Isotherm adsorption-desorption-method

The competitive adsorption isotherms were determined experimentally for the separation of chiral epoxide enantiomers at 25 °C by the adsorption-desorption method [37]. A mass balance allows the knowledge of the concentration of each component retained in the particle, q, in equilibrium with the feed concentration, < In fact includes both the adsorbed phase concentration and the concentration in the fluid inside pores. This overall retained concentration is used to be consistent with the models presented for the SMB simulations based on homogeneous particles. The bed porosity was taken as = 0.4 since the total porosity was measured as Ej = 0.67 and the particle porosity of microcrystalline cellulose triacetate is p = 0.45 [38]. This procedure provides one point of the adsorption isotherm for each component (Cp q. The determination of the complete isotherm will require a set of experiments using different feed concentrations. To support the measured isotherms, a dynamic method of frontal chromatography is implemented based on the analysis of the response curves to a step change in feed concentration (adsorption) followed by the desorption of the column with pure eluent. It is well known that often the selectivity factor decreases with the increase of the concentration of chiral species and therefore the linear -i- Langmuir competitive isotherm was used ... 

Nitrogen adsorption/desorption. One of the most common techniques used for analyzing size distributions of mesopores is the nitrogen adsorption/desorption method. The method is capable of describing pore diameters in the 1.5 nm to 100 nm (or 0.1 micron) range. Pore size distribution can be deteimined from either the adsorption or the desorption isotherm based on the Kelvin equation ... 

The adsorption-desorption method is a popular and commonly used method for characterization of surface and structural properties of porous materials, allowing the determination of their surface area, pore-size distribution, pore volume and adsorption energy distribution. Nitrogen is often used for the adsorbent gas but other adsorbent gases such as argon can also be used. According to this method, adsorption-isotherm (amount of adsorbed gas versus relative pressure [pressure/saturation vapor pressure of the adsorbent]) is drawn and the data are analyzed by assuming capillary condensation. 

The gas adsorption-desorption method involves the analysis of adsorption-desorption isotherms of an inert gas as a function of partial pressure (Table 15.3f) [175]. 

Specific surface area (SSA), total pore volume and average pore diameter were measured by N2 adsorption-desorption isotherms at 77K using Micromeritics ASAP 2020. The pore size was calculated on the adsorption branch of the isotherms using Barrett-Joyner-Helenda (BJH) method and the SSA was calculated using the Brunauer-Emmett-Teller (BET) method. 

The adsorption of fully and partially hydrolyzed (88%) polyvinyl alcohol (PVA) on 190-1lOOnm monodisperse polystyrene latex particles was investigated. The effect of molecular weight was investigated for 190 nm-size particles using the serum replacement adsorption and desorption methods. The adsorption density at the adsorption-isotherm plateau followed the relationships for the fully hydrolyzed... 

The determination of adsorption isotherms at liquid-solid interfaces involves a mass balance on the amount of polymer added to the dispersion, which requires the separation of the liquid phase from the particle phase. Centrifugation is often used for this separation, under the assumption that the adsorption-desorption equilibrium does not change during this process. Serum replacement (6) allows the separation of the liquid phase without assumptions as to the configuration of the adsorbed polymer molecules. This method has been used to determine the adsorption isotherms of anionic and nonionic emulsifiers on various types of latex particles (7,8). This paper describes the adsorption of fully and partially hydrolyzed PVA on different-size PS latex particles. PS latex was chosen over polyvinyl acetate (PVAc) latex because of its well-characterized surface PVAc latexes will be studied later. 

Adsorption Isotherms. The adsorption isotherms were determined using the serum-replacement adsorption or desorption methods (7). For the adsorption method, the latex samples (50 or 100 cm 2% solids) containing varying amounts of PVA were equilibrated for 36 hours at 25°C, placed in the serum replacement cell equipped with a Nuclepore membrane of the appropriate pore size, and pressurized to separate a small sample of the serum from the latex. For the desorption method, the latex samples (250 cm 2.5% solids) were equilibrated for 36 hours at 25°C and subjected to serum replacement with DDI water at a constant 9-10 cm /hour. The exit stream was monitored using a differential refractometer. The mean residence time of the feed stream was ca. 25 hours. It was assumed that equilibrium between the adsorbed and solute PVA was maintained throughout the serum replacement. For both methods, the PVA concentration was determined using a An-C calibration curve. 

Figure 1. Adsorption isotherms of fully hydrolyzed PVA samples on 190nm polystyrene particles (o) Vinol 107 (A) Vinol 325 (D) Vinol 350 open points by adsorption method and shaded points by desorption method.

