Pore structure size measurement

When characterizing pore size, a few points should be mentioned. First off, some separator materials of construction have an asymmetrical pore structure. For instance, fiber materials such as an AGM separator may have very different pore structures when measured in the various planes of orientation. For prevention of shorts, the pores should be characterized in the plane of the separator that is parallel to the surface of the electrodes. One method that is used for pore characterization is based on the first bubble method, where the void space of the separator is filled with a given liquid and then gas pressure is applied on the surface until a breakthrough occurs. Based on the pressure required and the intrusion liquid, the pore size can be quantified. 

A microscopic description characterizes the structure of the pores. The objective of a pore-structure analysis is to provide a description that relates to the macroscopic or bulk flow properties. The major bulk properties that need to be correlated with pore description or characterization are the four basic parameters porosity, permeability, tortuosity and connectivity. In studying different samples of the same medium, it becomes apparent that the number of pore sizes, shapes, orientations and interconnections are enormous. Due to this complexity, pore-structure description is most often a statistical distribution of apparent pore sizes. This distribution is apparent because to convert measurements to pore sizes one must resort to models that provide average or model pore sizes. A common approach to defining a characteristic pore size distribution is to model the porous medium as a bundle of straight cylindrical or rectangular capillaries (refer to Figure 2). The diameters of the model capillaries are defined on the basis of a convenient distribution function. 

Porosity and surface area are routinely measured by nitrogen absorption-desorption, mercury intrusion, and low-angle X ray. The electron microscope (EM) provides direct visual evidence of pore size and pore-size distribution. Thus, a combination of EM and conventional methods of pore-size measurement should provide reliable information on the pore structure of polymers. 

The physical characterisation of membrane structure is important if the correct membrane is to be selected for a given application. The pore structure of microfiltration membranes is relatively easy to characterise, SEM and AFM being the most convenient method and allowing three-dimensional structure of the membrane to be determined. Other techniques such as the bubble point, mercury intrusion or permeability methods use measurements of the permeability of membranes to fluids. Both the maximum pore size and the pore size distribution may be determined.1315 A parameter often quoted in manufacturer s literature is the nominal... 

The pore size of Cs2.2 and Cs2.1 cannot be determined by the N2 adsorption, so that their pore sizes were estimated from the adsorption of molecules having different molecular size. Table 3 compares the adsorption capacities of Csx for various molecules measured by a microbalance connected directly to an ultrahigh vacuum system [18]. As for the adsorption of benzene (kinetic diameter = 5.9 A [25]) and neopentane (kinetic diameter = 6.2 A [25]), the ratios of the adsorption capacity between Cs2.2 and Cs2.5 were similar to the ratio for N2 adsorption. Of interest are the results of 1,3,5-trimethylbenzene (kinetic diameter = 7.5 A [25]) and triisopropylbenzene (kinetic diameter = 8.5 A [25]). Both adsorbed significantly on Cs2.5, but httle on Cs2.2, indicating that the pore size of Cs2.2 is in the range of 6.2 -7.5 A and that of Cs2.5 is larger than 8.5 A in diameter. In the case of Cs2.1, both benzene and neopentane adsorbed only a little. Hence the pore size of Cs2.1 is less than 5.9 A. These results demonstrate that the pore structure can be controlled by the substitution for H+ by Cs+. 

One of the early questions raised on TUD-1 dealt with its pore structure did it have intersecting or nonintersecting pores At the University of Utrecht, one conclusive characterization was carried out with a silica TUD-1 with Pt inserted, which was analyzed by 3-D TEM (transmission electron microscopy) (9). The Pt anchors (not shown) were used as a focal point for maintaining the xyz orientation. As shown in Figure 41.2, the TUD-1 is clearly amorphous. While not quantitatively measured for this sample, the pores appear rather uniform, consistent with all porosimetry measurements on TUD-1 showing narrow pore size distributions. 

In order to verify the conditions of this averaging process, one has to relate the displacements during the encoding time - the interval A between two gradient pulses, set to typically 250 ms in these experiments - with the characteristic sizes of the system. Even in the bulk state with a diffusion coefficient D0, the root mean square (rms) displacement of n-heptane or, indeed, any liquid does not exceed several 10 5 m (given that = 2D0 A). This is much smaller than the smallest pellet diameter of 1.5 mm, so that intraparticle diffusion determines the measured diffusion coefficient (see Chapter 3.1). This intrapartide diffusion is hindered by the obstades of the pore structure and is thus reduced relative to D0 the ratio between the measured and the bulk diffusion coeffident is called the tortuosity x. More predsely, the tortuosity r is defined as the ratio of the mean-squared displacements in the bulk and inside the pore space over identical times ... 

The pore structure of a solid can contribute to the disintegration, dissolution, adsorption, and diffusion of a drug material [26,27]. Because of this, porosity and pore size distribution measurements have been used extensively to study tablets [28-30], granules [31,32], and excipients [33]. The following classification system of pore sizes has been developed based on the average pore radii [6] ... 

