Effect of salt on the micelles of cetyl pyridinium chloride

Surface tension, conductance, viscosity, dynamic light scattering (DLS) and small angle neutron scattering (SANS) measurements were carried out in order to investigate the effect of added salt NaCl and NaBr on micellization and structure of cetylpyridinium chloride (CPyCl) micelles. Critical micelle concentration (CMC) of CPyCl decreases with increase in salts concentration, while CMC and counter ion dissociation (β) increases with increase in temperature. At a given CPyCl concentration, the micelle size increases with increasing salt concentration above a threshold value. Micellar growth in presence of salts is clearly reflected by an increase in viscosity, hydrodynamic diameter (d) from DLS and aggregation number from SANS measurements. The effect of NaBr on CPyCl micelles was more pronounced in comparison to NaCl. The difference in solution properties of Br− and Cl− is that Cl− counter ions being more hydrated are less effective to neutralize the charge on the micellar surface.