Phase transitions and molecular dynamics of n-hexadecanol confined in silicon nanochannels

TL;DR

This study investigates how nanoscale confinement in silicon channels affects the phase transitions, crystal structures, and molecular dynamics of n-hexadecanol, revealing lowered transition temperatures and preferred crystal forms.

Contribution

It provides new insights into the phase behavior and molecular arrangements of n-hexadecanol under nanoscale confinement using combined x-ray and infrared spectroscopy.

Findings

Transition temperatures are lowered by approximately 20K under confinement.

Confinement favors the beta-form crystal structure, suppressing the gamma-form.

Molecular dynamics of CH2 vibrations remain unaffected by confinement.

Abstract

We present a combined x-ray diffraction and infrared spectroscopy study on the phase behavior and molecular dynamics of n-hexadecanol in its bulk state and confined in an array of aligned nanochannels of 8 nm diameter in mesoporous silicon. Under confinement the transition temperatures between the liquid, the rotator RII and the crystalline C phase are lowered by approximately 20K. While bulk n-hexadecanol exhibits at low temperatures a polycrystalline mixture of orthorhombic beta- and monoclinic gamma-forms, geometrical confinement favors the more simple beta-form: only crystallites are formed, where the chain axis are parallel to the layer normal. However, the gamma-form, in which the chain axis are tilted with respect to the layer normal, is entirely suppressed. The beta-crystallites form bi-layers, that are not randomly orientated in the pores. The molecules are arranged with their long axis perpendicular to the long channel axis. With regard to the molecular dynamics, we were able to show that confinement does not affect the inner-molecular dynamics of the CH_2 scissor vibration and to evaluate the inter-molecular force constants in the C phase.