Nanorheology of dioctyl phthalate confined between surfaces coated with long alkyl chains.

To shed light on the mechanism related to the high viscosity of a precipitated calcium carbonate (PCC) dispersion in dioctyl phthalate (DOP), the viscosity of DOP in a nanometer space was investigated using the shear resonance measurement. We used mica surfaces modified with dioctadecyldimethylammonium bromide (DODA) as a model of a PCC surface which bears long alkyl chains and bare mica surfaces as a reference. We found that a resonance peak for DOP of high water content (1164 ppm) confined between the DODA-modified surfaces gradually decreased upon approach from a distance of 320.6 nm and disappeared at ca. 57 nm. This indicated that the viscosity of DOP increased with decreasing distance, and a highly ordered state appeared at a large separation of 57 nm. This highly ordered state, however, cannot be observed for DOP of low water content (469 ppm) confined between the DODA-modified surfaces. The resonance peak for DOP of high water content (1164 ppm) between bare mica surfaces gradually decreased upon approach from a distance of 358.4 nm and disappeared at 10.7 nm, which is much shorter than the value for the DODA-modified surfaces. These results could be correlated with the high macroscopic viscosity of a dispersion of PCC coated with long alkyl chains in DOP. We propose the interpenetration of the long alkyl chains and the hydrogen-bonding network of DOP molecules mediated by water molecules as a plausible model for interpreting the high viscosity of coated PCC dispersions.