Estimation of surface free energy and solubility parameters of MgAl layered double hydroxides.

The characteristic parameters, surface free energy (γ) and solubility parameters (δ), of Mg ₂ AlNO ₃ , Mg ₃ AlNO ₃ , Mg ₄ AlNO ₃ , Mg ₃ AlCl, and Mg ₃ AlCO ₃ layered double hydroxides (LDHs) were estimated using contact angle and liquid phase exfoliation (LPE) methods. The γ parameters include total (γ _s ^t ), dispersive (γ _s ^d ), and polar (γ _s ^p ) ones. The δ parameters include the Hildebrand parameter (δ _t,s ) and the Hansen dispersive (δ _d,s ), polar (δ _p,s ), and hydrogen-bonding (δ _h,s ) ones. Results show that the γ _s ^t , γ _s ^d , and γ _s ^p of the five LDHs are ∼50.9-56.9, 35.2-39.4, and 15.6-18.8 mJ/m ² , respectively, and the δ _t,s , δ _d,s , δ _p,s and δ _h,s of the LDHs except for the Mg ₃ AlCO ₃ LDH are ∼31.4-32.9, 17.2, 9.5-10.0, and 24.5-26.1 MPa ^1/2 , respectively. The compositions of LDHs have no obvious impact on all the characteristic parameters. In comparison with van de Waals attraction-bonded materials, the electrostatic attraction-bonded LDHs exhibit higher γ _s ^t and δ _t,s values and, especially, much larger δ _h,s values. The characteristic parameter matching principle and the mixed-solvent strategy are suitable for the LPE of LDHs. Non-toxic glycol/PEG200 mixed liquids are found to be a good medium for the direct LPE of LDHs, producing monolayer nanosheets. To the best of our knowledge, this is the first report on the γ and δ parameters of LDHs. This work not only deepens the understanding of the feature of LDHs, but also provides effective liquid media for the direct LPE of LDHs.
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