Colloidal behavior of cobalt monooleate soap in apolar organic solvents

Abstract

A new type of metal soap-cobalt hydroxy monooleate soap, exhibiting an extraordinary thickening ability in apolar organic solvents, has been prepared using the metathesis method. Viscometric measurements showed that cobalt soap solutions have an unusual rheological behavior: (1) The soap solutions revealed a marked non-Newtonian behavior. (2) The extremely high intrinsic viscosity value (=5500 cm 3 g −1) indicated the formation of long polymer chains by the soap molecules. (3) The presence of additives such as acetylacetone and pyridine greatly reduced the solution viscosity, even to the same level as the solvent alone. By using static light-scattering intensity measurements, we determined the weight average molecular weight of soap polymer chains ( M w = 3.41 × 10 6 g mole −1) and the z-averaged radius of gyration ( 〈R g 2〉 z 1 2 = 320 nm ) at very low soap concentrations. By using the transient electric birefringent technique we also determined the rotational diffusion coefficient of the soap chains ( D r = 32 ± 3 s −1) at infinite dilution. A wormlike chain model with a persistence length of 108 ± 8 nm for soap polymer chains has been established. If we take into account chain flexibility in the Doi-Edwards theory of rotational diffusion coefficient of rodlike polymers in the semidilute regime, D r has a 1/( c 2 L e 9) dependence where L e is an effective chain length. Based on these physicochemical measurements, the colloidal behavior of cobalt monosoap in apolar media could be ascribed to the formation of polymer chains through the OH bridging between the cobalt atoms. An open-type mechanism of the soap association process is being proposed to account for the supramolecular formation.