Hydrogen-Bonded Dimer Dissociation under MOF Nanoconfinement Enables Thermal Transition Tailoring of Medium-Chain Fatty Acids

Significant modulation of phase transition temperatures (<i>T</i>_tr) in organic phase-change materials (PCMs) remains a long-standing challenge due to their intrinsically stable intermolecular interactions. We provide direct evidence that metal-organic framework (MOF) nanoconfinement and host-guest interactions dissociate hydrogen-bonded dimers of medium-chain fatty acids into monomeric species. These monomers engage in site-selective hydrogen bonding with the μ₃-O sites in UiO-66, reconstructing thermodynamic pathways of medium-chain fatty acids and enabling significant <i>T</i>_tr control. Specifically, the melting point of lauric acid (C₁₂) confined in UiO-66 is reduced from 44 to 2 °C, and that of capric acid (C₁₀) is reduced from 28 to -18 °C, exceeding the modulation by conventional porous hosts. In contrast, longer-chain fatty acids, such as myristic acid (C₁₄) and palmitic acid (C₁₆), retain dimeric structures and exhibit limited change in <i>T</i>_tr, revealing a chain-length-dependent, selective molecular recognition within the MOF pores. Our findings demonstrate a supramolecular approach to modulate phase-transition behavior by controlled dissociation of hydrogen-bonded dimers under nanoconfinement, offering a molecular-level strategy for designing advanced thermal energy storage materials.