Phase Behavior of Heptaethylene Glycol Dodecyl Ether and Its Aqueous Mixture Revealed by DSC and FT-IR Spectroscopy

The phase behavior of heptaethylene glycol dodecyl ether (C<INF>12</INF>E<INF>7</INF>) and its aqueous mixture was investigated by means of differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectroscopy in the temperature range from −20 to 70 °C. Phase boundaries among various phases including mesomorphic phases were determined on the basis of DSC thermograms, from which the binary phase diagram of this mixture system was constructed. According to the phase diagram, the mixture with the composition of 31 wt % H<INF>2</INF>O exhibits the following phase sequence with increasing temperature: solid → H<INF>1</INF> (normal hexagonal) → V<INF>1</INF> (normal bicontinuous-type cubic) → L<INF>α</INF> (lamellar) → liquid. FT-IR measurements for the mixture with this composition revealed the following features concerning the conformational structure of the C<INF>12</INF>E<INF>7</INF> molecule and the interaction between the surfactant and D<INF>2</INF>O in each phase. In the solid phase, the polyoxyethylene (POE) chain of the surfactant molecule has a rather extended structure with a trans-rich conformation, in contrast to the case of pure C<INF>12</INF>E<INF>7</INF>, in which a helical structure is dominant. When the solid phase transforms to the H<INF>1</INF> phase, the hydrogen-bond interaction between C<INF>12</INF>E<INF>7</INF> molecules and between C<INF>12</INF>E<INF>7</INF> and water are both weakened, and the fraction of gauche-conformer in the POE chain increases, whereas the alkyl chain adjacent to the POE chain remains, taking a rather trans-zigzag conformation. The enhanced fraction of gauche-conformer in the POE chain suggests that the POE chain has a helical structure in this phase. The increase in temperature within the H<INF>1</INF> region causes a conformational change in both the POE chain and the alkyl chain of the surfactant to a more disordered structure. Among the phases of V<INF>1</INF>, L<INF>α</INF>, and liquid, no significant difference is seen in the conformational structure and interaction with water molecules of the surfactant.