Solvent adsorption equilibrium

The separation of bi-naphthol enantiomers can be performed using a Pirkle-type stationary phase, the 3,5-dinitrobenzoyl phenylglycine covalently bonded to silica gel. Eight columns (105 mm length) were packed with particle diameter of 25 0 fiva. The solvent is a 72 28 (v/v) heptane isopropanol mixture. The feed concentration is 2.9 g for each enantiomer. The adsorption equilibrium isotherms were determined by the Separex group and already reported in Equation (28) [33]. 

The hydroxyl-terminated polymer was then added to the dispersion and the mixture allowed to stand for 24 hr to achieve adsorption equilibrium. Further n-heptane was then added to give a 4 1 (by volume) n-heptane ethanol solvent composition, and the sample... 

It was shown earlier that the choice of a standard state can be based on the analysis of the adsorption equilibrium. Assuming that adsorption is a substitution process of a solvent molecule by the solute, that the adsorbate and solvent have the same size (n = 1), and that the adsorption layer can be treated as a separate phase, the equilibrium constant can be... 

The relevance of LSC data to reverse osmosis stems from the physicochemical basis (adsorption equilibrium considerations) of liquid-solid chromatography (52), and the principle that the solute-solvent-membrane material (column material) Interactions governing the relative retention times of solutes in LSC are analogous to the interactions prevailing at the membrane-solution Interface under reverse osmosis conditions. The work already reported in several papers on the subject (53-58) indicate that the foregoing principle is valid, and hence LSC data offer an appropriate means of characterizing interfacial properties of membrane materials, and understanding solute separations in reverse osmosis. 

In a recent paper (70 Hartman studies the effect of surface relaxation on the habit of corundum and hematite. The habits observed on natural and synthetic crystals of these systems did not agree with calculated relaxed equilibrium or growth habits. Hartman concluded that these observations could be understood by invoking specific solvent adsorption on (111) faces. 

Polystyrene-supported copper(I) chloride exhibits rapid and reversible adsorption of carbon monoxide 109). For the adsorbent prepared by using toluene as solvent, the equilibrium molar ratio of adsorbed CO to the charged copper(I) chloride is 0.83 at 20 °C under 1 atm. 

A cyclic adsorption process for citrus oil processing in supercritical carbon dioxide (SC-C02) was studied with silica gel adsorbent. Based on the adsorption equilibrium properties, where adsorbed amounts decreased with the increase in the solvent density and oxygenated compounds were selectively adsorbed on silica gel, a continuous cyclic operation between the adsorption step at 8.8 MPa and 313 K, and the desorption step at 19.4 MPa and 313 K was demonstrated Highly concentrated fraction of oxygenated com pounds was continuously obtained for the desorption and blowdown step. The proposed system showed the feasibility of the continuous operation for citrus oil processing. 

Adsorption equilibria is important fundamental property to design and develop the adsorption process. We have measured the adsorption equilibrium constant of limonene and linalool in SC-C02 by an impulse response technique [9]. Figure 1 shows the adsorption equilibrium constant at a temperature of 313 K - 333 K and a pressure of 11.8 MPa - 23.5 MPa. Linalool was adsorbed more selectively than limonene. Adsorption equilibrium constants were correlated linearly in log-log plot as a function of the density of SC-C02 independent of pressure and temperature. Adsorbed amounts decreased with the increase in the solvent density for both limonene and linalool. These results suggest the possibility of a process where oxygenated compounds are selectively adsorbed on the adsorbent at a lower pressure and then desorbed at a higher pressure. 

Figure 1 Relation between adsorption equilibrium constant and solvent density.

The results of the adsorption experiments are presented in table 2. The equilibrium solubilities at the same conditions were determined and reported in a previous work [10]. The density values used were those of pure C02 [12] because nimesulide is only a trace component (max. solubility 0.0446 g/1 at 22 MPa). As it can be seen, the sorbate capacity of activated carbon decreases with increasing pressure and density of C02, regarding the values at similar concentrations as well as the total adsorption capacity at saturation concentrations. The higher solvent power of C02 at increasing pressure/density could be an explanation for the adsorption equilibrium shift in direction to lower coverage. 

An insight into the extrathermodynamic, modelistic contributions to / can be gained by regarding chemisorption as a displacement of a number v of adsorbed solvent molecules from direct contact with the electrode surface by the given adsorbing species S. In an aqueous solution the adsorption equilibrium can be written ... 

