Phase transition behavior of a series of even n-alkane C(n)/C(n+2) mixtures confined in microcapsules: from total miscibility to phase separation determined by confinement geometry and repulsion energy.

The phase behaviors of binary consecutive even normal alkane (n-alkane) mixtures (n-C(n)H(2n+2)/n-C(n+2)H(2n+6), with mass ratios of 90/10 and 10/90) with different average carbon numbers n¯ both in the bulk state (abbreviated as C(n)/C(n+2)) and in nearly monodisperse microcapsules (abbreviated as m-C(n)/C(n+2)), have been investigated by the combination of differential scanning calorimetry and temperature-dependent X-ray diffraction. The phase behavior of n-alkane mixtures gradually shifts from complete phase separation, partial miscibility to total miscibility in both bulk and microcapsules with the increase of average carbon numbers n¯. There are critical points for average carbon numbers of C(n)/C(n+2), where the corresponding mixtures exhibit coexistence of a triclinic phase (formed by alkane with a longer chain) and an orthorhombic ordered phase (formed by the two components of mixtures). Due to the confinement from hard shells of microcapsules, the critical points of m-C(n)/C(n+2) are smaller than those of C(n)/C(n+2). Such a phase behavior originates from the delicate combined action of confinement and repulsion energy for the encapsulated n-alkane mixtures with different average carbon numbers n¯. When n¯ is less than the critical point, the repulsion energy between the two kinds of molecules exceeds the suppression effect of confinement, and phase separation occurs in microcapsules. It is believed that the average carbon number is another important factor that exerts strong negative influence on the phase separation of m-C(n)/C(n+2) systems.