Porous properties

Figure 4.19. Shock pressure versus density Hugoniot states for initially porous quartz. Density of starting material is indicated on various curves. Porous properties of stishovite are represented by curves with 1.75, 2.13, and 2.65 Mg/m, initial density, whereas coesitelike properties are represented by 0.2-0.8 Mg/m curves (after Simakov and Trunin (1990)).

Dourdain, S. Bardeau, J. F. Colas, M. Smarsly, B. Mehdi, A. Ocko, B. M. Gibaud, A. 2005. Determination by x-ray reflectivity and small angle x-ray scattering of the porous properties of mesoporous silica thin films. Appl. Phys. Lett. 86 113108. 

Hoegger and Freitag modified the Hjerten s procedure and prepared a variety of monolithic acrylamide-based CEC columns [118]. Their approach allowed them to adjust both rigidity and porous properties of the monoliths and to achieve excellent separations of model compounds as well as selected pharmaceuticals. 

This technology was extended to the preparation of chiral capillary columns [ 138 -141 ]. For example, enantioselective columns were prepared using a simple copolymerization of mixtures of O-[2-(methacryloyloxy)ethylcarbamoyl]-10,11-dihydro quinidine, ethylene dimethacrylate, and 2-hydroxyethyl methacrylate in the presence of mixture of cyclohexanol and 1-dodecanol as porogenic solvents. The porous properties of the monolithic columns can easily be controlled through changes in the composition of this binary solvent. Very high column efficiencies of 250,000 plates/m and good selectivities were achieved for the separations of numerous enantiomers [140]. 

The morphology of the monoliths is closely related to their porous properties, and is also a direct consequence of the quality of the porogenic solvent as well as the percentage of crosslinking monomer and the ratio between the monomer and porogen phases. The presence of synergistic effects of these reaction conditions was verified using multivariate analysis [65]. 

Because the monoliths allow total convection of the mobile phase through their pores, the overall mass transfer is dramatically accelerated compared to conventional porous structures. Based on the morphology and porous properties of the molded monoliths, which allow fast flow of substrate solutions, it can be safely anticipated that they would also provide outstanding supports for immobilization of biocatalysts, thus extending the original concept of monolithic materials to the area of catalysis. 

Table 1. Porous properties and enzymatic activities of monolithic poly(2-vinyl-4,4-dimethyl-azlactone-co-acrylamide-co-ethylene dimethacrylate) reactors3... 

Membranes could now be cast with appropriate porous properties such that the previously mentioned "tailoring" operation could be carried out, i.e., the underwater heating of the membrane to a temperature which would provide adequate desalination. 

Porous property of these particles was considered to be based on crystallization mechanism of calcium phosphate, in which fine crystals formed in the bottom of fluidized bed attached to the fluidized seeds by driving force of supersaturation and fluid conditions. 

The polymerization time as a polymerization parameter for adjustment of the porous properties of thermally initiated copolymers has recently been characterized [111]. A polymerization mixture comprising methylstyrene and l,2-bis(p-vinylbenzyl)ethane as monomers was subjected to thermally initiated copolymerization for different times (0.75, 1.0, 1.5, 2, 6, 12, and 24h) at 65°C. The mixtures were polymerized in silanized 200pm I.D. capillary columns as well as in glass vials for ISEC and MIP/BET measurements, respectively. 

Even if MIP and BET are widely accepted regarding the characterization of HPLC stationary phases, they are only applicable to the samples in the dry state. In order to investigate the impact of polymerization time on the porous properties of wet monolithic columns, ISEC measurements of 200 jm I.D. poly(p-methylstyrene-co-l,2-bis(vinylphenyl)ethane) (MS/BVPE) capillary columns (prepared using a total polymerization time ranging from 45 min to 24 h) have been additionally evaluated (see Table 1.2 for a summary of determined e values). On a stepwise decrease in the time down to 45 min, the total porosity (St) is systematically increasing to about 30% in total (62.8% for 24 h and 97.2% for 45 min). This is caused by a simultaneous increase in the fraction of interparticulate porosity (e. ) as well as the fraction of pores (Cp). The ISEC measurements are in agreement with those of the MIP as well as BET analyses, as an increase in should be reflected in an increase in 8p and as the relative increase in the total porosity (caused by decreasing the polymerization time... 

Influence of the Polymerization Time on the Porous Properties of Monolithic MS/BVPE Networks, Considering Capillary Columns (80x0.2 mm I.D.) for ISEC and Glass Vial Bulk Polymers for MIP and BET Measurements... 

There is no recommendation of one of the introduced methods (MIP, BET, or ISEC) as the most accurate, reliable, and universally valid technique for the determination of the porous properties of a stationary phase. MIP, BET, and ISEC have rather to be regarded as three independent methodologies, those results complement one another to yield a precise estimation of the porosity of an investigated column packing. The most important characteristics, limitations, and methodological strengths of MIP, BET, and ISEC are intended to be discussed in this section. 

Porous properties of the mesoporous materials were mainly controlled by the silicate content and heat-treating conditions. 

The experimental method of mercury porosimetry for the determination of the porous properties of solids is dependent on several variables. One of these is the wetting or contact angle between mercury and the surface of the solid. 

Zeolites (3 were treated with a NaBO, solution, and the porous properties of boronated samples were investigated by sorption measurements with benzene and nitrogen as adsorbate, TEM, SEM and composition analysis. It is shown that the micropores are converted into the mesopores and the mesopores are developed into larger mesopores due to the extraction of framework silicon by base. The small atom size of boron and the poor stability of boron in framework should be responsible for the silicon removal in a large amount. The dissolution of silicon also causes the corrosion of outer surface of particles and the decrease of particle size. 

It is well known that the elements in framework of zeolite molecular sieves greatly influence the properties and behaviors of these materials [1-3], The introduction of heteroatoms into the framework has become one of most active fields in study of zeolites. The investigations were mostly focused on the methods to introduce heteroatoms into the framework (for examples, hydrothermal synthesis and post-synthesis), the mechanisms for incorporations, the effect of heteroatoms on the acid-base properties and the catalytic features of modified samples [1-10]. Relatively less attention was paid to the effect of treatment process on the porous properties of samples although the incorporation of heteroatoms, especially by the so-called post-synthesis, frequently changes the distribution of pore size. Recently, we incorporated Al, Ga and B atoms into zeolites (3 by the post-synthesis in an alkaline medium named alumination, galliation and boronation, respectively. It was found that different trivalent elements inserted into the [3 framework at quite different level. The heteroatoms with unsuitable atom size and poor stability in framework were less introduced, leading to that a considerable amount of framework silicon were dissolved under the action of base and the mesopores in zeolite crystal were developed. As a typical case, the boronation of zeolites (3 and the accompanied formation of mesopores are reported in the present paper. 

Porous properties of this monolithic polymer were controlled by addition of ammonium sulfate. Micrographs shown in Fig. 6.6 clearly demonstrate the changes in... 

---