Probe molecules of different sizes

In Sections 2.2 and 2.3, it is described that the network structures of polymer gels, especially microscopic inhomogeneity, may be detected through the diffusion behaviour of probe molecules of different sizes, which can be obtained by using time-dependent diffusion NMR spectroscopy. In this section, it will be... 

In the original paper one of the major advantages put forward in favour of the Os method over the contenq)ary t method was that it allows a similar type of analysis of adsorption isotherms of other adsorptives, besides nitrogen, to be made. This is of particular importance in the case of activated carbons where it is customary to make use of a range of probe molecules of different size, shape, polarizability and polarity in order to carry out a more complete characterization. A number of authors have since demonstrated the general feasability of doing this and reference data for the adsorption of neopentane and butane, for... 

A method of using GCMC simulation in conjunction with percolation theory [74,75] has been suggested for simultaneous determination of the PSD and network connectivity of a porous solid [76]. In this method, isotherms are measured for a battery of adsorbate probe molecules of different sizes, e.g., CH4, CF4, and SFg. As illustrated in Fig. 9a, the smaller probe molecules are able to access regions of the pore volume that exclude the larger adsorbates. Consequently, each adsorbate samples a different portion of the adsorbent PSD, as shown in Fig. 9b. By combining the PSD results for the individual probe gases with a percolation model, an estimate of the mean connectivity number of the network can be obtained [76]. 

A striking example of molecular sieving in a stable, continuous b-oriented silicalile-1 film (having pores of about 0.55 nm) on an electrode was recently demonstrated with redox probe molecules of different sizes (Fig. 13).[100] Specifically, the smaller complex Ru(NII3)63+ with a diameter of ca. 0.5 nm was shown to travel through the film, while the larger complex Co(phen)32+ with a diameter of ca. 1.3 nm was completely excluded from the zeolite film and thus from redox processes. 

An additional difficulty in the determination of actual TOF values for zeolite catalysed reactions deals with the accessibility by reactant molecules to the narrow micropores in which most of the potential active sites are located. The didactic presentation in Khabtou et al.[37] of the characterization of the protonic sites of FAU zeolites by pyridine adsorption followed by IR spectroscopy shows that the concentration of protonic sites located in the hexagonal prisms (not accessible to organic molecules) and in the supercages (accessible) can be estimated by this method. Base probe molecules with different sizes can also be used for estimating the concentrations of protonic sites located within the different types of micropores, which are presented by many zeolites (e.g. large channels and side pockets of mordenite1381). The concentration of acid sites located on the external surface of the... 

Many techniques have been developed to study coking, both chemical (adsorption of probe molecules having different sizes (2), solvent extraction (3)) and physical (X-ray diffraction (4), electron microscopy (5), IR (6), NMR (7)). Among these techniques 129-Xe NMR applied to microporous systems such as zeolites (8), coupled with HRADS techniques makes it possible to clarify the location of the coke and the blocking of the zeolite micropores. 

Over the past 30 years many organic molecules of different size, shape and polarity have been used as molecular probes. A high proportion of the experimental isotherms on porous carbons have been analysed by application of the Dubinin-Radushkevich (DR) equation or, in a few cases, by the Dubinin-Astakhov (DA) equation. So far, the more sophisticated Dubinin-Stoeckli (DS) treatment (Stoeckli, 1993) has been applied by very few other investigators. 

The roughness of the silica surface could introduce the steric hindrance of the surface accessibility similar to the effect of the micropores. In the discussion above, we assume the ideal tubular geometry of the silica surface. The use of different probe molecules for the BET measurement of silica surface area (such as N2, Ar, Kr, benzene, etc.) leads to significant difference in the surface area values for the same silica sample. It was suggested that silica surfaces possess the property of fractals [16] this essentially means that molecules of different size will see a different surface area. 

Mokiailnr probe method. When the membrane pores reach the molecular dimensions, the molecular sieving effect becomes operative and the membrane can discriminate gas molecules with a diameter difference as low as 0.02 nm. Some methods utilizing molecules of different sizes as molecular probes have been used to estimate the pore size of a membrane. In these cases, the membranes can be characterized by their permeabilities or accessible micropore volumes to different gases. The pore size can be inferred from the permeation rates or micropore volumes of different gases whose molecular dimensions are known or can be calculated. 

