Measurement of isotherms

This experiment is typical of many that are conducted using classical adsorption techniques, such as the measurement of isotherms and control of evacuation temperature and pressure. There is a high probability that such a treatment will increase the number of OH groups on the surface, but there is no direct evidence of this. An improvement could be made if the sample were continuously weighed during the cycle in a vacuum microbalance (160), but even this would not be entirely free from ambiguity. 

Many new adsorbents have been developed over the past 20 years including carbon molecular sieves, new zeolites and aluminophosphates, pillared clays and model mesoporous solids. In addition, various spectroscopic, microscopic and scattering techniques can now be employed for studying the state of the adsorbate and microstructure of the adsorbent. Major advances have been made in the experimental measurement of isotherms and heats of adsorption and in the computer simulation of physisorption. 

Measurements of isotherms are commonly obtained in the laboratoiy by performing batch experiments. As has been pointed out recently by Biirgisser et al. (1993), another alternative to the classical batch experiment that allows simple and rapid measurements of an entire possibly nonlinear adsorption isotherm relies on column experiments and the use of techniques borrowed from nonlinear chromatography (Burgisser et al., 1993 Schweich et al., 1983). Experimentally, the column experiment (column is packed with the material in question) is carried out near the relevant solid/solution ratio, and the percolating solution may easily be adjusted to the composition of interest (e.g., composition of groundwater). 

In many cases the measurement time of gas adsorption after a pressure change of the sorptive can be reduced substantially by measuring only two points in the initial part of the kinetic curve and extrapolation of the equilibrium value. The method is useful in the case of very slow adsorption processes and for the stepwise measurement of isotherms. 

Single-component isotherm parameters cannot always predict elution profiles with satisfied accuracy [122, 123], Therefore, to be able to predict accurate overloaded multi-component elution profiles where competition occurs competitive adsorption isotherm parameters are often necessary. Measurement of isotherms from a mixture is also often necessary because the pure enantiomers are not always accessible in large quantities. However, there exist only a small number of reports on the determination of multi-component adsorption isotherm parameters. FA can be used to determine binary isotherm data but it is time-consuming. The PP method is an alternative method to determine isotherm parameters from binary mixtures. It has been reported that the PP method works well up to weakly non-linear conditions [118, 119],... 

In most cases, chromatography is performed with a simple initial condition, C(f = 0,z) = q t = 0,z) = 0. TTie column is empty of solute and the stationary and mobile phases are under equilibrium. There are some cases, however, in which pulses of solute are injected on top of a concentration plateau (see Chapter 3, Section 3.5.4). The behavior of positive concentration pulses injected xmder such conditions is similar to that of the same pulses injected in a column empty of solute and they exhibit similar profiles. Even imder nonlinear conditions (high plateau concentration), a pulse that is sufficiently small can exhibit a quasi-linear behavior and give a Gaussian elution profile. Its retention time is linearly related to the slope of the isotherm at the plateau concentration. Measuring this slope is the purpose of the pulse method of measurement of isotherm data. Large pulses may also be injected and they will give overloaded elution profiles similar to those obtained with a column empty of solute. 

Elution by characteristic points (ECP) A method for the measurement of isotherms using the relationship derived in the ideal model between the mobile phase concentration and its retention time on the diffuse boimdary of a band. The method is accurate only if the column efficiency exceeds several thousand theoretical plates (preferably 5000). 

If Langmuir adsorption occurs, then a plot of 0 versus p for a partieular isotherm will display the form of equation (A 1.7.3). Measurements of isotherms are routinely employed in this manner in order to determine adsorption kinetics. 

A quantitative measure of isothermal crystallization is the crystallization half-time, Crystallization times expressed as the reciprocal of (crystallization rate) are shown in Figure 9.4 as a function of crystallization temperature. These results indicate that the fastest crystallization for the PDS homopolymer can be obtained at approximately 47°C. For the sake of comparison, Figure 9.4 also includes spherulitic growth rates from HSOM. 

Measurements of isotherms as a source of the heat of adsorption are rare for single crystal substrates. Special techniques of experimentation are required to record isotherms under vacuum conditions. One such measurement is known for CO on Ni(llO) [70Kli] which shall illustrate the case. The coverage of... 

The difficulty with all the analyses cited lies in the fact that there are at present no a priori methods for determining for a given gas-particle system the phenomenological coefficients C.., Cj, or the various accommodation coefficients which arise for the thermal-force problem. Of course, C, and can be obtained from independent experimental measurements of isothermal drag force, heat transfer, and thermal transpiration, respectively [2.103], However,... 

