Porosity instrumentation

A new air-flow unit based on the ASTM D 3574 - Test G method is now available from Fluid Data, 1201 North Velasco, Angleton, TX 77515 (409-849-2344). This unit. Fluid Data s Model FPI Foam Porosity Instrument, is reported to be easy to read, computer-compatible, and more portable than its predecessors. It uses a digital electronic display instead of a rotameter, has automatic calibration, and prints out consecutively numbered results. An RS-232 serial port allows data to be transferred from the unit and stored in a computer. This unit is illustrated in Figure 10.1. 

Figure 10.1 Fluid Data Model FPI Foam Porosity Instrument (used with permission of Fluid Data).

The instrument was operated at room temperature with Burdick and Jackson distilled in glass THF as the eluting solvent. The sample column bank consisted of six y-Styragel coluums with the following porosity designations 10, 10, 500, 500, 100, lOOA. The flow rate was adjusted to 0.6 ml/mln. A 2.2- milliliter syphon was used to monitor retention volume. 

Measurement of the punch and die forces plus the relative displacement of the punches can provide raw data which, when suitably processed and interpreted, facilitate the evaluation of many tableting parameters. Many of the workers first involved in instrumenting tablet presses concentrated on deriving relationships between the applied force (FA) and the porosity (E) of the consolidating mass. 

Repeat chromatograms in GPC should agree within 2%. However, the chromatogram is sensitive to such experimental conditions as (a) resolution of the columns, (b) range of porosities of the column packings, (c) flow rate of solvent, and (d) age of detectors. It is recommended that the laboratory use control charts to determine the optimal conditions of the instrument. 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

The most fundamental characteristic of a catalyst is its chemical composition, which is decisive for its specific usage (Table 5.1). The properties of a catalyst, namely activity and selectivity, have been discussed in Chapter 2. The physical properties of the catalyst are also important for its successful application. They are investigated by both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. 

Gel Permeation Chromatography. A Water Associates model 200 gel permeation chromatograph fitted with five Styragel columns having nominal porosity designations 107, 107,106, 1.5 X 105, and 1.5 X 104 A was used for the analysis of molecular weight distribution in TFE at a temperature of 50.0 0.5°C and a flow rate of 1.00 =t 0.05 ml/min. Further details concerning instrumental and operational parameters, sample preparation and injection, and data acquisition and reduction have been reported elsewhere (I). 

Most commercial instruments using gas adsorption for surface area and porosity determination are based on the BET isotherm. In Eq. (1.45), the monolayer capacity Vm can be used to calculate the surface area on the basis of the area occupied by each adsorbed gas molecule. According to Eq. (1.45), a plot of p/[Va(p0 - p)] versus p/po is linear. From the slope and the intercept, Vm can be obtained. Thus, the specific surface area S can be obtained as... 

The compaction properties of pharmaceutical formulations can be studied experimentally using a variety of techniques, ranging from instrumented production presses to compaction simulators, and methods of analysis. The results are usually plotted as porosity-axial stress functions, which is of interest to compare different materials. However, there are some drawbacks on this type of evaluation. As mentioned by Cunningham et al. [20], this approach is deficient once it considers only the average stress along the direction of compaction, ignoring radial stress transmission and friction. 

NMR spectra were recorded, in CDCl with DRX-500MHz spectrometer (TMS as the reference). Size exclusion chromatography (SEC) was performed using a LDC Analytical refractoMonitor IV instrument [RI detector, two columns SDV 8x3(X) (5 pm and 104 A porosity) and SDV 8 x 300 (5 pm and 100 A porosity), eluent - toluene (0.7 ml/min)]. Parallel measurements were effected with Wyatt Optilab 903 apparatus... 

The perceived crispiness of chips and many cereal-based products is the key determinant characterising product quality. Most crispy products are characterized by a cellular, lamellar or puffed structure often described as solid foam. The crispiness of a product is closely related to its structure and the extent of porosity. However, sensory evaluation of crispiness is difficult to correlate with instrumental parameters mainly because crispiness is not a clearly defined attribute (Roudaut et al. 2002). 

In this context, the SPM techniques (and especially STM and AFM) appear, a priori, ideally suited for the direct visualization of the porous structure of materials at scales which are not so readily accessible by other means (e.g., scaiming and transmission electron microscopies). However, the performance of such a task is confronted with two major limitations. The first one arises from the fact that detection with SPM is exclusively restricted to the outermost surface of the sample. Accordingly, this implies that only the most external porosity of the material can be probed, whereas no information on the bulk (inner) porosity, which might not be identical to the former, is revealed. The second drawback is related to the finite dimensions of the probing tip, which limits the size of the voids (pores) physically accessible (and thus detectable) by the tip on the sample surface. Obviously, pores significantly smaller than the tip diameter will pass uimoticed to the instrument when the surface is scanned. As a specific example, the tips normally employed in AFM are not sharp enough to provide access to the whole mesopore range (between 2 and 50 nm). 

A series of silica gels were synthesised from TEOS, selecting the experimental conditions (pH and water/TEOS ratio) in order to obtain samples with different porosity. Porous texture characterisation of these samples was done by gas adsorption (N2 and CO2 adsorption at 77 K and 273 K, respectively) (Quantachrome. Autosorb 6). The samples were degassed at 623 K under vacuum, until 10 torr. Water adsorption studies were carried out at 298 K in an automatic volumetric gas adsorption instrument (Belsorb 18). Experimental data was corrected for adsorption on inner wall of apparatus. Additionally, a blank experiment on all bulbs used showed that water adsorption on the inner surface of glass was negligible. Previous to water adsorption... 

Figure 1 Instrumental setup for simultaneous 2-dim detection porosity and inner surface density.

Micromeretics GeoPyc 1360 measures envelope density this includes the volume of pores and surface crevices. When absolute density is inputted, specific pore volume and percent porosity are determined. The instrument uses a flowing dry medium, DryFlo , to surround the powder and generate the displacement volume. 

SEC System. A Spectra Physics IsoChrom pump controlled the THE flow in the SEC part of the 2D instrument. Two SDV 5-jLtm SEC columns with 1000 and 10 A porosity (PSS) were used for size separation of the HPLC fractions. UV (SP 8450, Spectro Physics) and RI (Shodex SE 61, Diisseldorf, Germany) detection allowed lor conventional or multiple-detection data processing of detector traces. Eor the polyester analysis the SEC columns (50 and 100 A) were operated in acetone as eluent. 

However, the real problem with Heckel plots is identifying a truly rectilinear section. When pressure-porosity diagrams were first devised, instrumented... 

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