Experimental determination of effective charges in aqueous suspensions of colloidal spheres

We determined the low frequency conductivity σ, the phase behaviour and the shear modulus G of colloidal fluids, respectively solids prepared from deionised aqueous suspensions of highly charged spherical particles. Conductivity measures the number of freely moving small ions Z*σ and thus relates to the ion condensation process in the electric double layer under conditions of finite macro-ion concentrations. Phase behaviour and elasticity data are consistently described by a Debye–Hückel pair potential assuming pair-wise additive macro-ion interactions. Like Z*σ, also the effective charges Z*G derived from the elasticity data scales with the ratio of macro-ion radius to Bjerrum length. Z*G is, however, smaller than Z*σ. These findings are discussed in terms of recent experimental and theoretical results which suggest a reduction of the charge by both counter-ion condensation and many body terms in the effective particle interaction at finite concentration.