Exclusion critical, adsorption modes

LAC at the critical point can be classified as the fifth type of NELC. Operating in the region between size exclusion and adsorption modes of LC by changing the composition of the bicomponent mobile phase, retention becomes independent of polymer size and the separation is accomplished exclusively by composition (20). This technique was applied to the characterization of block polymers (2J). 

The theory of adsorption at porous adsorbents predicts the existence of a finite critical energy of adsorption e, where the macromolecule starts to adsorb at the stationary phase. Thus, at > the macromolecule is adsorbed, whereas at e < e the macromolecule remains unadsorbed. At e = Ec the transition from the unadsorbed to the adsorbed state takes place, corresponding to a transition from one to another separation mechanism. This transition is termed critical point of adsorption and relates to a situation, where the adsorption forces are exactly compensated by the entropy losses TAS = AH [2, 7]. Accordingly, at the critical point of adsorption the Gibbs free energy is constant (AG = 0) and the distribution coefficient is Kj = 1, irrespective of the molar mass of the macromolecules. The critical point of adsorption relates to a very narrow range between the size exclusion and adsorption modes of liquid chromatography. It is, therefore, very sensitive towards temperature and mobile phase composition. 

In aqueous mobile phases, a new parameter, pH comes into play, which leads to ionic effects and thus an ionic exclusion, expulsion and attraction. The variation of pH enables transition from exclusion to adsorption mode through critical region [125]. 

Fig. 6. Experimental chromatograms (a) and shape of calibration dependences (b) for nonfunctional oligobutadicnes of different molecular weight in the exclusion, critical and adsorption separation modes.

In a similar way, one can achieve the separation according to functionality in other solvent mixtures, e.g. chloroform-acetone (Fig. 17). Except of slight differences in retention times for molecules of different functionality, which can be caused either by the difference in the interaction in the mobile phase or a change in the water content of the adsorbent, the form of FTD chromatograms for both eluents at critical conditions is similar. This indicates that for the critical conditions to be realized it is sufficient to have any two solvents suitable for the detection method, one of which works in the exclusion and the other in the adsorption mode. 

In the pharmaceutical industry, most of the critical membrane filtration operations, such as sterile and virus filtration, are performed in the direct flow filtration mode where a feed solution passes directly through a membrane. As the solution passes through the membrane, particles are retained by size exclusion or adsorption. Direct flow filtration can be operated under constant flow or constant pressure modes. 

The critical diagram M vs retention time for a short RP-18 column of 60 mm length is shown in Fig. 7A. At acetonitrile concentrations > 47 vol.% in the solvent mixture, the retention time decreases as the molar mass of the PEO calibration sample increases retention corresponds to a size-exclusion mode. The reverse behaviour, i.e. the adsorption mode, is obtained at acetonitrile concentrations < 45 vol.%. The critical point of adsorption is operating at an eluent composition of acetonitrile-water 46 54 v/v, where separation is accomplished exclusively with respect to the functional endgroups- (see Fig. 7B). For the alkoxy-terminated PEOs, two distinctively different fractions are obtained, due to the formation of polyethylene glycol as an unwanted by-product. In the case... 

Fig. 1. Plots of log M vs. retention time for polystyrene standards at various temperatures. Polystyrene is eluted under critical conditions at a temperature of 95 °C, at 115°C in exclusion mode, and at 85 °C in adsorption mode. Mobile phase Dimethylformamide. Column packing Nucleosil C18. Symbols 115°C (inverted triangles), 95°C (triangles), 85°C (circles), 70°C (squares) [6]...

Let us consider the separation of polymethylmethacrylate (PMMA) on a nonmodified silica column as an example. In THE (medium polar eluent) the PMMA eludes in size exclusion mode because the dipoles of the methylmethacrylate (MMA) are masked by the dipoles of the THE. Using the nonpolar toluene as the eluent on the same column, the separation is governed by adsorption because the dipoles of the carbonyl group in the PMMA will interact with the dipoles on the surface of the stationary phase. The separation of PMMA in the critical mode of adsorption can be achieved by selecting an appropriate THF/toluene mixture as the eluent. In this case all PMMA samples... 

Fig. 5. Transition from the exclusion to the adsorption separation mode through critical conditions for polystyrene standards at a varying composition of the binary eluent (CCI4—CHC13). (Column Si-300, flow rate u = 0.5 rnl/min, volume of.santple 10 pi, UV detector, >, = 275 nm, t = 27 °C)...

Fig. 7. Interrelation between molecular weight and retention volume for macromolecules of different functionality at chromatography in the exclusion (1-3), the critical (4), and the adsorption (5) separation modes 59). In the general case, the distribution coefficient Kd is a function of the pore size D, the chain length N, the interaction energy with the pore wall of the backbone segments 0 and the terminal segment 0f containing the functional group (zones 1,2 and 3 correspond to the cases shown in Fig. 2)...

As it was shown in Section 3.2, close to the critical conditions the distribution coefficient Kd is a function of chain length, pore size D and the energy of interaction of units with pore walls, 0. For a chosen molecule and adsorbent, Kd = Kd(0), and, therefore, by changing 0 one can successively achieve the transition from the adsorption to the exclusion mode and vice versa, finding in this way the critical conditions necessary for separation according to the functionality. 

For a given polymer with fixed X0 and Ax, Eqs. (3.15) and (3.16) describe a non-linear but monotonous variation of 0 with the composition of the mobile phase Cb (Fig. 10). The procedure of finding the critical conditions then becomes very simple it is necessary to find two solvents, in one of which (a) the adsorption and in the other one (b) the exclusion mode is operative and then, by changing their ratio, to find the point C where there is no retention volume dependence on the molecular weight of the polymer. The only requirement imposed on the choice of the solvents is that they both should be good solvents. In practice, however, it is... 

To explain the separation procedure of polymer blends using chromatography at the critical point of adsorption, the behavior of blends of polystyrene (PS) and polymethyl methacrylate (PMMA) in different chromatographic modes is shown in Fig. 24. With silica gel Si-100 as the stationary phase, the mobile phase comprised mixtures of MEK and cyclohexane. In pure MEK a size exclusion mode was operating for both components. Under these conditions PS and... 

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