Reactive perfluoropolyoxyalkylene oligomers. II. Isothermal equilibrium on silica gel: Macromolecular aggregation

• We examine anomalies observed during the adsorption/desorption process of reactive fluorinated oligomers onto silica gel.
• Over-aggregation inside the pores justifies the apparent irreversibility of the adsorption of bifunctional species.
• Silica activity collapses at high concentration because of the formation of clusters larger than pore diameter.
• Monofunctional species, for which bulky cluster formation is precluded, do not show any anomaly.

It has been demonstrated in a previous work that the adsorption/desorption equilibrium of mono- and bis-methylol terminated perfluoropolyether (PFPE) oligomers is described very well by the classic Langmuir model, though the systems under investigation were not perfectly single-component. More precisely, it has been determined that the affinity vs. the solid phase (silica gel) increases proportionally with the number of functional groups per molecule, while the increase in the molecular weight (i.e. the length of the PFPE chain) exerts an opposite effect. The present study, focused on an in-depth understanding of some anomalies observed during the adsorption/desorption process, indicates that the aggregation in solution of these functional oligomers plays a very important role by ruling the dimension of the real species which are under dynamic equilibrium with the silica gel's active sites. The hypothesis that an over-aggregation is promoted, when oligomers confined inside the pores are considered, adequately describes the apparent irreversibility of the adsorption of bifunctional oligomers. Moreover, experimental evidences indicate that there is also a limiting concentration of di-functional species in a non-confined solution, above which the Langmuir model progressively loses its validity, and the activity of the solid phase progressively diminishes. This unexpected result may be explained on a basis of a similar phenomenology. These two peculiar effects show a different dependence on the concentration, being over-aggregated pores active even at relatively low concentrations, confirming that huge size confinement boosts over-aggregation of solutes. Consistently, monofunctional species that can form only smaller aggregates behave as expected on the basis of the classic Langmuir model without any anomaly.

Apparent irreversibility of the adsorption of bifunctional oligomers and progressive loss of efficiency of the silica gel: over-aggregation inside the pores (“size confinement effect”) and clusters formation at high concentration level (“size exclusion effect”).Figure optionsDownload as PowerPoint slide