Surface acidity, measurement

The results from surface acidity measurements by NH3 TPD in the catalysts show that the total number of acid sites was proportional to the accessible zeolite area (Tables 1 and 2). The same trend was found also for the Bronsted acid sites quantified by FT-IR of Py. 

Although pyridine is not specific because it interacts with both Bronsted and Lewis acid sites, the conversion of Lewis sites into Bronsted ones cannot be ruled out since the alkylation reaction generates water. So, the pyridine adsorption can still be useful for surface acidity measurements. Thus, the surface acidity of catalysts is given in Table 1 as the amount of... 

An alternative treatment of the reaction scheme can also be given as a natural extension of the work of Bell in concentrated mineral acid. The usual approach is to apply the Hammett acidity function, from which the solid surface acidity measurement has developed as given at the beginning of the article, to see if linearity is observed between and log k of the acid-catalyzed reaction. The assumption usually criticized is the constancy of the activity factor for two different bases. 

Mangipudi et al. [63,88] reported some initial measurements of adhesion strength between semicrystalline PE surfaces. These measurements were done using the SFA as a function of contact time. Interestingly, these data (see Fig. 22) show that the normalized pull-off energy, a measure of intrinsic adhesion strength is increased with time of contact. They suggested the amorphous domains in PE could interdiffuse across the interface and thereby increase the adhesion of the interface. Falsafi et al. [37] also used the JKR technique to study the effect of composition on the adhesion of elastomeric acrylic pressure-sensitive adhesives. The model PSA they used was a crosslinked network of random copolymers of acrylates and acrylic acid, with an acrylic acid content between 2 and 10%. 

The specific surface area of the fresh and used catalysts was measured by nitrogen adsorption method (Sorptometer 1900, Carlo Erba Instruments). The catalysts were outgassed at 473 K prior to the measurements and the Dubinin equation was used to calculate the specific surface area. The acidity of investigated samples was measured by infrared spectroscopy (ATI Mattson FTIR) by using pyridine (>99.5%, a.r.) as a probe molecule for qualitative and quantitative determination of both Bronstcd and Lewis acid sites (further denoted as BAS and LAS). The amounts of BAS and LAS were calculated from the intensities of corresponding spectral bands by using the molar extinction coefficients reported by Emeis (23). Full details of the acidity measurements are provided elsewhere (22). 

Emersion has been shown to result in the retention of the double layer structure i.e, the structure including the outer Helmholtz layer. Thus, the electric double layer is characterised by the electrode potential, the surface charge on the metal and the chemical composition of the double layer itself. Surface resistivity measurements have shown that the surface charge is retained on emersion. In addition, the potential of the emersed electrode, , can be determined in the form of its work function, , since and represent the same quantity the electrochemical potential of the electrons in the metal. Figure 2.116 is from the work of Kotz et al. (1986) and shows the work function of a gold electrode emersed at various potentials from a perchloric acid solution the work function was determined from UVPES measurements. The linear plot, and the unit slope, are clear evidence that the potential drop across the double layer is retained before and after emersion. The chemical composition of the double layer can also be determined, using AES, and is consistent with the expected solvent and electrolyte. In practice, the double layer collapses unless (i) potentiostatic control is maintained up to the instant of emersion and (ii) no faradaic processes, such as 02 reduction, are allowed to occur after emersion. 

Amino acid measurements in ALH84001 are almost certainly the result of Antarctic ice contamination. Amino acids are readily soluble in water but PAHs are practically insoluble. Isotopic measurements of 14C show that terrestrial carbon is incorporated into the meteorite during extended stays in the Antarctic ice fields. In addition, microbial activity on the exposed surfaces provides an additional source of biogenic organic material that may be incorporated over time. 

The surface pressure-area (tc-A) isotherm measurements and LB film transfer were performed with the use of a KSV 5000 minitrough (KSV Instrument Co., Finland) operated at a continuous speed for two barriers of 10 cm2/min at 20°C. The buffer used in the present work was composed of 10 mM MES, 2 mM ascorbic acid sodium salt, and a given concentration of salt or polymers (pH =7.0). The accuracy of the surface pressure measurement was 0.01 mN/m. Monolayers of the PS I were transferred at 10 mN/m on hydrophobic substrate surface by horizontal lifting method. 

