Stearic acid desorption

The calculations for (W0 — Wc) obtained by comparing the desorption rate constants of Ci6 and Ci8 sulfates and phosphonates and palmitic and stearic acids are summarized in Table II. 

Important trends in N2 isotherm when the PS beads are used as a physical template are shown in Table 1 and Fig. 2. In Table 1, PI is the alumina prepared without any templates, P2 is prepared without ]4iysical template (PS bead), P3 is prepared without chemical template (stearic acid), and P4 is prepared with all templates. For above 10 nm of pore size and spherical pore system, the Barrett-Joyner-Halenda (BJH) method underestimates the characteristics for spherical pores, while the Broekhoff-de Boer-Frenkel-Halsey-Hill (BdB-FHH) model is more accurate than the BJH model at the range 10-100 nm [13]. Therefore, the pore size distribution between 1 and 10 nm and between 10 and 100 nm obtained from the BJH model and BdB-FHH model on the desorption branch of nitrogen isotherm, respectively. N2 isotherm of P2 has typical type IV and hysteresis loop, while that of P3 shows reduced hysteresis loop at P/Po ca. 0.5 and sharp lifting-up hysteresis loop at P/Po > 0.8. This sharp inflection implies a change in the texture, namely, textural macro-porosity [4,14]. It should be noted that P3 shows only macropore due to the PS bead-free from alumina framework. 

Figure 2. Niuogcn adsorption-desorption isotherms of the porous alumina prepared using stearic acid and/or PS bead as a template (curve are shitted for clarity). The inset shows the corresponding pore size distribution from the desorption branch.

Tn 1922 Adam (I) published the third paper in his extraordinary series on surface film structure. He observed that fatty acid monolayers greatly expanded on alkaline subphases. He also suggested that fatty acid anions desorbed or dissolved from the monolayer into the alkaline subphase. In 1933 he and Miller (2) showed that the composition of the subphase buffer significantly affected the monolayer thus palmitic and stearic acid monolayers were more condensed on 2N sodium hydroxide than on 2N potassium hydroxide. The expansion, desorption, and cation selectivity of ionizing monolayers are the subjects of this investigation. 

Direct study of the desorption of stearic acid from platinum and from NiO was carried out by Timmons and Zisman [10]. Their findings are shown in Table 10-5. The methylene iodide contact angle and the surface potential measurements indicate that a stearic acid monolayer adsorbed on platinum can be removed completely by heating to 130 C or by extraction with diethyl ether. But if the adsorbent is nickel oxide, heating to 150 C or extraction with diethyl ether fails to restore the original contact angle behavior or the surface potential of the adsorbent surface. 

Figure 10-7. Desorption of stearic acid monolayer from iron. A Depletion by diethyl ether. B Thermal desorption at 373 K. C Exchange with inactive stearic acid. Data by Timmons, Patterson and Lockhart [8].

Figure 10-8. Desorption of stearic acid monolayers. (a) Depletion by carbon tetrachloride. (b) Exchange with inactive stearic acid. Data by C. 0. Timmons [20].

Mica was the least reactive surface studied, probably because it is nearly molecularly smooth and the basal surface of its lattice is essentially a layer of oxygen atoms [3,9]. Furthermore, it is not readily wetted by water. It did not adsorb stearic acid in wet or dry helium or in dry air [20]. In the presence of wet air, it initially adsorbed about 0.2 monolayer, but adsorption decreased to zero after 40 hours. Desorption to zero may indicate that the water and air, or other impurities on the surface, diffused or dissolved in the n-hexadecane solution. 

Initial adsorption-desorption humps were observed in most of the experiments. Perhaps the stearic acid molecules rearranged at the surface to occupy greater area. Another possibility is that some stearic acid was slowly displaced by trace quantities of water or other polar contaminants. 

Figure 3.4 FMC data for adsorption of stearic acid and isostearic acid onto Mg(OH)2 and A1(0H)3 from heptane and toluene adsorption data, desorption data. The dashed lines represent vertically adsorbed theoretical monolayer levels. M magnesium hydroxide, A aluminium hydroxide, hept heptane, tol toluene.

Figure 7.7 Schematic mechanism for initial solubilization. Mixed micelle desorption and diffusion (steps 4 to 5) are assumed to control stearic acid solubilization. From Chan et al. [31].

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. 

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