Polymer HPLC thermodynamics

The most straightforward way for understanding the retention phenomena in polymer HPLC is afforded by thermodynamics. It is widely accepted that the retention of any kind of substance in... 

The thermodynamic quality of mobile phase may be so poor that macromolecules precipitate within column or at least exhibit a tendency to phase separation. This is a specific feature of certain procedures of polymer HPLC (Sections 16.3.7,16.5.3, and 16.5.6). 

In order to explain the dependences of log M vs. Vr depicted in Figure 16.3b through d and to nnderstand particnlar processes in the polymer HPLC columns, the qualitative thermodynamic consideration, Eqnation 16.6 can be nsed. 

It can be concluded that adsorption is an important tool for controlling retention volumes of samples in polymer HPLC. Eluent composition and temperature are the most feasible variables to affect adsorption in the given polymer-column packing systems. The thermodynamic quality of eluent plays less important role. 

In conclusion, the enthalpic partition processes in the columns for polymer HPLC substantially differ from the adsorption processes. Enthalpic partition can be employed for the separation of polymers of the low-to-medium polarity in combination with the alkyl bonded phases on silica gels. The extent of the enthalpic partition and consequently also of the polymer retention is controlled primarily by the thermodynamic quality of eluent toward separated species and by the extent of the bonded phase solvation. 

For polymer HPLC, important parameter is viscosity of polymer solution. It increases with the molar mass of macromolecules and also with the improving thermodynamic quahty of solvent that is with the size of macromolecules in solution. High viscosity of polymer solutions prevents execution of liquid chromatography measurements at elevated concentration, usually above 1% or 10 mg.mL for polymers with intermediate molar mass. Oligomers can be treated at even tentime higher concentration. On the other hand, extreme solution viscosity of ultra-... 

It can be concluded that the adsorption processes extensively affect retention volumes in coupled methods of polymer HPLC. Eluent nature (composition) and temperature are the most common tools employed in control of adsorption in the particular polymer - colunm packing system. The thermodynamic quality of eluent likely plays less important role. The statement, which can be formd in the literature ... addition of a nonsolvent to eluent increases polymer adsorption... is misleading. The nonsolvent can be either a desorli or an adsorb for the given polymer so that a nonsolvent present in the solvent mixture can correspondingly either decrease or increase the extent of adsorption of macromolecules. Considerations on the role of conformational entropy of macromolecules in the adsorption processes may help explain some unexpected results in coupled methods of polymer HPLC. 

Stated that depending on the thermodynamic quality of solvent, the same macromolecule can assnme various conformations and consequently also different sizes in solution Similar thoughts are to be applied also considering role of solvents in liquid chromatography systems, in which macromolecules necessarily interact with eluent molecules. However, the third component has to be taken into account in polymer HPLC, namely the column packing and its interactions not only with the macromolecules of sample but also with the eluent moleeules. 

For practical reason, however it is useful to retain the term eluent strength also in polymer HPLC with the nonpolar column packings and for the enthalpic partition retention mechanism. This means that a thermodynamically good mobile phase is termed strong and a poor mobile phase is denominated weak. 

In conclusion, two distinct properties of mobile phases are to be distinguished in polymer HPLC namely their strength toward the coluiim packing and their thermodynamic quality toward both the column packing and the separated macromolecules. 

Pores (mean diameter usually from 6 to 100 nm) play a cmcial role in hplc, as they provide the surface where retention and hence separation occur. The chemical stmcture of the surface determines the thermodynamic environment for the analyte in thin surface layer (stationary phase) and in such a way determines the mechanism of retention. Flexible macromolecules approaching the internal surface of a solid particle change their spatial conformations because of steric interaction, which plays an important role in any mode of polymer chromatography. This interaction restricts the fluctuation motion of a macromolecule, decreases its... 

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