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 (∼298 K) 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 (∼4.5 nm) is seen at X=0.17. Data analysis assuming arbitrary shapes and sizes of clusters gives a limiting value of cluster size (∼4.5 nm) that is not very sensitive to X. It is suggested that the cluster size determined may not be the same as the usual critical-point fluctuations far removed from the critical point (TL). 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.