Phase transitions in monolayers of medium-chain alcohols on water studied by sum-frequency spectroscopy and ellipsometry

Abstract

Monolayers of medium-chain alcohols CH₃(CH₂)_m–1 OH (m= 9–14) have been studied by sum-frequency vibrational spectroscopy and ellipsometry in the vicinity of the two-dimensional solid–liquid phase transition. Gauche defects are present in the solid phase and increase only slightly in the monolayer just above the phase transition. A model of the chains as freely rotating rigid rods permits a calculation of the area per molecule and chain tilt in the liquid phase. The density of the hydrocarbon chains in the liquid monolayer phase is less than that in the solid monolayer phase but significantly higher than in a bulk liquid alkane. The area per molecule, chain tilt and volume per CH₂ group in the liquid phase all increase with increasing chain length.