Comparison of phase behavior between water soluble and insoluble surfactants at the air-water interface

The surface phase behavior of 2-hydroxyethyl myristate (2-HEM) has been studied in Langmuir monolayers by measuring surface pressure (π)-area (A) isotherms with a film balance and observing monolayer morphology with a Brewster angle microscope (BAM). These results are compared with the phase behavior of 2-hydroxyethyl laurate (2-HEL) in Gibbs monolayers studied by measuring π-time (t) curves and observing monolayer morphology. The π-A isotherms of 2-HEM show a first-order phase transition from a liquid expanded (LE) phase to a liquid condensed (LC) phase in the temperature range between 5 and 35 °C whereas the π-t curves of 2-HEL represent a similar phase transition in the temperature range between 2 and 25 °C. The critical surface pressure, π_c necessary for the phase transitions increases with increasing temperature in both the cases. The LC domains formed in 2-HEM show circular shapes, which are independent of the temperature. In contrast, the circular domains having stripe texture formed at lower temperatures show a shape transition to fingering domains with uniform brightness at 15 °C. The amphiphile, 2-HEM having 13-carbon chain has higher line tension than 2-HEL that has 11-carbon chain as tail. Thus, for 2-HEM, this high line tension always dominates over other factors giving rise to circular domains at the all studied temperatures.