Structure and thermal behavior of a layered silver hydroxyalkanecarboxylate

The structure and thermal behavior of silver 16-hydroxyhexadecanoate (Ag-HHDA) have been investigated by using various analytical tools. The X-ray diffraction pattern was composed of a series of peaks that could be indexed to (0k0) reflections of a layered structure. Diffuse reflectance infrared Fourier transform spectroscopy revealed that the alkyl chains in Ag-HHDA as prepared were in an all-trans conformational state. Upon heating the sample, noticeable structural changes took place particularly in two temperature regions. The first structural change occurring at approximately 380 K was a partially irreversible one in which the binding state of carboxylate to silver converted from bridging into unidentate. A second dramatic structural change occurring at approximately 480 K was a totally irreversible process that could be associated with the decomposition of Ag-HHDA. All of these thermal characteristics of Ag-HHDA are comparable to those of silver stearate (Ag-STA). Separately, we have endeavored without success to intercalate polar molecules into the OH-group terminated layers in Ag-HHDA; the exfoliation of Ag-HHDA in various polar solvents was also unsuccessful. This is indicative of the presence of a rather stronger H-bond in Ag-HHDA, but the comparable thermal characteristics of Ag-HHDA and Ag-STA dictate that the thermal behavior of silver alkanoate is determined exclusively by the silver-to-carboxylate group interaction. This is in sharp contrast to the case of two-dimensional self-assembled monolayers for which the terminal functionalities play a crucial role in determining the structure and thermal stability of entire monolayers.