A 3-D phase diagram of the HPC/H2O/H3PO4 tertiary system against various temperatures was established. Four distinct phases—the completely separated phase (S), the cloudy suspension phase (CS), the liquid crystalline miscible phase (LC), and the isotropically miscible phase (I)—were identified. The S phase shrank as the temperature increased, revealing that the HPC solubility increased with temperature, regardless of the LCST (lower critical solution temperature) characteristic. The addition of H3PO4 suppressed the formation of LC phase. However, as the temperature was raised sharply from 50 to 70 °C, the LC phase could only be maintained at high H3PO4 concentration region; it was a triangular shape, and the top apex of the triangle was the temperature-invariant L* point (HPC/H2O/H3PO4 38/9/53 wt%). The CS phase expanded considerably into the H2O-rich but H3PO4-poor region when the temperature continued to increase over 48 °C. The LCST points of the CS phase that contained 0 and 15 wt% of H3PO4 were 34 and 38 °C, respectively. These CS results demonstrate that H3PO4 suppresses the occurrence of LCST behavior. Additionally, the binodal curve exhibits a weak or even zero dependence of binodal temperature on the HPC concentration at HPC concentrations of less than 30 wt% in a pure water system. A hypothesis concerning the sequential desorption of water molecules was proposed to explain such behavior.
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The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract no. NSC_98-2221-E-027-003-MY3. Ted Knoy is appreciated for his editorial assistance.
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