Evidence of clustering in an aqueous electrolyte solution: a small angle x-ray scattering study

The work reported here was motivated by a desire to verify the existence of structure specifically MP-rich clusters-induced by sodium bromide (NaBr) in the ternary liquid mixture 3-methylpyridine (MP) + water (W) + NaBr. We present small angle x-ray scattering (SAXS) measurements in this mixture. These measurements were obtained at room temperature (∼ 298K) in the one-phase region (below the relevant lower consolute points, TLs) at different values of X, (i.e., X = 0.02 - 0.17) where X is the weight fraction of NaBr in the mixture. Cluster-size distribution, estimated on the assumption that the clusters are spherical, shows systematic behaviour in that the peak of the distribution shifts towards larger values of cluster radius as X increases. The largest spatial extent of the clusters (∼ 45 A) is seen at X = 017. Data analysis assuming arbitrary shapes and sizes of clusters gives a limiting value of cluster size (∼ 45 A) that is not very sensitive to X. The influence of the additional length scale due to clustering is discussed from the standpoint of crossover from Ising to mean-field critical behaviour, when moving away from the TL. (author)