Phase behavior of dodecane—tetradecane binary system confined in SBA-15

Abstract

Phase behavior of dodecane–tetradecane (n-C₁₂H₂₆–C₁₄H₃₀, n-C₁₂–C₁₄) binary system in bulk and confined in SBA-15 (pore diameters 8 nm; 15.9 nm) has been investigated by differential scanning calorimetry and transmission electron microscopy. The bulk system possesses some special phases relating to the rotator phase in normal alkanes. Dodecane–tetradecane mixtures confined in SBA-15 (8 nm) are a system miscible both in solid and liquid states with a phase diagram of a smooth curve. Dodecane–tetradecane system confined in SBA-15 (15.9 nm) exhibits not only solid–liquid (s–l) in all compositions but solid–solid transition in mole fractions of tetradecane 0.1–0.6, which forms a phase diagram of “loop line” shape. Melting temperatures of n-C₁₂–C₁₄/SBA-15 (8 nm) are lower than those of n-C₁₂–C₁₄/SBA-15 (15.9 nm) in all mole fractions. The evolution of the phase diagram of n-C₁₂–C₁₄ confined in 8 nm, 15.9 nm pore sizes of SBA-15 and in bulk, respectively, shows a dramatic effect of confinement on phase behavior of normal alkane mixtures. The s–l phase boundary lines of n-C₁₂–C₁₄/SBA-15 (8, 15.9 nm) are fitted as $T_{\text{m,r}}^{x}$ being [$xT_{{\text{m} ,r}}^{A} + \left( {1 - x} \right)T_{\text{m,r}}^{B} - D$], where D is a polynomial ∑ a _i x ⁱ, i = 1, 2,···, n (A = C₁₄, B = C₁₂).