Sol-gel transitions in aqueous suspensions of synthetic takovites. The role of hydration properties and anisotropy

To reach a better understanding of the behavior of suspensions of charged c olloidal platelets, takovite samples with a general formula Ni1-x, Al-x(OH) (2), An(x/z)(z-), yH(2)O, where An is an interlayer anion, were synthesized . These materials belong to the family of layered double hydroxides (also r eferred to as anionic clays). They can be easily prepared in the laboratory by coprecipitating mixed nickel and aluminum salts by NaOH. Their charge c an be adjusted by changing the synthesis conditions, while the hydration pr operties can be modified by using various interlayer anions. On the basis o f oscillatory measurements, rheological phase diagrams were obtained by plo tting the sol-gel transition as a function of solid content and ionic stren gth for three monovalent anions: F-, Cl-, and NO3-. In all cases, the phase diagrams exhibit a negative slope, i.e., the sol-gel transition shifts tow ard lower solid contents for increasing ionic strength. The nature of the i nterlayer anion has a strong influence, the sol-gel line moving toward high er solid content in the order takovite F- < takovite Cl- < takovite NO3-. T hese differences cannot be assigned to changes in the surface charge of the samples as zeta potential measurements reveal a constant charge for the th ree anions. Postsynthesis hydrothermal treatments were then performed in or der to modify the size and anisotropy of takovite NO3-. The aspect ratio of the particles appears as a key parameter controlling gel formation as it h as a very pronounced effect on the rheological phase diagrams. Particles wi th higher aspect ratio exhibit a sol/gel transition at lower solid contents and with a reduced dependence on ionic strength.