Phase Behavior of Aqueous Mixtures of Some Polyethylene Glycol Decyl Ethers Revealed by DSC and FT-IR Measurements

Abstract

Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR) were used to investigate the phase behavior of aqueous binary mixtures of three polyoxyethylene type nonionic surfactants, penta-, hexa-, and octaethylene glycol decyl ethers (C10E5, C10E6, and C10E8, respectively) in the temperature range −40–70°C. Many thermotropic transitions among various phases assumed by these mixture systems, including mesomorphic phases, were detected in DSC thermograms, from which theT–Xphase diagrams were constructed with the aid of polarized optical microscopic observation to characterize the mesomorphic phases. It was revealed by FT-IR measurements that the phase compounds are formed in a solid phase between these surfactants and water molecules; the compositions of the compounds for the three surfactant species are expressed as C10E5· 10H2O, C10E6· 12H2O, and C10E8· 16H2O, respectively. The stoichiometry of the phase compounds clearly demonstrates that just two water molecules are bound per oxyethylene unit of polyoxyethylene (POE) chain of the surfactants in the solid phase, probably due to the hydrogen bonding. It was also found that the phase state of the mixture is reflected in the wavenumber of the absorption maximum associated with the OH stretching vibration, νOH, although no significant difference was appreciable in νOHamong mesomorphic phases. The νOHdecreased in the orderL(liquid phase) >H1(normal hexagonal phase) ≈V1(normal cubic phase) ≈Lα(lamellar phase) > solid phase, which indicates that the hydrogen bonding between water and the POE chain of the surfactants becomes stronger in the sequence liquid < mesophase < solid.