Two-Step Freezing in Alkane Monolayers on Colloidal Silica Nanoparticles: From a Stretched-Liquid to an Interface-Frozen State
Two-Step Freezing in Alkane Monolayers on Colloidal Silica Nanoparticles: From a Stretched-Liquid to an Interface-Frozen State
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胶体二氧化硅纳米颗粒上烷烃单层的两步冷冻：从拉伸液体到界面冷冻状态
DOI：
10.1021/acs.jpcb.6b00119
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发表时间：
2016
影响因子：
3.3
通讯作者：
Wang Dujin
中科院分区：
文献类型：
--
作者：
Gao Xia;Huber Patrick;Su Yunlan;Zhao Weiwei;Wang Dujin
采用差示扫描量热法（DSC）和变温固态13 C核磁共振（VT solid-stat 13 C NMR）研究了典型的软硬杂化纳米复合材料（ann-octadecane C18/SiO2-nanoparticle）的结晶行为，并与SiO2纳米粒子的负载量进行了比较.对于具有高二氧化硅负载的复合材料，观察到本体冷冻前的两个潜热峰，表明相当大一部分C18分子涉及本体C18中未知的两个相变。结合NMR测量以及文献中报道的平面无定形二氧化硅表面上的烷烃和烷醇的实验，这种相行为可以归因于向2D液体状单层的过渡和随后的冷却时的无序到有序的过渡。第二个转变的结果在形成的烷烃分子的分子长轴平行于纳米粒子的表面法线的界面冻结单层。加热时，观察到相反的相序列，然而，具有相当大的热滞后雅阁符合一级相变的特征。一个热力学模型，考虑平衡的界面结合，链拉伸弹性，和熵效应定量占所观察到的行为。互补的同步辐射为基础的广角X射线衍射（WAXD）实验，使我们能够记录这种特殊的界面冻结行为对周围的烷烃熔体，特别是在散装C18中不存在的旋转相的成核的强烈影响。
The crystallization behavior of an archetypical soft/hard hybrid nanocomposite, that is, ann-octadecane C18/SiO2-nanoparticle composite, was investigated by a combination of differential scanning calorimetry (DSC) and variable-temperature solid-state13C nuclear magnetic resonance (VT solid-state13C 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 C18molecules involve two phase transitions unknown from the bulk C18. 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 C18.