Adsorbents surface areas

The general type of approach, that is, the comparison of an experimental heat of immersion with the expected value per square centimeter, has been discussed and implemented by numerous authors [21,22]. It is possible, for example, to estimate sv - sl from adsorption data or from the so-called isosteric heat of adsorption (see Section XVII-12B). In many cases where approximate relative areas only are desired, as with coals or other natural products, the heat of immersion method has much to recommend it. In the case of microporous adsorbents surface areas from heats of immersion can be larger than those from adsorption studies [23], but the former are the more correct [24]. 

Fused silica capillary columns of various internal bores and of lengths in the range 25 to 50 m are mainly employed for analytical separations. A variety of polar and non-polar column types are available including those open tubular types with simple wall coatings (WCOT), those with coatings dispersed on porous solid-supports to increase adsorbent surface area (SCOT) and porous layer open tubular (PLOT) columns. Important stationary phases include polyethylene glycol, dimethylpolysiloxane and different siloxane copolymers. Various sample introduction procedures are employed including ... 

The competition model of Snyder assumes that the adso tion surface is completely covered by adsorbed eluent molecules forming a mono-layer. When solute molecules are adsorbed they displace olvent molecules. Due to the size differences one or more eluent mofecules are displaced by the solute molecules. The adsorbent surface is ijissumed to be homogeneous and each molecule tends to interact totally wRh the surface, i.e., it is adsorbed flatwise. Thus, the adsorbent surface area that the molecules require can be calculated from their molecular dimensions. Neglecting the interactions between solute and eluent molecules in the liquid and the adsorbed phase, the retention of an adsorbed molecule (expressed as net retention volume per unit weight of adsorbent K) can be related to the properties of the stationary phase, the eluent, and the sample by... 

Now if A is the adsorbent surface area and t the thickness of the adsorbed layer, then the volume of the adsorbed layer, or adsorption space is [4] ... 

This limitation could be overcome by varying adsorbent surface area so as to change solute k values independently of e°. However, this is a more complex select vity-optimization strategy. 

Adsorbents employed in this mode of chromatography are porous rigid materials with hydrophobic surfaces. In all modes of HPLC with positive analyte surface interactions (NP, RP, lEX) the higher the adsorbent surface area, the longer the analyte retention and in most cases the better separation. The majority of packing materials used in RP HPLC are chemically modified porous silica. The properties of silica have been studied for many years [15, 16], and the technology of manufacturing porous spherical particles of controlled size and porosity is well-developed. 

The column void volume, Vo, is dehned as the total volume of the liquid phase in the column and could be measured independently [18]. Total adsorbent surface area in the column, S, is determined as the product of the adsorbent mass and specific surface area. 

Figure 2-5. Illustration of the column shce for construction of mass balance. Mobile-phase flow F in mL/min analyte concentration c in mol/L n is the analyte accumulation in the shce dx in mol v is the mobile-phase volume in the shce dx expressed as VqIL, where L is the column length s is the adsorbent surface area in the shce dx, expressed as S/L, where S is the total adsorbent area in the column.

In a liquid binary solution, this accumulation is accompanied by the corresponding displacement of another component (solvent) from the surface region into the bulk solution. At equilibrium a certain amount of the solute will be accumulated on the surface in excess of its equilibrium concentration in the bulk solution, as shown in Figure 2-6. Excess adsorption E of a component in binary mixture is defined from a comparison of two static systems with the same liquid volume Vo and adsorbent surface area S. In the first system the adsorbent surface considered to be inert (does not exert any surface forces in the solution) and the total amount of analyte (component 2) will be no = VoCo. In the second system the adsorbent surface is active and component 2 is preferentially adsorbed thus its amount in the bulk solution is decreased. The analyte equilibrium concentration Ce can only be measured in the bulk solution, so the amount VoCe is thereby smaller than the original quantity no due to its accumulation on the surface, but it also includes the portion of the analyte in the close proximity of the surface (the portion U Ce, as shown in Figure 2-6 note that we did not define V yet and we do not need to define... 

