The
crystallization behavior of an archetypical soft/hard hybrid
nanocomposite, that is, an <i>n-</i>octadecane C<sub>18</sub>/SiO<sub>2</sub>-nanoparticle composite, was investigated by a combination
of differential scanning calorimetry (DSC) and variable-temperature
solid-state <sup>13</sup>C nuclear magnetic resonance (VT solid-state <sup>13</sup>C NMR) as a function of silica nanoparticles loading. Two
latent heat peaks prior to bulk freezing, observed for composites
with high silica loading, indicate that a sizable fraction of C<sub>18</sub> molecules involve two phase transitions unknown from the
bulk C<sub>18</sub>. Combined with the NMR measurements as well as
experiments on alkanes and alkanols at planar amorphous silica surfaces
reported in the literature, this phase behavior can be attributed
to a transition toward a 2D liquid-like monolayer and subsequently
a disorder-to-order transition upon cooling. The second transition
results in the formation of a interface-frozen monolayer of alkane
molecules with their molecular long axis parallel to the nanoparticles’
surface normal. Upon heating, the inverse phase sequence was observed,
however, with a sizable thermal hysteresis in accord with the characteristics
of the first-order phase transition. A thermodynamic model considering
a balance of interfacial bonding, chain stretching elasticity, and
entropic effects quantitatively accounts for the observed behavior.
Complementary synchrotron-based wide-angle X-ray diffraction (WAXD)
experiments allow us to document the strong influence of this peculiar
interfacial freezing behavior on the surrounding alkane melts and
in particular the nucleation of a rotator phase absent in the bulk
C<sub>18</sub>