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 Mg₂AlNO₃, Mg₃AlNO₃, Mg₄AlNO₃, Mg₃AlCl, and Mg₃AlCO₃ 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/m², respectively, and the δ_t,s, δ_d,s, δ_p,s and δ_h,s of the LDHs except for the Mg₃AlCO₃ 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.