This expression describes the analyte retention in binary system using only the total volume of the liquid phase in the column, Vq, and total adsorbent surface area S as parameters and the derivative of the excess adsorption by the analyte equihbrium concentration. It is important to note that the position of Gibbs dividing plane in the system has not been defined yet. 

In the derivations of the retention functions so far, the adsorbed phase volume or thickness of the adsorbed layer was not introduced. The adsorbent and column parameters (surface area and void volume) independently measured are not dependent on the eluent type composition. Measurement of the void volume and adsorbent surface area is discussed in the following references 18 and 23. 

The analysis of experimental excess adsorption isotherms using equation (2-50) had shown unusual results [22]. The adsorbed layer thickness of acetonitrile adsorbed from water on different types of reversed-phase adsorbents calculated as the ratio of adsorbed layer volume and adsorbent surface area appears to be on average equal to 14 A, which is equivalent to approximately five monolayers of acetonitrile molecules adsorbed on the hydrophobic surface. At the same time, the adsorbed layer thickness of methanol adsorbed from water on the same adsorbents is equal to only 2.5 A, which is equivalent to the monolayer-type adsorption. 

Vr(csi) is the analyte retention as a function of the eluent concentration, Vo is the total volume of the liquid phase in the column, y Cei) is the volume of adsorbed layer as a function of eluent composition, Kp(cei) is the distribution coefficient of the analyte between the eluent and adsorbed phase, S is the adsorbent surface area, and is the analyte Henry constant for its adsorption from pure organic eluent component (adsorbed layer) on the surface of the bonded phase. 

So far the solution of the mass-balance equation for models with a single dominating process (partitioning or adsorption) was discussed in Sections 2.8 and 2.9. In both cases the solutions have similar form, with the difference in the definition of the parameters (volumes of the mobile and stationary phases in the case of partitioning total volume of the liquid phase and adsorbent surface area in the case of adsorption model). 

All these characteristics are interrelated. Variations of porosity which include pore diameter can affect both the adsorbent surface area and the bonding density. The type of base material affects adsorbent surface chemistry. Therefore, in our discussion we combine these characteristics in two major classes geometry and surface chemistry. 

Adsorbent surface area, pore volume, and pore diameter are the properties of significant importance. HPLC retention is generally proportional to the surface area accessible for a given analyte (Chapter 2). Surface area accessibility is dependent on the analyte molecular size, adsorbent pore diameter, and pore size distribution. 

Surface area of HPLC adsorbents is probably the most important parameter, although it is almost never used or accounted for in everyday practical chromatographic work. As shown in the theory chapter (see Chapter 2), HPLC retention is proportional to the adsorbent surface area. The higher the surface area, the greater the analyte retention, although as we discuss later, depending on the surface geometry, analytes of a different molecular size could effectively see different surface areas on the same adsorbent. 

It is generally assumed that a nitrogen molecule occupies 16.4 on the polar silica surface. The adsorbent surface area is then calculated as a product of the total amount of nitrogen in the monolayer (n ) and the nitrogen molecular area (16.4 A ). 

Adsorbent surface area (S) is measured as a product of molecular area (o) of a probe substance and the number of the molecules (N) in complete adsorbed monolayer. On the fractal surface the total number of molecules in the monolayer is dependent on its roughness and could be expressed as... 

The calculation of the adsorbent surface area using BET theory involves the estimation of the molecular cross-sectional area of nitrogen molecule [15]. In general it is assumed to be equal to 16.2 per nitrogen molecule on the surface, but this value is the subject of intense criticism during the past 30 years [72]. 

Adsorbent surface area is calculated as the product of the monolayer capacity estimated from BET equation and nitrogen molecular area, con,- If the... 

Figure 3-19. Different ways of measuring the modified adsorbent surface area.

Adsorbent Surface area (mVg) Hydrophobicity coefficient" Phenol uptake (mg/g)... 

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