The development of microporosity during steam activation was examined by Burchell et al. [23] in their studies of CFCMS monoliths. A series of CFCMS cylinders, 2.5 cm in diameter and 7.5 cm in length, were machined from a 5- cm thick plate of CFCMS manufactured from P200 fibers. The axis of the cylinders was machined perpendicular to the molding direction ( to the fibers). The cylinders were activated to bum-offs ranging from 9 to 36 % and the BET surface area and micropore size and volume determined from the N2 adsorption isotherms measured at 77 K. Samples were taken from the top and bottom of each cylinder for pore structure characterization. 

In another article by Corma et al. (178), ITQ-7, a three-dimensional large-pore zeolite, was tested as an alkylation catalyst and compared with a BEA sample of comparable Si/Al ratio and crystal size. The ratio of the selectivities to 2,2,4-TMP and 2,2,3-TMP, which have the largest kinetic diameter of the TMPs, and 2,3,3-TMP and 2,3,4-TMP, which have the lowest kinetic diameter, was used as a measure of the influence of the pore structure. Lower (2,2,4-TMP + 2,2,3-TMP)/ (2,3,3-TMP + 2,3,4-TMP) ratios in ITQ-7 were attributed to its smaller pore diameter. The bulky isomers have more spacious transition states, so that their formation in narrow pores is hindered moreover, their diffusion is slower. The hydride transfer activity, estimated by the dimethylhexane/dimethylhexene ratio,... 

Another possible solution to the problem of analyzing multiple-layered membrane composites is a newly developed method using NMR spin-lattice relaxation measurements (Glaves 1989). In this method, which allows a wide range of pore sizes to be studied (from less than 1 nm to greater than 10 microns), the moisture content of the composite membrane is controlled so that the fine pores in the membrane film of a two-layered composite are saturated with water, but only a small quantity of adsorbed water is present in the large pores of the support. It has been found that the spin-lattice relaxation decay time of a fluid (such as water) in a pore is shorter than that for the same fluid in the bulk. From the relaxation data the pore volume distribution can be calculated. Thus, the NMR spin-lattice relaxation data of a properly prepared membrane composite sample can be used to derive the pore size distribution that conventional pore structure analysis techniques... 

Both Ksec <1 pore size distribution have been measured experimentcilly for hard-sphere column packing materials (9), but for soft gel packing materials there does not seem to be ciny reliable information presumably because the accepted method of pore structure characterisation in porous materials, mercury porosime-try, cannot be used. However, Ep Ccin be measured for gels without great difficulty from the column calibration curve (as is mcinife-st from Equation 12) provided the calibration is made on the basis of the peak mean position, i.e. the first moment of the peak... 

Determination of Pore Size Distributions. The shape and range of a GPC calibration curve are, in part, a reflection of the pore size distribution (PSD) of the column packing material. A consideration of the nature of PSDs for the ULTRASTYRAGEL columns to be used in this work is therefore appropriate. The classical techniques for the measurement of PSDs are mercury porisimetry and capillary condensation. The equipment required to perform these measurements is expensive to own and maintain and the experiments are tedious. In addition, it is not clear that these methods can be effectively applied to swellable gels such as the styrene-divinylbenzene copolymer used in ULTRASTYRAGEL columns. Both of the classical techniques are applied to dry solids, but a significant portion of the pore structure of the gel is collapsed in this state. For this reason, it would be desirable to find a way to determine the PSD from measurements taken on gels in the swollen state in which they are normally used, e.g. a conventional packed GPC column. 

Characterization. When silica gel is used as an adsorbent, the pore structure determines the gel adsorption capacity. Pores are characterized by specific surface area, specific pore volume (total volume of pores per gram of solid), average pore diameter, pore size distribution, and the degree to which entrance to larger pores is restricted by smaller pores. These parameters are derived from measuring vapor adsorption isotherms, mercury intrusion, low angle x-ray scattering, electron microscopy, gas permeability, ion or molecule exclusion, or the volume of imbibed liquid (1). 

The only proof of the above contention are the results in Figure 3 of the paper where reaction rate is approximately proportional to external area. Figure A here shows the pore size distribution (from mercury penetration (3)) of each particle size, confirming that little additional pore structure was opened up during coal grinding. This would rule out the possibility that the effect measured in Figure 3 was caused by opening up entrances to sealed pores. 

Mesostructured materials are granules containing individual platelets (crystals) associated in a fairly random manner. This type of configuration is always associated with a bi-porous structure, in which small particles (platelets) have pores, usually mesopores, different from the composite particle (secondary mesopores and macropores). The secondary pore structure controls access to the individual crystal mesoporosity. As a result, different mass transfer resistances to diffusion through bi-porous structures could be present. In order to evaluate the relative significance of both primary and secondary pore diffusion, usually two different particle sizes are employed in diffusion measurements. 

Besides specific surface area, silicas are also characterised by their porosity. Most of the silica s are made out of dense spherical amorphous particles linked together in a three dimensional network, this crosslinked network building up the porosity of the silica. Where the reactivity of diborane towards the silica surface has been profoundly investigated, little attention has been paid to the effect of those reactions on the pore structure. However different methods are developed to define the porosity and physisorption measurements to characterise the porosity parameters are well established. Adsorption isotherms give the specific surface area using the BET model, while the analysis desorption hysteresis yields the pore size distribution. 

---