Volumetric and fluorescence studies of aqueous solutions containing n-octylamine, cetyltrimethylammonium bromide, and salt

Densities of aqueous solutions containing n-octylamine, 0.0500 mol kg(-1) c etyltrimethylammonium bromide (CTAB), and sodium chloride/potassium chlorid e have been determined and used to derive the standard partial molar volume s of n-octylamine at 298.15 K. The effect of the additives (sodium chloride , potassium chloride, and n-octylamine) on the polarities of the micelles h ave also been investigated by static fluorescence spectra using pyrene as a probe at 298.15 K. It has been observed that the postmicellar transitions of sphere-to-rodlike, as indicated by the amplified changes in the standard partial molar volumes of n-octylamine, are affected strongly by the additi on of an electrolyte. The effect of sodium chloride on the postmicellar tra nsition of CTAB is much weaker than potassium chloride. Polarity of the int erior of the mixed micelle and therefore the solubilized site of pyrene var y with molality of the additives. This is considered to be caused by the di fference in chemical structure and hydrophobic nature of the mixed micelle. The results of fluorescence spectra reveal the possible microscopic causes for the effect of additives on the postmicellar transition of CTAB.