Bolt GH, De Boodt MF, Hayes ME, McBride MB (eds) (1991) Interactions at the soil coUoid-solution interface. NATO ASI Series—Applied Science—Series E vol 190, Kluwer, Dordrecht Bowman BT (1979) Method of repeated additions for generating pesticide adsorption-desorption isotherm data. Can J Soil Sci 59 435-437... 

Each type of pore is associated with a characteristic type of adsorption isotherm. The appropriate method of characterizing the porosity of an iron oxide is, therefore, to obtain the complete adsorption/desorption isotherm. There are six standard adsorption isotherms for gases (Fig. 5.3). Type I, with enhanced adsorption at low relative... 

To construct the adsorption isotherm, the adsorption, desorption, and calibration cycle shown in Fig. 15.11 is repeated for each data point required. Errors are not cumulative since each point is independently determined. Relative pressures corresponding to each data point are established by measuring the saturated vapor pressure using any of the preceding methods or by adding 15 torrs to ambient pressure. Thus, if X is the mole fraction of adsorbate in the flow stream, the relative pressure is given by... 

As indicated by XRD patterns, there exist just 2-3 broad peaks in the calcined acid-made materials (Fig. 3A). Moreover, the N2 adsorption/desorption isotherm shown in Fig. 3B, the calcined acid-made mesoporous silica indeed possesses a broad capillary condensation at the partial pressure p/p0 of ca. 0.2-0.4, indicating a broad pore size distribution with a FWHM ca. 1.0 nm calculated from the BJH method. This is attributed to the occurrence of partial collapse of the mesostructure during the high temperature calcination. The hexagonal structure completely collapsed when subjected to further hydrothermal treatment in water at 100 °C for 3 h. Mesoporous silica materials synthesized from the acid route are commonly believed to be less stable than those from the alkaline route [6,7]. 

Nitrogen adsorption - desorption isotherms were obtained from a volumetric adsorption analyzer ASAP 2010 manufactured by Micromeritics. The samples were first degassed for several hours at 350°C. The measurements were then carried out at -196°C over a wide relative pressure range from 0.01 to 0.995. The average pore diameter and the pore size distribution were determined by the B JH method from the adsorption branch of isotherm [18],... 

The nitrogen adsorption-desorption isotherms were obtained at 77K by AutoSorb-1 -C (Quantachrome). Prior to measurement, the samples were outgassed at 300°C for 3 h. The specific surface areas of the samples were determined from the linear portion of the BET plots. Pore size distribution was calculated from the desorption branch of N2 desorption isotherm using the conventional Barrett-Joyner-Halenda (BJH) method, as suggested by Tanev and Vlaev [15], because the desorption branch can provide more information about the degree of blocking than the adsorption branch. 

Specific surface areas and micropore volumes were obtained from nitrogen adsorption - desorption isotherms at -196°C using Micromeritics ASAP 2010. Prior to the measurements all powdered samples were degassed at 175 °C under vacuum 10 6 Torr for 6 hours. The total surface area was calculated using BET equation. The method of Horvath and Kawazoe was used to determine the pore size diameters of the product. 

Powdered, particulate MCM-41 molecular sieves (Si/Al = 37) with varied pore diameters (1.80, 2.18, 2.54 and 3.04 nm) were synthesized following the conventional procedure using sodium silicate, sodium aluminate and C TMAB (n = 12, 14, 16 and 18) as the source materials for Si, A1 and quaternary ammonium surfactants, respectively [13]. Each sample was subjected to calcination in air at 560 °C for 6 h to remove the organic templates. The structure of the synthesized material was confirmed by powder X-ray diffraction (XRD) and by scanning/transmission electron microscopy. Their average pore sizes were deduced from the adsorption curve of the N2 adsorption-desorption isotherm obtained at 77 K by means of the BJH method (Table 1). 

Moreover, it has been remarked that it is necessary to use more than one adsorbate for a correct characterization of the narrow porosity. Thus, in the case of CMSs and other carbon materials (i.e., highly activated carbons) with narrow micropores, N2 at 77 K is not a suitable adsorbate due to diffusion problems. Other adsorptives and conditions, like C02 at 273 or 298 K, avoid such problems. From all of these, it can be concluded that for a suitable characterization of the porosity of carbon materials by physical adsorption, the use of more than one adsorbate and the application of several theories and methods to the adsorption-desorption isotherms are recommended. 

Catalyst Characterization. Carbon contents were determined by the Carlo Erba method and sulfur content by high temperature combustion In O2 (ASTM-D1552-64). Surface area and pore volume distribution were measured via N2 adsorption desorption Isotherms (4). ESR measurements were carried out with a modified Varian Radical Assay Spectrometer at both 77 K and room temperature (3). 

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