The processes classified in the third group are of primary importance in elucidating the significance of electric variables in electrosorption and in the double layer structure at solid electrodes. These processes encompass interactions of ionic components of supporting electrolytes with electrode surfaces and adsorption of some organic molecules such as saturated carboxylic acids and their derivatives (except for formic acid). The species that are concerned here are weakly adsorbed on platinum and rhodium electrodes and their heat of adsorption is well below 20 kcal/mole (25). Due to the reversibility and significant mobility of such weakly adsorbed ions or molecules, the application of the i n situ methods for the surface concentration measurements is more appropriate than that of the vacuum... 

Both acid- and base-promoted reactions may be affected by acidic surfaces and, hence, by the factors which influence the surface acidity. Kinetic evidence for increased Br nsted acidity at clay surfaces has been presented by McAuliffe and Coleman (80) who studied the hydrolysis of ethylacetate and the inversion of sucrose. They noted that potentionmetrie pH measurements did not explain the catalytically effective H+-concentration at the clay surface. 

The H+ and NH forms of homoionic montmorillonite promote the hydrolysis of chloro-s-triazines to the hydroxy analogs (hydroxy-s-triazines) (73). Apparently, the surface acidity of these clays was extremely high, since no degradation was observed in control experiments conducted at pH 3.5 in homogeneous aqueous solution. Russell et al. (73) suggested that the hydroxy-s-triazine products were stabilized in the protonated form at the silicate surface. The IR spectra of these surface complexes agreed with the spectra obtained in 6N HC1, and it was inferred that the pH at the clay surface was 3 to 4 units lower than that measured in suspension. 

Some of the compounds described in this chapter were studied for specific physical properties. Surface tension measurements with solutions of 9-16 in 0.01 M hydrochloric acid demonstrated that these zwitterionic X5Si-silicates are highly efficient surfactants.21 These compounds contain a polar (zwitterionic) hydrophilic moiety and a long lipophilic z-alkyl group. Increase of the n-alkyl chain length (9-15) was found to result in an increase of surface activity. The equilibrium surface tension vs concentration isotherms for 9 and 16 were analyzed quantitatively and the surface thermodynamics of these surfactants interpreted on the molecular level. Furthermore, preliminary studies demonstrated that aqueous solutions of 9-16 lead to a hydrophobizing of glass surfaces.21... 

Surface photovoltage spectroscopy (SPS) in Fig. 6.5 was used to determine the surface acidity of JML-1 by measuring transition of electrons between the interface and the surface. The JML-I40 calcined at 550°C exhibited two peaks at 596 nm and 677 nm, whereas the sample without calcination had only one peak at 330 nm. The peak at 330 nm is assigned to the band-band electron transition and those at 596 nm and 677 nm are attributed to the surface-related transitions. The observation of these surface-related transitions indicates the presence of positive charges on the surface of the calcined sample, suggesting that the acidity of JML-1 catalyst is resulted from a large amount of SZ acidic sites on the silica surface. 

Encouraged by the results from static surface tension measurements, we next ran force-area curves on 3N, 6N, and ION acids. As Figure 33 indicates, there is pronounced discrimination between the racemic and enantiomeric monolayers, and those differences are sharply dependent on the subphase acidity, as had been impUed by the surface tension behavior. 

The aggregation behavior of C21-DA salt in dilute electrolyte medium appears to resemble that of certain polyhydroxy bile salts (25,16). That C21-DA, with a structure quite different from bile acids, should possess solution properties similar to, e.g., cholic acid is not entirely surprising in light of recent conductivity and surface tension measurements on purified (i.e., essentially monocarboxylate free) disodium salt aqueous solutions, and of film balance studies on acidic substrates (IX) The data in Figure 3 suggest that C21-DA salt micelles Incorporate detergents - up to an approximate weight fraction of 0.5 -much like cholate Incorporates lecithin or soluble... 

Micelles tend to aggregate, and there are many ways to measure their concentration, including surface tension measurements. The midpoint of the concentration range over which micellar aggregation occurs is called the critical micellar concentration (CMC). Below the CMC, added bile-salt molecules dissolve in the form of monomers above the CMC, added bile-salt molecules form micelles, leaving the monomeric concentration essentially constant. The pH at which CMC formation occurs is called the critical micellar pH, (CMpH). Table 1.1 lists values for some of the bile acids mentioned in this review. 

X ray photoelectron spectroscopy (XPS) is powerful in identifying species present at the surface/interface and atoms or functional groups involved in acid-base interactions [116]. Since XPS measures the kinetic energy of photoelectrons emitted from the core levels of surface atoms upon X ray irradiation of the uppermost atomic layers, it can be used to characterize surface acid sites, in combination with base probe molecules adsorption. 

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