state for the mixture of this class of surfactants and water. Differential scanning calorimetry (DSC) and Fourier transform The only description concerning this subject is seen in an infrared spectroscopy (FT-IR) were used to construct and characearly publication by Clunie et al. (14) for binary mixtures terize the phase diagram for a binary mixture of heptaethylene of hexaoxyethylene glycol octyl or decyl ether and water, glycol decyl ether (C 10 E 7 ) and water in the temperature range in which they claimed that the surfactant crystal and ice are from 060 to 80ΠC. Plots of the endothermic peak temperatures present separately as different phases in the solid state.
obtained by DSC measurements against compositions provided
It is believed that the solution properties as well as the eutectic solid-liquid phase boundaries with a eutectic composition mesomorphic phase behavior of this class of surfactants are of 34 wt% of H 2 O. On the other hand, heat of fusion per unit largely influenced by the interaction between hydrophilic weight of the mixture changed discretely at the composition corresponding to the ''eutectic'' composition. Furthermore, the IR spec-polyoxyethylene (POE) chains and water. For instance, the tra obtained for the mixture in the solid phase were well reproorigin of the clouding phenomenon has been attributed to a duced as a superposition of those for the mixture of 34 wt% H 2 O more or less sudden dehydration of the POE head group and pure components but were not reproduced by superimposing associated with the increase in temperature (20, 21) and the spectra obtained for the solid surfactant and ice. These obsermore recently to a temperature-dependent enthalpy-entropy vations indicate that a solid phase compound is formed between compensation for the interaction of the head group with C 10 E 7 and water with a stoichiometry of 1:14 and that the com-water . The participation of a water-POE interacpound and pure components exist as separate phases, rather than tion has also been suggested in the mesomorphic phase tranthe phases separating into surfactant and ice, which would be sition (17). Such a strong interaction between the POE chain expected if the C 10 E 7 /water mixture formed a true eutectic mixture and water molecule observed in liquid and liquid-crystalline system. It is estimated from the composition corresponding to the phases of the surfactant/water mixture would also appear in phase compounds that two molecules of water per oxyethylene unit are bound to hydrophilic polyoxyethylene chain of C 10 E 7 to a solid phase. If this is the case, the rather simple view form a hydrated compound. α§ 1997 Academic Press described by Clunie et al. (14) that the surfactant crystal Key Words: heptaethylene glycol decyl ether; nonionic surfacand ice form separate phases in the solid state is questiontant; surfactant/water mixture; phase diagram; phase compound; able, because the phase separation means that no interaction differential scanning calorimetry; Fourier transform infrared specoccurs between the surfactant and water molecules.
troscopy.
From the above point of view, in the present work we investigated the phase behavior of the mixture of heptaethylene glycol decyl ether (C 10 E 7 ) and water with special atten-