Effect of cations and anions on the formation of polypseudorotaxanes

Aqueous solutions of α-, β-, or γ-cyclodextrin form stable and crystallizable inclusion complexes (polypseudorotaxanes) when added to long chain polymers such as PEG, PPG, and alkanols. This process consists of the threading of several cyclodextrin molecules around a single chain, with a kinetic profile that depends on temperature and solvent nature. We have studied the effect of salts on this threading process for a polypropylene glycol derivative with β-cyclodextrin at 25 °C. What emerges is that changes in the threading time are strongly dependent on the nature of both cations and anions. The formation of a polypseudorotaxane is then a process where hydrophobic forces clearly play a dominant role, and in this paper, we show that the diversity of salt effects is related to the microscopic frequency dependent dielectric properties of the ion pair, that account for the dispersion potential experienced by the different ions in solution.