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 MgAlNO, MgAlNO, MgAlNO, MgAlCl, and MgAlCO layered double hydroxides (LDHs) were estimated using contact angle and liquid phase exfoliation (LPE) methods. The γ parameters include total (γ), dispersive (γ), and polar (γ) ones. The δ parameters include the Hildebrand parameter (δ) and the Hansen dispersive (δ), polar (δ), and hydrogen-bonding (δ) ones. Results show that the γ, γ, and γ of the five LDHs are ∼50.9-56.9, 35.2-39.4, and 15.6-18.8 mJ/m, respectively, and the δ, δ, δ and δ of the LDHs except for the MgAlCO LDH are ∼31.4-32.9, 17.2, 9.5-10.0, and 24.5-26.1 MPa, 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 and δ values and, especially, much larger δ 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.