Characterization of chain molecular assemblies in long-chain, lamellar copper alkylsulfonates: Self-assembled monolayer vs bilayer structure

Direct characterization of chain molecular structures in a series of layered organic/inorganic long-chain alkylsulfonates, Cu(C_nH _2n+1SO₃)₂·yH₂O (CuSO _3-n, where n=10, 12 and y = 4 for group I; n = 14, 16, 18 and y = 2 for group II), using combined application of Fourier transform infrared (FTIR) spectroscopy and powder X-ray diffraction (XRD) is reported. These compounds show a layered structure, as determined by XRD, consisting of alternating organic alkylsulfonate layers and inorganic copper(II) hydrate layers, with interlayer distances of up to 3.82 nm. It is worth noting that the layer-to-layer distance can be easily and rationally metered by choosing alkylsulfonates of different lengths: Shorter alkylsulfonates, group I, yield an interdigitated monolayer structure with shorter periodicity, whereas longer alkylsulfonates, group II, yield a noninterdigitated bilayer structure with longer periodicity. The goal was to correlate the structural changes by increasing the chain length, monitored by XRD, with the alkyl chain structure detected by FTIR spectroscopy. From the FTIR spectra, the distinguishable features between the two groups appear in two different wavenumber regions, i.e., the hydrogen-bonding region (2900-3500 cm^-1) and the sulfonate-stretching region (1000-1250 cm^-1), indicating that the hydrophilic layers of the two groups are completely different. Furthermore, a detailed investigation of the C-H modes, including the stretching, scissoring, and rocking modes, reveals that the chain-packing modes of the two groups are different, even though the alkyl chains in the CuSO_3-n series are all in an all-trans conformational state. The change in the monolayer to bilayer packing with the alkyl chain length, arising from the intermolecular interaction between the alkyl chains, is explained by the rearrangement of the alkyl chains from orthorhombic to hexagonal subcells with decreasing packing density.