Thermal Behavior of Long-Chain Alcohols on Sapphire Substrate
2015
Zhu, He | Dhinojwala, Ali
Structures of amphiphilic molecules at the liquid/solid and solid/solid interfaces are important in understanding lubrication, colloid stabilization, chromatography, and nucleation. Here, we have used interface-sensitive sum frequency generation (SFG) spectroscopy to characterize the interfacial structures of long-chain alcohols above and below the bulk melting temperature (Tₘ). The melting temperature of the ordered hexadecanol monolayer was measured to be around 30 °C above the bulk Tₘ, consistent with the transition temperature reported using X-ray reflectivity [Phys. Rev. Lett. 2011, 106, 137801]. The disruption of hydrogen bonds between the sapphire and the alcohol hydroxyl groups was directly measured as a function of temperature. The strength of this hydrogen-bonding interaction, which explained the monolayer thermal stability above Tₘ, was calculated using the Badger–Bauer equation. Below Tₘ, the ordered self-assembled monolayer influenced the structure of the interfacial crystalline layer, and the transition from the ordered monolayer to the bulk crystalline phases (α rotator phase, β crystalline phase, and γ crystalline phase) resulted in packing frustrations at the interface.
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