Efficient molecular packing of glycerol monostearate in Langmuir monolayers at the air-water interface

Abstract

Understanding the molecular packing of glycerol monoesters at the air-water interface is relevant for various industries where their emulsifying characteristics are exploited. In this study, we show that glycerol monostearate (GMS) forms closely packed Langmuir monolayers, with an area per molecule of 19Å2 inferred from the surface pressure isotherms, similar to the value for stearic acid. The order parameter calculated from molecular dynamics (MD) simulations indicates that the closely packed GMS molecules are fully connected by hydrogen bonding, which is important for applications as emulsifier. The GMS monolayer had a compressional modulus typical of a liquid-expanded (LE) phase at low surface pressures (<30mNm−1), and of a liquid-condensed (LC) phase at higher pressures. The LE-LC phase transition was captured with Brewster angle microscopy images. Using in situ infrared measurements, we observed that the GMS chains were progressively organized with increasing surface pressure in the monolayer, again consistent with MD simulations. The importance of the chain size was confirmed in subsidiary experiments, in which glycerol monocaprylate did not form stable monolayers owing to its shorter chain. In summary, the combination of surface-specific methods to characterize Langmuir monolayers and MD simulations could illustrate the relevance of physicochemical properties of GMS for its emulsifying characteristics.