Full adsorption-desorption

At the other extreme, with materials of specific surface area above 500 m2 g 1 one must be careful not to reduce the mass of sample by too much it must remain representative of the batch of adsorbent and it must be weighed with an accuracy consistent with the accuracy provided by the adsorption measurement. For these two reasons, it is usually unwise to use a sample mass under, say, 50 mg. If the full adsorption-desorption isotherm is to be determined, one can be limited by the capacity of adsorptive reservoir or dosing volume or by the automatic control range of the electronic microbalance (typically, between 50 and 100 mg with sensitivity >1 pg). It therefore often happens that the measurement of one isotherm cannot provide the best determination of the specific surface area and at the same time the best determination of the full adsorption-desorption isotherm. 

There are many aggregated powders (clays, pigments, cements, etc.) that appear to give normal Type II adsorption isotherms, although their full adsorption-desorption isotherms exhibit Type H3 hysteresis. However, unlike Type IV isotherms, there is no plateau at high pjp°- These isotherms are now termed Type lib, as indicated in Figure 13.1. 

Figure 6.6 shows the full adsorption-desorption isotherm of two batches of the mi-cronized powder shown earlier in Figure 6.4. 

It is to be recognized that the data provided by these simplified procedures have limited significance. Their main value is that eluants and conditions of no value can be eliminated from further consideration. Conclusions reached from these simplified procedures —as to the best eluants and conditions—should be confirmed by a supplementary test using the full adsorption-desorption procedure on the substance to be processed. This is especially necessary when the preliminary studies employ a synthetically prepared solution these studies, do not fully reflect the behavior of a substance when present in industrial or biological preparations containing coadsorbates. 

BET surface areas were measured using a multi-point Coulter SA 3100 instrument with data collected over the P/Po range of0.02-0.2. Adsorption of N2 at 77 K was carried out after outgassing the samples at 573 K. BJH pore distributions were determined using 45 data points over a full adsorption desorption isotherm. 

The occurence of the complete adsorption of macromolecules forms a base of the full adsorption-desorption liquid chromatography-like separation method, FAD (Berek and Nguyen, 1998). A very weak mobile phase is employed, which acts as an adsorb for n-1 sample constituents. They are completely retained within the appropriate full adsorption-desorption column packed with nonporous particles. The unretained sample constituent is directly forwarded to an online SEC column for its molecular characterization. In the next stage, eluent strength is stepwise increased and sample constituents are successively one-by-one desorbed and forwarded into the SEC column. Successful separation of up to six distinct polymers with help of FAD method was reported. In the FAD method, solvents are chosen so that they always act as a desorb just for one sample constituent. 

Nguyen, S.H. Berek, D. Chiantore, O. Reconcentration of diluted polymer solutions by full adsorption/desorption procedure-1. Eluent switching approach studied by size exclusion chromatography. Polymer 1998, 39 (21), 5127-5132. 

Recently Trathnigg et al. characterized fatty acid polyglycol esters using 2D-LC with LCCC as the first and RPLC as the second dimension [140]. Fractions from LCCC are transferred to RPLC using the full adsorption-desorption (FAD) technique [125], by which they are focused and reconcentrated before injection into the second dimension (Fig. 21). This is achieved by increasing the water content of the mobile phase behind the LCCC column. Monoester oligomers of up to 20 ox-yethylene units can be resolved to the baseline. 

Polymer Structure and Characterization, in separation science, the phenomenon of adsorption coupled with size-exclusion chromatography (SEC) has been applied to rapidly separate and characterize polymers. In this technique, called full adsorption-desorption/SEC coupling (FAD/SEC), all the components of a polymer blend are initially adsorbed onto an adsorbent. Then each component is displaced using a suitable small-molecule displacer and sent into the SEC column to characterize their size and mass distribution. This approach also finds application in characterizing copolymers having complex architecture (63). 

Du5 an Berek, Son Hoai Nguyen, and Gdrard Hild Molecular characterization of block copolymers by means of liquid chromatography /. Potential and limitations of full adsorption-desorption procedure in separation of block copolymers, Eur. Polym. J., 36(6)... 

Nguyen and co-workers [110, 111] used adsorption/desorption chromatography to physically separate binary component blends of PS with PMMA and PS with polyvinyl acetate (PVAc). This technique was coupled to SEC in order to separate the blend components by size. Full adsorption/desorption chromatography has been applied... 

Full linear model with adsorption-desorption 11... 