Surface fractals have a fractal boundary but a compact, nonfractal bulk. Common to all methods of fractal analysis is that yardsticks of different size probe the surface. Depending on the method, the probes may be molecules of different size. 

The molecular sieve behavior of a number of microporous activated carbons was studied using molecules of different sizes and shapes as molecular probes by... 

An experiment of adsorption from the gas-phase, performed in microcalorimeter coupled with volumetric line can give a profile of Qdi/ versus the amount adsorbed, integral heats of adsorption, adsorption isotherms (adsorbed amounts vs. equilibrium pressure) and irreversibly absorbed amount of a chemisorbed gas the same stands for the adsorption from the liquid-phase, where the adsorbate (titrant) is added to both sample and reference ceUs simultaneously. The profile of differential heats versus the uptake of probe gives the data concCTning the amount, strength and distribution of the active sites. Besides, the values of initial heats of adsorption characterize the strongest sites active in adsorption process. For the sake of acidic/basic characterization of solids surface, the most commonly used gas-phase probes are ammonia, pyridine or some amines for the interaction with acidic sites. SO2 and CO2 are the probes used to notice and characterize the basic sites. In microporous solids, the accessibility of active sites is not the same for the molecules of different sizes. Therefore, many different probes can be applied to study acidity or basicity of same solid materials this approach brings additional information. For example, acidity of zeolites can be characterized by adsorption of ammonia, but also by adsorption of pyridine (from the gas phase) and aniline (from the liquid phase) [20-22], Liquid microcalorimetry can be also used for the determination of acidic character of solid adsorbent the common liquid-phase probe is aniline dissolved in n-decane [40]. 

The volume of the solid phase Vp is usually measured by a pycnometric technique, which measures the excluded volume of a pycnometric fluid, whose molecules cannot penetrate the solid phase of PS. A simple example of a pycnometric fluid is helium [55], The pycnometric fluid fill in all void space (pores) accessible to it, and presumably do not adsorb on the surface of PS. In the case of microporous PSs, measurement of the volume accessible for guests with various sizes allows the determination of a distribution of micropores volume vs. the characteristic size of guest molecules. This approach lays the basis of the method of molecular probes. The essence of this method is in the following we have a series of probe molecules with different mean sizes (dl>d2>d3>---). The pycnometric measurements of the excluded volume will give a series The difference A V=Vpi-Vpi(i>j) corresponds to the volume of micropores with pycnometric sizes of d in a range dt[Pg.283]

In previous work [15,17,28] we have evaluated the relation between the flow ratio co, the pore size, and the ionic strength of the solution experimentally, by means of size-exclusion electrochromatography (SEEC). In SEEC the transport rates of the (neutral) macromolecules depend direcdy on co. As in conventional, pressure-driven SEC, the separation in SEEC is based on the differential accessibility of the (stagnant) mobile phase in the pores of the particles for macromolecules of different sizes. However, with increasing pore flow ratio in SEEC, the velocity difference between the mobile-phase fractions inside and outside the particles decreases. The retention ratio x (the retention time relative to a low-molecular-mass marker) for a probe molecule in SEEC is given by... 

The structural heterogeneity of activated carbon is a result of the existence of micropores, mesopores, and macropores of different sizes and shapes, randomly coimected in a pore network. In a pore network, some of the pores, that are large enough to accommodate the probe molecules, may be accessible only through smaller pores that only permit the passage of molecules having sufBciently small size. Therefore only a part of the available pores is... 

Effective density. The diameter and volume of the micropores were determined by the measurement of the density using as displacement molecules with different sizes of effective diameter, e g., helium (0.25 nm), water (0.264 nm), benzene (0.370 x 0.528 nm), and decaline (0.472 X 1.01 X 0.624 nm). All pycnometric fluids are non-polar, except water. This adsorbate was used for the sake of the little diameter of its molecule. In the case of CMSs studied - not including of oxygen surface groups [8] - water molecule is good molecular probe. 

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