Crystallization behavior of binary even-even n-alkane mixtures in microcapsules: effect of composition and confined geometry on solid-solid phase separation.

The crystallization behaviors of binary even-even normal alkane (n-alkane) mixtures (n-C(18)H(38)/n-C(20)H(42), abbreviated as C(18)/C(20)) with different compositions, both in the bulk state and in nearly monodisperse microcapsules, have been investigated by the combination of differential scanning calorimetry and temperature-dependent X-ray diffraction. The solid-solid phase separation, usually observed during the cooling process of bulk samples, is greatly suppressed and even eliminated after being microencapsulated, with the orthorhombic-ordered phase dominating in the low-temperature crystal. Such a crystallization transition is attributed to the special interaction between the two even n-alkanes and the confined environment in microcapsules. The triclinic ordered phase, solely formed by the single even n-alkanes (C(18) or C(20)), becomes less stable due to the weakening of the layered structure and the suppression of the terminal methyl-methyl interactions in the confined geometry, which favors the miscibility of the two components. Furthermore, besides the chain-length difference and the composition, the confined environment is proved to be another important factor to exert strong positive influence on suppressing the solid-solid phase separation of C(18)/C(20) binary system.