Thermodynamics of Polyethylene Glycol Intrusion in Microporous Water

Polymers can be used to augment the properties of microporous materials, affording enhanced processability, stability, and compatibility. Manipulating polymers to target specific properties requires detailed knowledge of how different polymer and microporous materials interact, including the factors that determine whether a given polymer intrudes into framework micropores or remains on the external particle surface in different environments. Here, we report a study of the thermodynamics of polyethylene glycol (PEG) intrusion into a representative hydrophobic zeolite (silicalite-1) and metal–organic framework [ZIF-67; Co(2-methylimidazolate)2] in water, both of which can be formed into colloidally stable aqueous dispersions—termed "microporous water"—with dry, guest-accessible pore networks. Through a combination of O2 carrying capacity measurements and isothermal titration calorimetry (ITC), we establish relationships between PEG intrusion behavior, polymer length, polymer end groups, and the structure of the microporous framework. In particular, we find that PEG intrusion is exothermic for silicalite-1 but endothermic for ZIF-67. Our results provide fundamental insights into polymer intrusion in microporous materials that should inform efforts to design composite solids and fluids with enhanced functionality.