New phase behavior of n-undecane–tridecane mixtures confined in porous materials with pore sizes in a wide mesoscopic range

Abstract

Solid–liquid phase behaviors of normal alkane mixtures, undecane–tridecane (n-C11H24–C13H28, C11–C13), in bulk and adsorbed in SBA-15 and controlled porous glass (CPG) are investigated using differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). The bulk C11–C13 mixtures display multiple phases due to the special rotator phase RI. In very small pores of SBA-15 (pore diameter d, 3.8 nm), the mixtures exhibit a curved melting boundary on heating. Inside SBA-15 (8.9 nm), the solid mixtures can form two-phase regions of mixed orthorhombic crystals with C11 and C13 packings (O11 + O13), and (RI + O13), which might include a possible RI phase in a narrow temperature range before melting. The solid mixtures confined in CPG (8.1 nm) also display two-phase regions (O11 + O13) and (RI + O13) and a possible RI phase in a small gap. The phase diagrams of the mixtures confined in SBA-15 (17.2 nm), CPG (46.4, and 300 nm) resemble to some extent that of the bulk system and appear in the close temperature range as well. The pore size exerts significant influence on the phase behavior of the C11–C13 mixtures under confinement. From the XRD analysis, the pore solid alkane molecules show effectively 2D arrangements confined in pores of diameter less than 20 nm while the lamellar ordering recovers in large pores of CPG (46.4, and 300 nm). The pore geometries of SBA-15 and CPG, the quasi one-dimensional vs. 3D connected network, also impact greatly on the phase behavior of the pore C11–C13 mixtures. Depression of the melting points and solid–solid transition temperatures of the pure C11 and C13 follows the Gibbs–Thomson equation confined in SBA-15, while it needs an extra term of d−2 in addition to the equation under confinement of CPG.