Templating Effects of Stearate Monolayer on Formation of Mg−Al-Hydrotalcite

The effect of a floating stearate monolayer on the formation of Mg−Al-hydrotalcite (Mg−Al-LDH) has been studied. A subphase was an aqueous solution of Mg(NO 3 ) 2 ·6H 2 O (1.6 × 10 - 3 M) and Al(NO 3 ) 3 ·6H 2 O (5.3 × 10 - 4 M) adjusted at pH = 10.5 by 1.0 M NaOH. Templating effects were studied in the following two ways. First, we spread a chloroform solution of stearic acid onto the above subphase and waited 6 h at zero surface pressure (0.51 nm 2 molecule - 1 ). After the surface was compressed to 20 mN m - 1 (0.26 nm 2 molecule - 1 ), the floating film was deposited onto mica as a Z-type film (method 1). Second, we spread the same chloroform solution onto the subphase and started to compress the surface to the molecular area (0.40−0.20 nm 2 molecule - 1 ) after 30 min. The floating film was maintained at the constant surface pressure for 6 h. Thereafter, it was deposited onto mica as a Z-type film (method 2). From the atomic force microscope (AFM) images of the films, it was concluded that the largest thin crystals (ca. 3 × 10 μm) with the thickness of 10 ± 1 nm were obtained according to method 2 when the deposition was done at 0.36 nm 2 molecule - 1 (0.78 mN m - 1 ). This optimum molecular area was close to the area occupied by one negative charge for 3:1 Mg−Al-LDH (0.34 nm 2 ). From the X-ray diffraction measurements and elemental analyses, the deposited films prepared by both methods were suggested to be Mg−Al−CO 3 2 - -LDH. As a comparison, a 0.1 mL portion of the subphase solution, which had been aged for 6 h, was cast onto mica and dried under the air. The AFM image of such a cast sample showed noncrystalline aggregates of small particles with the diameter of 0.2−3 μm. These results indicated that a stearate monolayer acted as a template for the crystallization of Mg−Al-LDH at an air−water interface.