Thermal Behavior of Long-Chain Alcohols on Sapphire Substrate

Abstract

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.

Thermal and Chemical Stability of n -Hexadecanethiol Monolayers on Au(111) in O 2 Environments

“…The increase in capacitance in this temperature range is interpreted as decrease in the monolayer thickness according to the parallel plate capacitor formula. The decrease in thickness has been reported for alkyl monolayers on different substrates as a consequence of the disordering of the monolayer by an increased formation of gauche defects at high temperatures [39,40]. From 403 K, the monolayer decomposition is evident in the CV profiles and the increase in capacitance is more pronounced as shown in Fig.…”

Solving the “Magic Angle” Challenge in Determining Molecular Orientation Heterogeneity at Interfaces

“…Although this challenge has been well-known for more than a decade, [22][23][24][25] no general solutions to it have been proposed, to the best of our knowledge. Details about this function are described in the supporting information.…”

Order and Melting in Self-Assembled Alkanol Monolayers on Amorphous SiO2

“…26 Moreover, we have recently explored the structure and temperature evolution of a C18OH SAM on single-crystal sapphire (0001), which has the same hexagonal symmetry as, and only a small lattice mismatch to, the monolayer. 27 A very recent nonlinear optics study of alkanol SAMs on single crystal sapphire 28 provides further insights into the temperature evolution. The study presented here, addressing C18OH monolayers on an amorphous substrate, should help in elucidating the role of the substrate's order and epitaxy in the SAM's structure and temperature dependence.…”