Utilized selective anisotropic solvent adsorption on specific crystal faces to favour the growth of morphologically polar crystals. Some additional reports of the study of crystal modification and nonlinear optical activity include those on anhydrous and hydrated sodium / -nitrophenolate (Brahadeeswaran et al. 1999), derivatives of 2-benzylideneindan-l,3-dione (Matsushima et al. 1992), straight-chain carbamyl compounds (Francis and Tiers 1992), benzophenone derivatives (Terao et al. 1990), a 1,3-dithiole derivative (Nakatsu et al. 1990), o -[(4 -methoxyphenyl)methylene]-4-nitro-benzene-acetonitrile (Oliver et al. 1990) and so-called lambda shaped molecules (Yamamoto et al. 1992). Hall et al. (1988) followed the thermal conversion of the centrosymmetric P2 /c) form of 2,3-dichloroquinazirin to the non-centrosymmetric Pc form by monitoring the development of an SHG signal. Consistent with the earlier observation, the centrosymmetric form was obtained under equilibrium conditions, while the non-centrosymmetric one could be obtained under more kinetic conditions. 

Sorption of VOCs involves the processes of adsorption and partitioning. Partitioning is the incorporation of the VOC into the natural organic matter associated with the solid and is analogous to the dissolution of an organic compound into an organic solvent. Adsorption is the formation of a chemical or physical bond between the VOC and the mineral surface of a solid particle (Rathbun, 1998). The equilibrium relation between aqueous and solid phase concentrations then is expressed as... 

Assumption 5 In the definition of the isotherm, the convention is adopted that the solvent (if pure) or the weak solvent (in a mixed mobile phase) is not adsorbed [8]. Riedo and Kov ts [9] have given a detailed discussion of this problem. They have shown that the retention in liquid-solid i.e., adsorption) chromatography can best be described in terms of the Gibbs excess free energy of adsorption. But it is impossible to define the surface concentration of an adsorbate without defining the interface between the adsorbed layer and the bulk solvent. This in turn requires a convention regarding the adsorption equilibrium [8,9]. The most convenient convention for liquid chromatography is to decide that the mobile phase (if pure) or the weak solvent (if the mobile phase is a mixture) is not adsorbed [8]. Then, the mass balance of the weak solvent disappears. If the additive is not adsorbed itself or is weakly adsorbed, its mass balance may be omitted [30]. 

In excess solution adsorption, the support material is submerged in excess amount of impregnation solution (the volume of impregnation solution is much higher than the pore volume of the support). The excess solution is filtered after adsorption equilibrium is reached. In many cases, competitive adsorption between solvent and solutes and/or between different solutes leads to a nonuniform distribution of active components throughout the support particles. This phenomenon can be utilized to enhance performance (normally selectivity) of certain types of catalysts. The distribution of the active components can also be tailored by the manipulation of the pore structure of the support, pH and viscosity of the solution. ... 

In particular, according to this adsorption mechanism, each adsorbate molecule A cuts from Sc a smaller cluster Sa of solvent molecules with equivalent to A dimensions and displaces it towards the bulk solution, where it is disintegrated into g monomeric solvent molecules. Thus, irrespective of the size of the original solvent clusters Sc the adsorbate molecules do not see either these clusters or the monomeric solvent molecules but only the clusters Sa, which have always dimensions equal to those of the adsorbate molecules. In this respect the adsorbed layer behaves as if it were composed of adsorbate A molecules and solvent clusters Sa with equal dimensions. For this reason, this adsorption mechanism is compatible with a value of n close to unity. Moreover, if it is analysed within the frames of classical thermodynamics, we obtain that the adsorption equilibrium may be described by the following equation [11] ... 

The most convenient method of study of adsorption at small coverage is gas chromatography. By this method it is possible to determine the constant of adsorption equilibrium (retention volume) and from the retention volumes at different temperatures to calculate the heat of adsorption and changing of differential standard entropy of adsorption. If the support for fullerene crystals is the adsorbent with inert and small specific surface area so the retention of compounds will be determined by intermolecular interaction of compounds with fullerene crystal surface. The deposition of fullerene crystals on support surface is quite difficult owing to small solubility of fullerene in organic solvents [21, 22] as well as small vapour pressure of fullerene [23]. 

In the measurements of the adsorption equilibrium and intracrystalline diffusion data, the injection sample loop was first filled with a sample solution (water as solvent) of a known sorbate concentration by a syringe. The sample was then injected into the column after a stable base line in the recorder had been obtained. For each adsorbate at a given temperature, about 4 to 6 samples of different adsorbate concentration (CG from about 0.015 to 0.06 g/ml) and at different carrier flow rate (Q from 0.5 to 2.0 ml/min) were injected to give the corresponding response peaks at the outlet of the column. The response peaks were recorded and then directly read from the recording chart and input to a DEC-20 computer for further analysis. Figure 2 shows some recorded response peaks from the silicalite LC column for ethanol, n-propanol and n-butanol. 

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