Monolayers of hydrophobic polymers, tethered to the water surface are suitable model systems for the investigation of polymer melt brushes . Especially they offer a convenient way to vary the area per tethered head group and to give the brush a well defined orientation and make it accessible to other experimental techniques. Simple measurements of isotherms already yield valuable information on the thermodynamics of these brushes. 

Maiipuii, V. O. Ratcliff, G. A. Measurement of isothermal vapor-liquid equilibria for acetone - n-heptane mixtures using modified GiUespie still J. them. Eng. Data 1972,17, 366-369... 

Miyamoto, S. Nakamura, S. Iwai, Y. Arai, Y. Measurement of isothermal vapor-liquid equilibria for hydrocarbon + monocarboxylic acid binary systems by a flow-type apparatus J. Chem. Eng. Jpn. 2000,45. 857-861... 

A new calorimeter for simultaneous measurements of isotherms and heats of adsorption, Thermochimica Acta, 405 (2003), p. 31-41. 

The physical significance of each of these parameters was discussed by Pitzer (1986, 1989, 1990, 1998) and Tanger and Pitzer (1989). Hovey et al. (1990) have correlated the measurements of isothermal vapor-hquid compositions for H2O + KCl and H2O + NaCl using the equation of state (2.47). An equation of state (2.47), which was originally proposed by Pitzer (1986) was improved and used by Tanger and Pitzer (1989) to describe the pTx properties of NaCl(aq) and KCl(aq) solutions. The Pitzer-Tanger-Hovey (PTH) EOS (Hovey et al, 1990) for H2O + NaCl solutions is based on an expansion of pressure aroimd the critical point of pure water (see Equation (2.47)). Only a few simple terms of the expansion were used for the H2O + NaCl solution, and the equation of Haar et al. (1984) was used to calculate the properties of pure water. The theoretical basis for the PTH EOS Hovey et al. (1990) is given in the work by Pitzer (1986) and Pitzer et al. (1987). Hovey et al (1990) pre-... 

Another method of measurement of isotherm is the dilution method, which is performed by preparing several bottles containing raw water diluted to different levels and adding the same amount of adsorbent in the bottles. Then the amount adsorbed in each flask is determined by comparing the initial concentrations and the final concentrations. By this method the... 

Nitrogen adsorption isotherms were measured at 77K, using an automated instrument (Omnisorp lOOcx, Beckman Coulter Electronics Ltd). Adsorbent samples (approximately O.lg) were out-gassed at 200"C to a residual pressure of 10" mbar for at least 6 hours prior to the measurement of isotherms. Specific surface areas were derived from the isotherms using the BET equation [2]. Total pore volumes were obtained using the Gurvitsch Rule and the micropore volumes were determined using the Og method [2]. Mesopore volumes were subsequently determined by subtraction. 

Chemiluminescence has also been proposed as a novel tool in paper conservation studies [654,655]. CL phenomena can be used for assessing the thermal and oxidative degradation pathways of paper-based historical documents. In contrast with the usual accelerated degradation experiments in climatic chambers, measurement of isothermal CL is quick. The influence of all paper components (alkalinity, metal content, cellulose peroxides and carbonyl groups, moisture) and exposure to light will be investigated in the framework of the PAPYLUM project (ending October 2004). 

Special features The HTPl-V is most commonly used when the requirement is for both ideal and non-ideal gases and is therefore complanentary with the Hiden IGA-001. A typical application is the measurement of isothermal uptake of hydrogen in storage media combined with in situ surface area and porosity analysis... 

Figure 1 General scheme of station for the simultaneous measurement of isotherm and heat of adsorption.

Description of the Unit for Simultaneous Measurement of Isotherm and Adsorption Heat... 

A modern adsorption microcalorimeter was built for the simultaneous measurement of isotherms and adsorption heats, establishing its correct functioning through adequate calibration of both the calorimeter part and the volumetric equipment of the adsorption part. For this purpose, the microcalorimeter calibration constant was found with values that go from 134.11 0.19 WV" to 156.67 0.23 WV. The adsorption isotherm was determined for a type NAZSM-5 zeolite as a reference solid to establish the correct functioning of the equipment. Micropore volume and superficial area were determined to be 0.20 cmVg and 296 m /g, respectively. These results agree very well with those obtained with commercial equipment. Finally, the differential heats of adsorption, for the same solid, were measured. The analysis of results gives valuable information about the studied C02/NaZSM-5 system, which is in concordance with other studies in the literature. 

Cleaver B, Zani P (1978) Adiabatic thermal pressure coefficients of molten salts an indirect method for the measurement of isothermal compressibility. High Temp High Press 10 437... 

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