line is the baseline to be subtracted from the spectra. [Reprinted with permission from Joseph T. Buontempo and Stuart A. Rice, J. Chem. Phys. 98(7), 5835-5846 (April 1, 1993). Copyright 1993, American Institute of Physics.]...

Figure 1. Surface pressure-area curve of stearyl alcohol monolayer at pH 2.0, 25°C. Compression rate 6.5 A2/molecule/min.

Figure 1 shows the surface pressure—area curve of stearyl alcohol monolayer at pH 2.0 and 25 °C, where Ae is the area/molecule in the expanded state at zero surface pressure, Ac is the area/molecule in the condensed state at zero surface pressure, and A2o is the area/molecule at surface pressure of 20dynes/cm. Figure 2 shows the values of Ae, Ac, and A2o for various alkyl alcohols. From this diagram the average area per molecule in mixed monolayers was calculated using simple additivity rule. Figure 3 shows the excess molecular area when Ci6 alkyl alcohol... 

The results on the surface pressure-area curve of stearyl alcohol monolayer shown in Figure 1 and the area/molecule of other alkyl alco-... 

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

In the rapid absorption of O2 and CO2 into unstirred, chemically reactive solutions (20), monolayers of stearyl alcohol may decrease the... 

Compounds used to form Langmuir monolayers were stearic acid (>99.9%) and stearyl alcohol (>99.9%) purchased from Aldrich. Stearic acid known to bind with cadmium ions (CdCh, >99.9%, Aldrich) were mixed with stearyl alcohol at varying compositions from 100 0 to 50 30. Each solution was at the concentration of ImM in chloroform (>99%, Fluka). 

It is interesting in the in-situ investigation (Fig. 2b) that as the relative amount of the inactive stearyl alcohols to cadmium ion exceeds 20% the asymmetric carboxylate band suddenly decreases in its intensity and remains about the same afterwards. In Fig. 2c, it is clearly seen that the adsorbed amount of cadmium ions at 80% of stearic acid is reduced by more than half of those at larger compositions of the acid. By contrast, the ex-situ results showed a linear decrease in cadmium adsorption with the monolayer composition as well as a certain deviation from the theoretical calculation that is different to the case without adsorption of cadmium ions. 

Surface self-diffusion is the two-dimensional analogue of the Brownian motion of molecules in a liquid bulk. Measurements of self-diffusion have to be performed in complete absence of any Marangoni flow caused by surface tension differences. Such experimental conditions are best established in an insoluble monolayer where one part consists of unlabelled and the other of radio-tracer labelled molecules. The movement of molecules within the surface monolayer can be now observed by using a Geiger-Miiller counter. There are possible effects of liquid convective flow in the sublayer which was discussed for example by Vollhardt et al. (1980a). With e special designed apparatus Vollhardt et al. (1980b) studied the self-difihision of different palmitic and stearic acid and stearyl alcohol and obtained self-diffusion coefficients between l-i-4 lO cm /s. 

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