Triethylamine adsorption

Results similar to Figure 7 have been obtained with triethylamine adsorption from cyclohexane onto Hi-Sil, a silica gel with a high surface density of silanol groups (34). This is in contrast with Cab-0-Sil, a silica gel known to have a lower surface concentration of silanol groups, some of which are stronger acids... 

The temperature coefficient for both the pyridine and triethylamine adsorption was small, and will need checking by calorimetric measurements. 

Figure 4. Plots of 0 vs.. (o o o and —) and 8(AG,ds)/l r vs., ( and - - -) due to triethylamine adsorption on a Hg electrode at concentrations 0.002, 0.00126, 0.001, 0.000794, 0.000631, and 0.0005 mol dm (from top to bottom). Points are experimental data reprinted from J. Electroanal. Chem., 255, M. L. Foresti el al.. Adsorption of Triethylamine at the Mercury/Water Interphase from Charge and Interfacial Tension Measurements, p. 267, Copyright 1988, with permission from Elsevier Science. Curves were calculated from Eqs. (16), (21), and (23) using the following parameters r,i = r = 1, m = 1, fej = 2.02 V , = -0.57 V...

In very dilute base or, preferably, in the presence of weak bases, the homoannular enolate (16) is formed which can be adsorbed in either a cis (17) or a trans (18) manner. In this case the presence of a methyl group results in a slight favoring of trans adsorption, thus leading to the formation of a slight excess of the m-product as is observed on hydrogeneration of A" -3-keto steroids in the presence of triethylamine. ... 

Hydrophilic interaction chromatography on Asahipak NH2P or Excel-pak CHA-P44 with pulsed amperometric detection has been used to fractionate malto-oligosaccharides.266 The Asahipak NH2P is a polyvinyl alcohol support with a polyamine bonded phase, and the Excelpak is a sulfonated polystyrene in the Zn+2 form. Amine adsorption of sialic acid-containing oligosaccharides was performed on a Micropak AX-5 column (Varian) using acetonitrile-water-acetic acid-triethylamine.267... 

Selective removal of the iodine from fluorinated compounds was performed by 5% Pd/C catalyzed hydrogenolysis in the presence of triethylamine or sodium acetate.467 Ra-Ni and 1% NaOH were used for the cleavage of the C-I bond.468 The adsorption of chloroiodomethane was studied on a Pt(lll) surface. Dissociation began with C—I bond cleavage at about 150 K. Co-adsorbed deuterium atoms weaken the bonding between the starting compound and the surface and decrease the amount of dissociated molecules.469... 

Dichlorobenzidine and its salts are collected from air matriees using adsorption/filtration approaehes (Morales et al. 1981 NIOSH 1994) and recovered from the adsorbent using methanol eontaining a small amount of triethylamine (TEA). The addition of TEA eonverts any salt to the eorresponding amine, thus rendering it soluble in the organic solvent. Limits of deteetion in the low g/m (low to sub-ppb) range have been reported. The compoimd 4,4 -methylenebis(2-chloroaniline) was reported to interfere with 3,3 -dichlorobenzidine (Morales et al. 1981 NIOSH 1994). 

Triethylamine serves as a modifier to prevent both nonspecific adsorption and oxidation. 

Samples, even at moderate concentrations, injected into the HPLC column may precipitate in the mobile phase or at the column frit. In addition, the presence of other compounds (e.g., lipids) in the injection sample may drive the carotenoids out of solution or precipitate themselves in the mobile phase, trapping carotenoids. It is best to dissolve the sample in the mobile phase or a slightly weaker solvent to avoid these problems. Centrifugation or filtration of the samples prior to injection will prevent the introduction of particles that may block the frit, fouling the column and resulting in elevated column pressure. In addition to precipitation, other sources of on-column losses of carotenoids include nonspecific adsorption and oxidation. These can be minimized by incorporating modifiers into the mobile phase (Epler et al., 1993). Triethylamine or diisopropyl ethylamine at 0.1% (v/v) and ammonium acetate at 5 to 50 mM has been successful for this purpose. Since ammonium acetate is poorly soluble in acetonitrile, it should be dissolved in the alcoholic component of the mobile phase prior to mixing with other components. The ammonium acetate concentration in mobile phases composed primarily of acetonitrile must be mixed at lower concentration to avoid precipitation. In some cases, stainless steel frits have been reported to cause oxidative losses of carotenoids (Epler et al., 1992). When available, columns should be obtained with biocompatible frits such as titanium, Hastolloy C, or PEEK. 

A graph similar to Figure 7 resulted with adsorption of tri-ethylamine onto the same iron oxide, with AHa< s=-13.1 Kcal/mole and Tm corresponding to 167 A2. Using Drago s values of Cg and Eg for triethylamine.il.09 and 0.99 respectively jin units of (Kcal/mole)V2>we can write equation (5) as ... 

