Phase Separation and Surface Tension of Aqueous Hydrochloric Acid Solutions of N-Dodecyl-2-pyrrolidone

Abstract

Although N-dodecyl-2-pyrrolidone(D. P.) is almost insoluble in pure water, it dissolves clearly in aqueous solution of hydrochloric acid(HCI). The lowest concentration of HCI in which D. P. can clearly dissolve depends upbn temperature. It was found that there are two kinds of temperature of phase separatlon at a given concentration of HCI: the one is the temperature above which twophases appear, similarly to the cloud point observed for aqueous solution of nonionic surfactants; the other is the temperature below which two phases appear, similarly to the Krafft point observed for aqueotis solution of ionic surfactants.
It was concluded that D. P. in water forms N-dodecyl-2-pyrrolidone hydrochloride(D. P. HCl) in the presence of HCl, and gives a clear solution because of the hydroyphilic character of the latter. When the temperature is higher than the"cloudl point", phase separation occurs, with D. P. precipitating. These relations are shown in Fig.2. The enthalpy change of the hydroclhoride formation is calculated to be -0.88kcal/mol.
Surface tension of aqueous sblutionof D. P. was measured at various concentrations of HCl, and the results are shown in Fig.3. The adsorption of D. P. ontothe surface were examined on the basis of Langmuir s equation as shown in Fig.6. Criticahnicelle concentration(CMC), surface tension at CMC(γ cmc)and surface area of adsorbed molecule(A) changed remarkably with concentration of HCl near the "cloud point", as shown in Fig.7. It is, therefore, concluded that D. P. is changed to D. P. HCl near the "cloud point".