Enthalpies of mixing a non-ionic surfactant with water and with aqueous solutions of monovalent salts

The enthalpies of mixing n-octyl penta(oxyethylene glycol) (C8E5) with water and with aqueous solutions of the monovalent salts NaCl, KCl, CsCl and NaF have been studied at 298.15 K by calorimetry. For binary and ternary mixtures the heat of reaction was exothermic for all compositions. In the critical micelle concentration (c.m.c.) region, when different amounts of liquid surfactant are mixed with water, the enthalpy of mixing shows a transition to a less steep decrease as a function of the amount of surfactant. Apparent and partial molar enthalpies of the surfactant and partial molar enthalpies of the water are determined in the c.m.c. region and beyond the c.m.c. region. Marked changes in the enthalpic properties of the two components occur in the c.m.c. region. A more regular variation is seen over the whole composition range. In the experiments with salt present in the water the molar ratio of water to surfactant was held constant at different values, viz. 291.84, 60.80 and 12.16. With NaCl, KCl and CsCl the molality was varied between 0.1 and 2.1 m. With NaF the molality was varied between 0.1 and 0.5 m. The observed excess heat of mixing liquid surfactant with the various aqueous solutions was analysed in terms of apparent molar enthalpies of C8E5. An almost linear endothermic shift of the apparent molar enthalpy with the mole fraction of salt in the ternary mixtures was observed. Lines were fitted to the apparent enthalpic data using the method of least squares. The gradients of the lines show that the anion mainly determines the magnitude of the shift, and the magnitude of the shift decreases strongly with the vol.% surfactant.