There are shortcomings in this work, however, and we expect to solve these soon. Adsorption is a slower process than most of us realize (25), and at 25°C the adsorption of pyridine onto iron oxide takes about three days to reach equilibrium. The results of Figure 7 with pyridine and those with triethylamine were obtained in about one hour. However Fm was the same for the two temperatures, for the slopes are exactly equal for the two lines. We are now using a flow microcalorimeter to measure the evolution of heat upon adsorption and we are adding a UV sensor to detect concentration changes this combination should give accurate heats of adsorption and desorption. We will then be able to compare these direct measurements of heats of adsorption with those obtained from the temperature coefficients of adsorption isotherms. 

Additional evidence of the importance of adsorption of basic molecules on nonacidic sites at room temperature has been given by Derkaui and coworkers. These researchers studied the adsorption of triethylamine on silica between 294 and 486 K (71,94) and the adsorption of triethylamine, benzene, cyclohexane, and isooctane on graphitized thermal carbon black between 293 and 383 K (95). 

In most recent calorimetric studies of the acid-base properties of metal oxides or mixed metal oxides, ammonia and n-butylamine have been used as the basic molecule to characterize the surface acidity, with a few studies using pyridine, triethylamine, or another basic molecule as the probe molecule. In some studies, an acidic probe molecule like CO2 or hexafluoroisopropanol have been used to characterize the surface basicity of metal oxides. A summary of these results on different metal oxides will be presented throughout this article. Heats of adsorption of the basic gases have been frequently measured near room temperature (e.g., 35, 73-75, 77, 78,81,139-145). As demonstrated in Section 111, A the measurement of heats of adsorption of these bases at room temperature might not give accurate quantitative results owing to nonspecific adsorption. 

Tables XIII I76-I79), XIV (I80-I83), and XV present a survey of micro-calorimetric studies performed for silica, alumina, and silica-alumina, respectively. Silica displays relatively low heats of adsorption for both basic probe molecules (e.g., ammonia, triethylamine, n-butylamine, pyridine, and trimethylamine) and acidic probe molecules (e.g., hexafluoroisopropanol), indicating that the surface sites on silica are both weakly acidic and basic. Most of the adsorption over silica is considered mainly to be due to hydrogen bonding and van der Waals interaction. Infrared and gravimetric adsorption measurements of pyridine adsorbed on SiO at 423 K have shown that more than 98% of the pyridine adsorbed was hydrogen bonded (62). The differential heats of ammonia 18, 74, 85, 105, 140, 147) and triethylamine (18, 71, 94. 105, 176) on silica show a considerable decrease as the adsorption temperature is increased.

When these bases are compared in terms of their respective proton affinities, the order of basic strength is ammonia < n-butylamine < pyridine < trimethylamine < piperidine < triethylamine, which is the same order observed with microcalorimetric measurements. In fact, plots of the initial differential heat of adsorption of ammonia, pyridine, trimethylamine, and triethylamine on silica-alumina and on silica as a function of the proton affinity give linear correlations, as can be seen in Fig. 7 (18, 105). 

The adsorption of ammonia, pyridine, trimethylamine, and triethylamine on silica and silica-alumina was studied microcalorimetrically by Cardona-Martinez and Dumesic (18, 105). The calorimetric results of this study were correlated successfully in terms of Drago parameters for each catalyst. These parameters describe well the acidic properties of silica and the strongest sites (Lewis acid sites) on silica-alumina and may allow the prediction of heats of adsorption for a wide range of basic molecules with known Drago parameters on these sites. Parameters to describe the strength of the Brpnsted sites could not be determined because the contribution from these sites could not be studied independently. 

Previous attempts to estimate Drago parameters for solid surfaces met with limited success. Fowkes and co-workers (198-201) calculated Q and Ex values for SiOj, TiOj, and Fe Oj using a combination of UV and IR spectroscopies and a flow calorimeter. They determined heats of adsorption of pyridine, triethylamine, ethyl acetate, acetone, and polymethylmethacrylate (PMMA) in neutral hydrocarbon solutions. However, their results did not provide consistent Q/Ea parameters for the surface acid sites. It should be noted that the heats determined were for high surface coverages, and these values provide a lower bound for the actual acid strength distribution. 

Figure 3.24 Experimental adsorption isotherms of nucleotides on ODS silica, (a) Nucleotides on ODS. 1, Guanosine 2 3 -cyclic monophosphate 2, adenosine 2 3 -cyclic monophosphate 3, adenosine monophosphate. Spherisorb ODS-2 100 mM phosphate buffer pH 7, 25° C. (b) 1, a-MSH 2, benzyl-dimethyl-dodecyl ammoniiun bromide (BDDAB) on Spherisorb ODS-2,25°C, eluted with 15% v/v aqueous methanol (—) or with 21% aqueous acetonitrile, 0.2% formic acid and 0.4% triethylamine, pH 7 (—). Reproduced mth permission from from J.-X. Huang and Cs. Horvath, ]. Chromatogr., 406 (1987) 275, (a) Fig. 6, (b) Fig. 7.

Remedy, change the mobile and/or stationary phase, change the pH or use a different method (e.g. ion-pair chromatography). A change in pH may also be required in adsorption chromatography addition of acetic or formic acid if a sample is acidic, or pyridine, triethylamine or ammonia if it is basic. 

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