In the case of the full 2D problem with linear surface adsorption-desorption reactions (1), (2), (131) and (132), we present two tests. 

Table 5 Full linear surface adsorption-desorption problem parameter values at the Case A2 diffusive transport with surface reaction...

Figure 6 Volume concentrations (linear surface adsorption-desorption reactions, Case B2) Comparison between concentration obtained using our effective problem (eff), average of the section of the concentration from the original problem (full) and the concentration coming from the simple average (moy) at t = 240 s.

FIGURE 4.4 High-resolution N2 adsorption-desorption isotherms at 77 K (a) normal and (b) semilogarith-mic scale, for three porous carbon materials. Open symbols adsorption full symbols desorption. ACF1 ACF prepared from Nomex aramid fiber by physical activation with C02 [29] AC1 and AC2 activated carbons prepared from Spanish Anthracite by chemical activation with KOH [21]. 

The Langmuir model describes, for a uniform surface and a non-self-interacting adsorbate, the relationship between amount adsorbed and exposure concentration. The parameters of the model are the maximum amount adsorbed as a full monolayer and the equilibrium constant for the adsorption-desorption process which indirectly reflects the strength of the adsorbate-substrate interaction. For the present situation the analysis is modified in the following ways ... 

From an experimental standpoint, the availability of liquid nitrogen and the range of commercial equipment now available make it relatively easy to determine full nitrogen adsorption-desorption isotherms at 77 K. This is an additional reason why nitrogen is now internationally accepted as the standard BET adsorptive (IUPAC Sing et al., 1985), with the convention that routine work, it is assumed that the nitrogen monolayer is in a close-packed liquid state at 77 K, irrespective of the actual structure of the BET monolayer. 

A clearer picture of the sorption of water vapour by montmorillonite was obtained by Cases et al. (1992). Their adsorption-desorption isotherms of water on sodium montmorillonite are shown in Figure 11.7. The wavy nature of the adsorption and desorption branches (At and Dlr respectively) of the full hysteresis loop in Figure 11.7 is evidently similar to that of the water isotherm in Figure 11.6 and is indicative of a complex mechanism. However, it was established that the scale of the hysteresis loop depended on the maximum relative pressure reached before the pressure was reduced. This dependency is illustrated by the appearance of the partial sorption isotherms also plotted in Figure 11.7. Here, a small hysteresis loop (desorption branch D3) was the result of (p/p°)ma < 0.25, in contrast to much larger loop (desorption branch D2) when the adsorption was taken to (p/p°)max = 0.35. 

Figure 1.30. Adsorption-desorption of water vapour on a Wyoming montmorillonite (full curves). Also given is the 001 spacing obtained by X-ray diffraction(dashed curves) filled circles, adsorption open circles, desorption. (Redrawn from J.M. Cases, I. Berend, G. Besson, M. Frangois, J.P. Uriot, F. Thomas and J.E. Poirier. Langmuir 8 (1992) 2730.

The BET surface areas of the zeolite samples were determined by N2 adsorption-desorption at -196 C in a Micromeritics ASAP 2010 equipment. Prior to the determination of the adsorption isotherm, the calcined sample (0.5 g) was outgassed at 400 C under a residual pressure of 1 Pa in order to remove moisture. The adsorption data were treated with the full BET equation. The t-plot method using the universal t-curve was applied in order to obtain an estimation of the micropore volume, microporous surface and external surface area [7]. 

The main advantage of this noethod is the speed with which the measurements of specific surface areas (greater than 1 m /g) are conducted, from one or more points of the adsorption isothermal line, it cannot, however, be used to obtain the full shape of the adsorption-desorption isotherm. 

The gas adsorption/desorption. The determination of pore size and pore size distribution from gas adsorption/desorption isotherms is known from other types of adsorbents a hysteresis loop occurs between the adsorption and desorption curves when a full isotherm is measured. 

Cinke et al. [76] have studied the effects on the specific surface area of the nanotubes of a purification process consisting of debundhng the nanotubes (subjecting them to a dimethylformamide/ethylene diamide treatment), followed by an HCl treatment and wet oxidation. The area for the pristine HiPco nanotubes is 577 m /g that for the tubes subjected just to the wet oxidation and HCl treatment is 968 m /g and that for tubes subjected to the full two-step process is 1587 m /g. This study also found an increase in the size of the hysteresis loops in adsorption-desorption cycles as a result of the purification process. 

---