Metastable and Stable Morphologies during Crystallization of Alkanes in Miniemulsion Droplets

Undercooling and crystallization of different alkanes in stable nanodroplets with narrow size distributions were analyzed by using DSC measurements. The crystal structure was determined by X-ray measurements. Alkane droplets with a defined size between 100 and 500 nm in water were prepared by using the miniemulsion process. The required undercooling to obtain crystallization in such droplets is significantly increased compared to that for the bulk material since the nucleation mechanism is shifted from heterogeneous to homogeneous nucleation. For the even alkanes (C18−C24), a structure change from the triclinic in the bulk to orthorhombic structure in small droplets (100 nm) was detected and attributed to confinement effects inside the droplets. An intermediate rotator phase is of less relevance for the nanosized droplets. For odd alkanes, a strong temperature shift of the crystallization point compared to the bulk system was detected, but no structure change was observed. Both in bulk and in miniemulsion droplets an orthorhombic structure was formed.