Solvent role in polymer adsorption

COHEN STUART ET AT. Solvent Role in Polymer Adsorption... 

The Role of the Solvent in Polymer Adsorption Displacement and Solvency Effects... 

The role of the solvent in polymer adsorption has been the subject of much discussion. For example, theories have made predictions about the effect of the polymer/solvent interaction (i.e. Flory Huggins x parameter) on adsorption. For many systems, x parameters had already been tabulated so that a number of adsorption studies focused attention on this parameter. In spite of much effort, available data are ambiguous, sometimes verifying and sometimes contradicting the trends predicted by theory. 

Such displacement effects, although often very pronounced, have not yet been studied systematically. They will be the subject of the present paper. We will discuss the adsorption of polymer from a mixture of two solvents and we will see that in some cases drastic effects occur as a function of the mixture composition. Also, we explore some consequences and practical applications of displacement. It turns out that displacement studies not only increase our insight on the role of the solvent in polymer adsorption but can also be used to determine the segmental adsorption energy. So far, experimental data for this quantity were very scarce. Some illustrative experiments will be discussed briefly. 

The role of the solvent Is very Important In the adsorption of polymers on solid substrates, and the Importance of the acid-base considerations In polymer adsorption from solvents has been emphasized recently (24) So for good adhesion, the polymer should have the right acid-base characteristics with respect to the substrate, and should cover the maximum area of the substrate. 

The exact role of the solverrt thermodynamic quality in the polymer adsorption is not well rmderstood though numerous experiments indicate that it plays by far less important role than the solvent strength. Therefore, polymer adsorption in the chromatographic columns is in practice almost exclusively controlled by adjustment of eluent strength. 

A logical division is made for the adsorption of nonelectrolytes according to whether they are in dilute or concentrated solution. In dilute solutions, the treatment is very similar to that for gas adsorption, whereas in concentrated binary mixtures the role of the solvent becomes more explicit. An important class of adsorbed materials, self-assembling monolayers, are briefly reviewed along with an overview of the essential features of polymer adsorption. The adsorption of electrolytes is treated briefly, mainly in terms of the exchange of components in an electrical double layer. 

The process of polymer adsorption is quite different in many aspects from that of small molecules the latter has been studied extensively in the past. These differences in their adsorption characteristics arise in turn from the obvious flexibility of the larger polymer molecules, so that in addition to the nsnal adsorption factors considered, such as the adsorbate-adsorbent, adsorbate-solvent, and adsorbent-solvent interactions, a major aspect to be understood is the conformation of polymer molecnles at the interface and its role in dispersion. Polymers have a large number of functional groups, each of which can potentially adsorb at the surface, whereas smaller molecules are mostly monofunctional. 

This idea of a favorable solvation sheath is in agreement with observations made during preferential adsorption studies in ternary systems exhibiting cosolvency. Maximum coil expansion is usually found to occur at a mixed solvent composition where there is not preferential adsorption. In other words, the driving force is an attempt to maintain the most favorable composition, thereby minimizing polymer-polymer contacts. However, the balance of interactions giving rise to cosolvency and to inversion in preferential sorption are different, so that both phenomena have to be studied separately. This has been exemplified by a series of systems in which the molecular sizes of the liquid solvents and the nature of their interactions are varied and each plays its role in determining maximum sorption and inversion in preferential sorption. 

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. 

The role of mobile phase in the adsorption and enthalpic partition processes is substantially different. As explained, solvent quality toward macromolecules only marginally affects their adsorption on a given surface. Solvent strength controls the adsorption based retention of polymer species in the particular column pack-... 

---