SCATTERING AND OSMOTIC-PRESSURE OF DILUTE D LATEX SOLUTIONS WITH HIGHSURFACE-CHARGE

We measure the osmotic pressure of bromopolystyrene particles suspensi ons in the dilute sol regime (0.5-6.5% in volume fraction) i.e. for av erage distances between particles in the range between 150 to 400 nm. The osmotic pressure varies linearly with the concentration and is abo ut 400 times higher than the perfect gas pressure due to the particles alone. The explanation is that the main contribution to the osmotic p ressure in the absence of salt is due to the (uncondensed) counter-ion s. This observation allows an experimental definition of an effective charge. The measured pressures are in agreement with theoretical value s determined by the non linearized Poisson-Boltzmann Cell model. Struc ture factors of these solutions are obtained by light and ultrasmall a ngle X-ray scattering (USAXS): a strong liquid order is observed. The theoretical curves calculated with the HNC integral equation fit very well this short range order. The effective charge determined by fittin g the shape of the observed structure factor is compatible with the va lue determined directly from the osmotic pressure. Both values are muc h lower than the effective charge determined by pH and conductivity me asurements.