Surfactant and Water Ordering in Triacontanol Monolayers at the Water−Hexane Interface

Our view of molecular ordering in Langmuir monolayers at the water−vapor interface influences our understanding of molecular ordering at other interfaces, including liquid−liquid interfaces for which structural information is scarce. We present a comparative study of a monolayer of a long-chain alkanol at the water−vapor and water−hexane interfaces using X-ray reflectivity to highlight significant differences between these two interfaces. The molecules in the Langmuir monolayer form an ordered phase of nearly rigid rods. In contrast, at the water−hexane interface, the triacontanol molecules form a condensed phase with progressive disordering of the chain from the −CH 2 OH to the −CH 3 group. Surprisingly, at the water−hexane interface, the density in the headgroup region is 10−15% greater than either bulk water or the ordered headgroup region found at the water−vapor interface. It is conjectured that this higher density is a result of water penetration into the headgroup region of the disordered monolayer.