Investigation on Molecular Aggregates of Alkylammonium Perfluorocarboxylates in Aqueous Solutions.

Abstract

The aggregation behavior of three alkylammonium perfluorocarboxylates (CnH2n+1NH3+CmF2m+1COO-; n+m=15, n=2,4,6) were investigated by rheology and polarization microscopy. The amphiphiles with n=8,10 were quite stable and hardly soluble in water and could not be studied by the method. The above three amphiphiles formed quite large molecular aggregates in diameter of up to 3μm in the solutions at lower concentration of 1.5 mmol dm-3. At higher concentrations of 50-100 mmol dm-3, on the other hand, the first two amphiphiles formed fine crystallites which were strongly connected to one another, resulting in high viscosity of the solutions, while the third formed stable leaflet-like crystals of larger size depending on the concentration and on the temperature. The high complex viscosity over the concentration range 20-50 mmol dm-3 decreases with increasing the oscillating frequency applied to the solution for the first amphiphile, which results from loosening of strong connections among the crystallites due to increasing shear rate.