FATTY-ACIDS IN LAYERED METAL-HYDROXIDES - MEMBRANE-LIKE STRUCTURE AND DYNAMICS

Layered metal hydroxides consist of sheets of positively charged edge-sharing M(OH)6 octahedra as exemplified by the hydrotalcite family of materials. Anions occupy positions in the interlayers. In this study, we focus on the incorporation of long-chain fatty acids (CH3(CH2)nCOOH, n = 10, 12, 14) in between these layers. These materials mimic membranes and exhibit interesting packing and dynamic characteristics. The packing is strongly influenced by the charge distribution of the metal hydroxide layer. In the case of LiAl2(OH)6+ (25 angstrom2/unit charge), a monomolecular film is formed, whereas a bimolecular film is formed in Mg3Al(OH)g+ (33 angstrom2/unit charge). In the lithium aluminum hydroxide, the carboxylic acid is ion-exchanged into the layers, but in magnesium aluminate it is intercalated as the carboxylic acid form. On the basis of diffraction, calorimetry, and vibrational spectroscopy, the dynamics of the films as a function of temperature has been explored. For LiAl2(OH)6+, the spacing between the layers increases with temperature and retains an all-trans packing of the alkyl chains. In the Mg3Al(OH)g+ sample, the spacing between the layers decreases with temperature and the bimolecular film becomes disordered through the formation of kinks and gauche blocks at temperatures exceeding 130-degrees-C. The control of packing density and dynamics of the alkyl chains via the charge density of the metal hydroxide layer results in the formation of novel, membrane-like materials.