Abstract
The intercalation behavior of various saturated and unsaturated fatty acids into layered double hydroxide (LDH) was investigated by the reconstruction method using Mg-Al-LDH with Mg/Al=3. The fatty acids used were both saturated (butyric acid (number of carbons, NC=4), capric acid, NC=10, lauric acid, NC=12, palmitic acid, NC=16 and stearic acid, NC=18) and unsaturated acids (crotonic acid, NC=4, decenoic acid, NC=10 and oleic acid, NC=18). Intercalation of these fatty acid anions into LDH was confirmed by a shift of the basal reflection at about 2θ=12° toward lower angles, except for crotonic acid. Intercalation became more difficult with fatty acids of smaller NC. At concentrations >1mM the d-spacings of the basal reflections of the saturated fatty acid intercalated samples increased with increasing NC of the fatty acids, from 3.20nm in capric acid to 5.17nm in stearic acid. By contrast, the d-spacings of the unsaturated fatty acid intercalated samples showed no clear trend with the NC values, but were sensitive to the fatty acid concentration, i.e. from 3.55 to 4.85nm in decenoic acid samples and from 3.85 to 4.80nm in oleic acid. The changes in the d-spacings suggest that the fatty acid anions are intercalated as double layers, and then as micelles at higher fatty acid concentrations. The difference in the d-spacing changes between the saturated and unsaturated fatty acids is considered to be resulted from differences in the molecular shape, i.e. the straight configuration of saturated fatty acids compared with the bent configuration of unsaturated fatty acids.
