Ultrafast to Ultraslow Dynamics of Self-Assembled Monolayers at the Air/Water Interface

Approximately 70% of the Earth’s surface is the air/water interface. On a water surface, an oil droplet spontaneously spreads to form a molecular single layer: the Langmuir monolayer. The non-bulk molecular dynamics of interfacial water and surfactants in a model Langmuir monolayer provide important insights for understanding 2D phase transitions, dynamics of biological water, the solvation and conformational dynamics of proteins, and mechanisms of trans-membrane activities. However, it has proven difficult to measure monolayer and interfacial water molecular dynamics.

Here we employ a new technique, reflection enhanced two-dimensional infrared (2D IR) spectroscopy, on a carbonyl stretching mode of tricarbonylchloro-9-octadecylamino-4,5-diazafluorenerhenium(I) (TReF18) monolayers at the air/water interface. We studied the Langmuir monolayer at two surface densities in the liquid expanded phase zone. The low density (LD) monolayer has a mean molecular area of 90 Å2, and the high density (HD) monolayer has a mean molecular area of 90 Å2. Comparison to experiments on a water soluble version of the metal carbonyl head groups (TReBDC) shows that water hydrogen bond rearrangement dynamics slow from 1.5 ps for bulk water to 3.1 ps for interfacial water. Longer time scale fluctuations were also observed and attributed to fluctuations of the number of hydrogen bonds formed between water and the three carbonyls of TReF18.

Figure 1. Structural relaxation of the probe at the air-water interface (left) and in bulk water (right). The randomization time of bulk water has been previously found to be ~1.5 ps.

At the higher surface density, two types of TReF18 minor structures are observed in addition to the main structure. The enhancing reflection method can take usable 2D IR spectra on the monolayer within 8 seconds, enabling us to track the fluctuating minor structures’ appearance and disappearance on a tens of seconds timescale. 2D IR chemical exchange spectroscopy further shows these structures interconvert in 30 ps. Finally, slow 2D spectral lineshape evolution was observed to reveal that it takes the monolayers hours to reach macroscopic structural equilibrium. Therefore, in addition to the ultrafast microscopic dynamics on the picosecond timescale, the macroscopic monolayer structure also undergoes ultraslow fluctuation and evolution on the multiple timescales from seconds to hours.

Figure 2. Top: Appearance of additional on-diagonal peaks showing new structures in HD monolayer. Bottom: Appearance of cross peaks reveals the interconversion time between the different structures.