Pores volume

The pore volume defines the possible run time/elution volume of the separation. The higher it is, the higher the peak capacity is and the chance of a good separation. 

Lower flow rates are showing higher plate numbers. Low flow rates allow particularly bigger molecules to diffuse better into the pores. In the case of high flow rates, these are, as a tendency, shifted toward lower elution volumes. 

A higher temperature reduces viscosity and, therefore, allows a better diffusion. Efficiency increases. Proteins, however, are temperature sensitive and, therefore, the temperature increase is hmited. 

A lower viscosity increases efficiency. Viscosity can be influenced by the type of salt, the salt concentration, and the organic modifier. 

For a given pore volume the surface area increases with decreasing pore size. Approximate values are  

Microporous adsorbents Mesoporous adsorbents Macroporous adsorbents  

From mercury penetration the surface area is determined knowing the surface tension and contact angle of mercury and the total volume of penetrated mercury. Measurements agree with the BET measurements below surface areas of 100 m /g. 

The specific pore volume can be determined from nitrogen adsorption measurements if the adsorbent is meso- or microporous. For macroporous adsorbents with pore diameters above 1000 A, the pore volume can be determined by mercury penetration measurements by integrating the pressure volume curve. The total pore volume of meso- and microporous adsorbents can be calculated by assuming that, in the range 0.95 pjpo 1, all pores in the adsorbent are filled with condensed gas. The total pore volume is then simply calculated as  

The volume of micropores can be obtained using the Dubinin equation [78], 

The migration of molecules between the stationary phase and the mobile phase is driven by random movement or diffusion, a factor which is deleterious to high resolution in all forms of chromatography. Since resolution in SEC is solely dependent on diffusion, unlike the other forms of chromatography, optimisation of stationary phase particles is important to improve mass transfer. Factors deleterious to mass transfer can be divided into three separate types those attributable to stagnant mobile phase in the pores of the particles, those caused by differential penetration of the solute molecules into the stationary phase and, finally, longitudinal diffusion between the particles (Snyder and Kirkland, 1979). 

Even within the best supports available there wiU be some irregularities in the particle shapes which will lead to non-uniform channels through which the molecules will permeate. Optimisation of the physical parameters of the column, the type of stationary phase and the method of packing are normally directed at overcoming these effects. 

The mobile phase can affect resolution through direct interaction with both the stationary phase and the solute. Silica columns are best used 

By altering the pH in conditions of low ionic strength, the polarity of the stationary phase can be altered to suit the particular requirements of the solute. This has been called non-ideal SEC and is useful for certain proteins (Kopaciewicz and Regnier, 1982). 

Mobile phase flow rates of around 0.5-1.0 ml/min are recommended for the resolution of a variety of macromolecules on a number of stationary phases. However, flow rates of 0.1-0.5 ml/min are recommended for use with TSK SW and H types. In general, for larger molecules (polynucleotides, proteins), the mass transfer term is much larger and the flow rate has to be correspondingly reduced to maintain resolution. 

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Fig. XVI-2. Comparison of the pore volume distribution curves obtained from porosimeter data assuming contact angles of 140° and 130° with the distribution curve obtained by the isotherm method for a charcoal. (From Ref. 38.)...

Physical properties affecting catalyst perfoniiance include tlie surface area, pore volume and pore size distribution (section B1.26). These properties regulate tlie tradeoff between tlie rate of tlie catalytic reaction on tlie internal surface and tlie rate of transport (e.g., by diffusion) of tlie reactant molecules into tlie pores and tlie product molecules out of tlie pores tlie higher tlie internal area of tlie catalytic material per unit volume, tlie higher the rate of tlie reaction... 

For each group of pores, the pore volume 6v is related to the core volume by means of a model, either the cylinder or the parallel-sided slit as the case may be. Allowance is made for the succession of film thicknesses corresponding to the progressive thinning of the multilayer in each pore, as desorption proceeds. Thus for group i, with radius rf when the film thickness is tj j > i) and the core volume is the pore volume 6vf will be given by... 

Each of the procedures described in Section 3.6 for the calculation of pore size distribution involves a value of the pore area y4f for each successive group of pores. In the Roberts procedure 6A, can be immediately obtained from the corresponding pore volume and pore radius as (for... 

Since in practice the lower limit of mercury porosimetry is around 35 A, and the upper limit of the gas adsorption method is in the region 100-200 A (cf. p. 133) the two methods need to be used in conjunction if the complete curve of total pore volume against pore radius is to be obtained. 

Fig. 3J0 Plot of cumulative pore volume against logarithm of r the effective pore radius, (o) For charcoal AY4 A by mercury intrusion O by capillary condensation of benzene, (b) For zinc chloride carbon AYS A by mercury intrusion O by capillary condensation of benzene x by capillary condensation of benzene, after mercury intrusion followed by distillation of mercury under vacuum at temperature rising to 350°C. (Courtesy...

Fig. 3J1 Comparison of pore volume size distributions for Clear Creek sandstone" (courtesy Dullien.) Curve (A), from mercury porosimetry curve (B), from photomicrography (sphere model).

The increase in pore volume brought about by high intrusion pressures may be caused by fracture of the pore walls that gives access to pores... 

In Unger and Fischer s study of the effect of mercury intrusion on structure, three samples of porous silica were specially prepared from spherical particles 100-200 pm in diameter so as to provide a wide range of porosity (Table 3.16). The initial pore volume n (EtOH) was determined by ethanol titration (see next paragraph). The pore volume u (Hg, i) obtained from the first penetration of mercury agreed moderately well with u fEtOH),... 

Values of pore volume of samples of porous silica, determined by ethanol titration (v (EtOH)) and by mercury porosimetry (v (Hg, i) and v (Hg, ii)) ... 

Evaluation of pore volume by displacement of mercury and another fluid... 

That the uptake n, at saturation does indeed approximate to the pore volume of the adsorbent is confirmed by the agreement, frequently obtained, between the quantity and the pore volume calculated from the... 

Comparison of the pore volume obtained (a) by the Gurvitsch rule and (b) from the densities in mercury and in another fluid... 

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

Thus, the porosity of an aerogel is ia excess of 90% and can be as high as 99.9%. As a consequence of such a high porosity, aerogels have large internal surface area and pore volume. 

Typical pore size distributions for these adsorbents have been given (see Adsorption). Only molecular sieve carbons and crystalline molecular sieves have large pore volumes in pores smaller than 1 nm. Only the crystalline molecular sieves have monodisperse pore diameters because of the regularity of their crystalline stmctures (41). 

This principle is illustrated in Figure 10 (45). Water adsorption at low pressures is markedly reduced on a poly(vinyhdene chloride)-based activated carbon after removal of surface oxygenated groups by degassing at 1000°C. Following this treatment, water adsorption is dominated by capillary condensation in mesopores, and the si2e of the adsorption-desorption hysteresis loop increases, because the pore volume previously occupied by water at the lower pressures now remains empty until the water pressure reaches pressures 0.3 to 0.4 times the vapor pressure) at which capillary condensation can occur. 

For spherical particles the average diameter of the pores, defined as four times the pore volume divided by the surface area, can be shown to be... 

Tar Sands. Tar sands (qv) are considered to be sedimentary rocks having natural porosity where the pore volume is occupied by viscous, petroleum-like hydrocarbons. The terms oil sands, rock asphalts, asphaltic sandstones, and malthas or malthites have all been appHed to the same resource. The hydrocarbon component of tar sands is properly termed bitumen. 

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