Two-dimensional melting of alkane monolayers ionically bonded to mica

The structure and phase transitions of an alkyl monolayer tethered to a mica surface have been studied by X-ray, infrared (IR) spectroscopy, and differential scanning calorimetry. All results indicate that the alkyl chains attend an all-trans conformation at low temperatures, leading to a two-dimensional crystalline film that undergoes a first-order transition to an isotropic liquid upon heating. Although the molecules are fixed to the surface at one end, which restricts their translational freedom, they undergo a melting process. It seems that the trans-gauche transformation is enough to destroy the two-dimensional lattice. A model in which the molecules assume a tilted upright position to the mica surface can explain the results obtained. To minimize the conformational entropy and maximize the packing density of the molecules, the chains attached to different mica platelets interdigitate to build an organic interlayer. A crystal-crystal transition was also observed, which led to an increase in the thickness of the organic interlayer. This increase was attributed to a change in the tilt angle of the chains to the mica surface. The phase behavior of the alkyl monolayer is quite similar to that of bulk alkanes but the transition temperatures are higher probably because of the chain end fixation. The solid-liquid transition influenced the IR spectrum but not the film thickness, whereas the solid-solid transition did the contrary. However, both phase transitions were observed in the differential scanning calorimetry. It was possible to differentiate between bonded and intercalated molecules by thermogravimetric analysis, because their decomposition temperatures were different.