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

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 20 K. While bulk n-hexadecanol exhibits at low temperatures a polycrystalline mixture of orthorhombic β and monoclinic γ forms, geometrical confinement favors the more simple β form: only crystallites are formed, where the chain axis is parallel to the layer normal. However, the γ form, in which the chain axis is tilted with respect to the layer normal, is entirely suppressed. The β crystallites form bilayers 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 innermolecular dynamics of the CH₂ scissor vibration and to evaluate the intermolecular force constants in the C phase.