Temperature effects on adsolubilization of aromatic solutes partitioning to different regions in cationic admicelles

This study examines the effect of temperature on the adsolubilization partition coefficient of sparingly soluble polar solutes. The adsolubilization of polar solutes, i.e. diphenylmethane, diphenylether, 4-bromotoluene and p-tolunitrile into cetylpyridinium chloride admicelles adsorbed on precipitated silica at pH of 8 were examined. The investigated adsolubilization temperatures were 20, 35, 50 and 65 °C. It was found that the admicelle/aqueous partition coefficients for the solutes are governed by molecular packing density in the admicelles, solute–surfactant headgroup interaction and the aqueous solubility of the solute. As temperature increased, the loosening molecular packing and increasing solute–surfactant headgroup attraction enhances the ability of the solutes to partition into the admicellar phase, increasing the partition coefficient. On the other hand, increasing temperature increases the aqueous solubility of the solute, which would reduce the partition coefficient. For slightly polar solutes, e.g. diphenylmethane and 4-bromotoluene, the partition coefficient tended to decrease as temperature increased, implying that the impact of increasing aqueous solubility plays the dominant role. For the more polar diphenylether, the impact of increasing solubility appears to be offset by strong attractive force between solute and surfactant headgroups, resulting in nearly unchanged partition coefficient values even with increasing temperature. For the highly polar solute p-tolunitrile, the partition coefficient initially decreased with increasing temperatures in the examined range (thus the solubility effects dominated) and then increased at higher temperatures in the examined range, suggesting that looser molecular packing and the solute–surfactant attraction dominate here.