Estimation of surface free energy and solubility parameters of Mg-Al layered double hydroxides

The characteristic parameters, surface free energy (gamma) and solubility parameters (delta), of Mg2Al-NO3, Mg3Al-NO3, Mg4Al-NO3, Mg3Al-Cl, and Mg3Al-CO3 layered double hydroxides (LDHs) were estimated using contact angle and liquid phase exfoliation (LPE) methods. The gamma parameters include total (gamma(t)(s)), dispersive (gamma(d)(s)), and polar (gamma(p)(s)) ones. The delta parameters include the Hildebrand parameter (delta(t,s)) and the Hansen dispersive (delta(d,s)), polar (delta(p,s)), and hydrogen-bonding (delta(h,s)) ones. Results show that the gamma(t)(s), gamma(d)(s), and gamma(p)(s) of the five LDHs are similar to 50.9-56.9, 35.2-39.4, and 15.6-18.8 mJ/m(2), respectively, and the firs, delta(d,s), delta(d,s) ,delta(p,s)and delta(h,s) of the LDHs except for the Mg3Al-CO3 LDH are similar to 31.4-32.9, 17.2, 9.5-10.0, and 24.5-26.1 MPa1/2, 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 gamma(t)(s) and delta(t,s) values and, especially, much larger delta(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 gamma and delta 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. (C) 2019 Elsevier Inc. All rights reserved.