Temperature-controlled poly(propylene)glycol hydrophobicity on the formation of inclusion complexes with modified cyclodextrins. A DSC and ITC study

Abstract

The study highlighted the main forces driving the formation of hydroxypropyl-cyclodextrins (HP-CDs) + poly(propylene) glycol 725 g mol⁻¹ inclusion complexes. The temperature parameter was chosen as the variable to modulate the hydrophobicity of the polymer, and consequently ITC experiments as functions of temperature as well as DSC measurements were done in a systematic way. The polymer is not included into HP-α-CD, it is strongly bound to HP-β-CD and it is floating in HP-γ-CD. The stability of the inclusion complexes is entropy controlled. The gain of the entropy is a unique result compared to the opposite literature findings for inclusion complexes based on polymers and CDs. This peculiarity is ascribable to the removal of water molecules from cages during complexation and this effect compensates the entropy loss due to constraints caused by the CD threading. In spite the host–guest van der Waals contacts are optimized, the enthalpies for the inclusion complex formation are positive and reveal the large heat required for dehydrating the propylene oxide units. All the macrocycles enhanced the polymer solubility in water. Increasing the affinity of the macrocycle to the macromolecule makes more expanded the one-phase area of the binodal curve. A new thermodynamic approach was proposed to predict quantitatively the binodal curve as well as the dependence of the enthalpy of separation phase on the macrocycle composition. The agreement between the experimental data and the computed values was excellent.