Adsorption critical

The elution behavior of various polymers near their critical adsorption point with silica gel packings and various eluents has been studied (12). It was of interest to apply hybrid column systems composed of active ( critical ) packings (silica gels) in combination with nonactive (nonadsorptive) PS/DVB and DVB-based gels. Some PS/DVB and DVB gels exhibited rather strong... 

It is evident from these results that the interactive properties of the investigated SEC PS/DVB or DVB gels are very different. Because polar electroneutral macromolecules of PMMA were more retained from a nonpolar solvent (toluene) than from polar ones (THF, chloroform), we conclude that the dipol-dipol interactions were operative. Columns No. 1 and No. 2 were very interactive and can be applied successfully to LC techniques that combine exclusion and interaction (adsorption) mechanisms. These emerging techniques are LC at the critical adsorption point (18), the already mentioned LC under limiting conditions of adsorption (15,18), and LC under limiting conditions of desorption (16). In these cases, the adsorptivity of the SEC columns may even be advantageous. In most conventional SEC applications, however, the interactive properties of columns may cause important problems. In any case, interactive properties of SEC columns should be considered when applying the universal calibration, especially for medium polar and polar polymers. It is therefore advisable to check the elution properties of SEC columns before use with the... 

The adsorption of HPAM on sand (Figure 4) is not detected below a threshold value of Ca2+ due to strong electrostatic repulsion between the polyelectrolyte and the highly charged negative surface. This threshold value, which was also observed in the case of monovalent ions (9), represents the point where the critical adsorption energy is overcome, and once this value is surpassed, adsorption increases sharply. This form of adsorption behavior is in line with predictions of theories on polyelectrolyte adsorption (20). 

The critical adsorption energy. A critical adsorption energy is predicted by many theories (2-6). Its value is dependent on conformational properties of the polymer and usually estimated as a few tenths ofkT (7). Yet, a method to determine x c experimentally has never been suggested. Displacement studies provide such a method. Inspection of Equation 5 bears out that Xsc is obtained from a displacement isotherm, provided that Xgd and the solvency terms vanish. This condition is met by taking as the displacer a molecule which is (nearly) identical to the repeating unit of the polymer, i.e. the polymer is displaced by its own monomer. Such... 

In modern statistical theories (see Scheutjens and Fleer, 1980, and Fleer and Lyklema, 1983) it is found that there does exist a certain critical adsorption free energy, customarily called xs, above which the moleculq adsorbs strongly and below which it does not adsorb. Typical values of xs, (crit) range from 0.2 - 0.4 kT. 

At very low adsorption densities, metal ion adsorption exhibits Langmuiriaij behavior. However, above some critical adsorption density, r (i.e., when approximately 0.01% of surface sites are occupied), the intensity of metal lon/interactions decreases with increasing surface-site occupancy. A corollary of this model is that K has a unique value at adsorption densities below r for a specific adsorbate and adsorbent. By extension, 0a[Pg.181]

The principle of the liquid chromatography under critical conditions (LC CC) was elucidated in Section 16.3.3. The mutual compensation of the exclusion—entropy and the interaction—enthalpy-based retention of macromolecules can be attained when applying in the controlled way the interactions that lead to either adsorption or enthalpic partition. The resulting methods are called LC at the critical adsorption point (LC CAP) or LC at the critical partition point (LC CPP), respectively. The term LC at the point of exclusion-adsorption transition (LC PEAT) was also proposed for the procedures employing compensation of exclusion and adsorption [161]. It is anticipated that also other kinds of enthalpic interactions, for example the ion interactions between column packing and macromolecules can be utilized for the exclusion-interaction compensation. 

Dietrich, S., (1991), Fluid interfaces - wetting, critical adsorption, van der Waals tails, and the concept of the effective interface potential , in Taub, H., Torzo, G., Lauter, HJ. and Fain, S.C., (eds), Phase. Transitions in Surface Films 2, NATO Advanced Science Series, Physics, Vol. 267, 391-423. 

Zheligovskaya et al. [55] have simulated the adsorption of quasirandom adsorption-tuned copolymers (ATC). The critical adsorption energy as well as some characteristics of the adsorbed single chains (statistics of trains, loops, and tails) were studied. All these properties were compared with those... 

Molecular sieve, 5A 225 Air and light gas analysis H2, 02, N2, (CH4, CO, NO, SFe co-elute) Synthetic calcium aluminosilicate (zeolite), having an effective pore diameter of 0.5 nm, C02 is adsorbed strongly 5A usually gives the best results of all synthetic zeolites should be activated before use and used above critical adsorption temperature 21.6% (mass/mass) water capacity... 

Spectra, with adequate resolution for solute identification, have been obtained from as little as 10 ng of material, but the sample size will depend strongly on the extinction coefficient of the solute at the critical adsorption wavelengths of the respective compounds. 

There exists a critical adsorption energy given by... 

We first note the gradual behaviour of the adsorption isotherms, and this is typical for super-critical fluid adsorption in pores of all sizes. For sub-critical adsorption which we will consider later, there is a possibility of phase transition for certain pore widths. 

Relation and difference between sub- and super-critical adsorption... 

When a polymer is added in a surfactant system, there are two critical concentrations CAC and CMC2. CAC is the critical adsorption concentration at which surfactant starts to adsorb on the polymer chains it is lower than the critical micelle concentration (CMC). CMC2 is the surfactant concentration at which micelles are formed when polymer is present it is higher than CMC (Li et al 2002). Both CAC and CMC2 are on the order of magnitude of CMC. 

Therefore, in the limiting case of large aggregates, the equation of state for the surface layer (2.101) and the respective adsorption isotherm (2.102), expressed via the molar area of monomers and the critical adsorption of aggregation, can be written in the following form... 

Eq. (2.111) predicts that above the critical adsorption F, or above the corresponding critical bulk concentration c, determined from Eq. (2.102) as... 

Figure 2.14 illustrates the effect of the aggregation number for the case where the surface layer is free of monomers (F < 10 mol/m ). In this case, when the aggregation number exceeds a certain value (n > 20), the curves also become independent of n. Here, if the critical adsorption F is not too small, the curves exhibit a characteristic Iracture which indicates the formation of clusters in the adsorption layer (cf Fig. 2.15). However, for n>50 (that is, for very large clusters) the shape of the curves becomes independent of n.

Here Fj and Fc are the partial and critical adsorption of monomers, respectively, and the average molar area co should be expressed via the equation... 

For arbitrary times, surface layer coverage and critical adsorption Fc, the set of equations (2.110) - (2.112) together with the transport equation has to be solved numerically. This model assumes that the aggregation process itself does not require additional time, i.e. there is always equilibrium between monomers and aggregates in the adsorption layer. To solve this set of equations numerically, first-order finite difference schemes can be applied as described in... 

Beyond this phase transition point, the creation and growth of condensed phase domains is observed. The shape of these domains is very similar to those of pure DPPC domains, as one can see from the images given in Fig. 4.51. The BAM images were taken at different times after the start of the penetration experiments and the letters correspond to the respective moments in the rt(t) curve. The aggregation of DPPC into condensed phase domains is induced even if the initial surface DPPC concentration is less than 50% of the critical adsorption T. ... 

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