Liquid/liquid phase separation of two-component systems in confined spaces

The phase separation of various triethylamine/water mixtures (mole fraction of triethylamine X) in silica pores was examined by differential thermal analysis and H NMR as a function of the pore size (d). The phase-separation temperature in confined systems shifted by /DeltaT/ to a lower temperature than that in the bulk phase, /DeltaT/ increased from zero for small pores to a maximum with increasing d, followed by a decrease according to the inverse power of the pore size. The pore size, giving rise to the maximal /DeltaT/, increased from 18 to 40 nm with increasing X from 0.1 to 0.4. This d-dependence in /DeltaT/ corresponds very much to those of the self-diffusion coefficient (D) of water in a confined mixture at 298 K (one-phase region). The smaller is D, the larger is /DeltaT/. This suggests that the translational molecular motion or the composition fluctuation as well as the selective adsorption of water should be related to the phase separation in confined spaces.