Crown ethers at the aqueous solution-air interface: 1. Assignments and surface spectroscopy
Petru Niga1, Wendy King, Jonas Hedberg
1School of Chemical Science and Engineering, Royal Institute of Technology, Stockholm, 100 44, Sweden.
Physical Chemistry Chemical Physics : PCCP
|February 8, 2011
Summary
Vibrational sum frequency spectroscopy revealed molecular details of crown ethers at air-water interfaces. Protonated crown complexes formed, influencing interfacial water structure and ordering.
Area of Science:
- Surface Chemistry
- Spectroscopy
- Supramolecular Chemistry
Background:
- Crown ethers are cyclic molecules known for their ability to bind cations.
- Understanding their behavior at interfaces is crucial for applications in separation and sensing.
- Aqueous interfaces present unique challenges due to solvent interactions.
Purpose of the Study:
- To investigate the molecular arrangement and conformation of 4-Nitro Benzo-15-Crown-5 (NB15C5) and Benzo-15-Crown-5 (B15C5) at the air-water interface.
- To probe the structure of interfacial water molecules interacting with adsorbed crown ethers.
- To identify specific vibrational modes indicative of molecular interactions and ordering.
Main Methods:
- Utilized vibrational sum frequency spectroscopy (VSFS), a surface-sensitive technique.
- Targeted specific vibrational modes including NO, CN, COC, CH (crown ethers), and OH (water).
- Analyzed spectral data to infer molecular conformation, arrangement, and interactions at the air-water interface.
Main Results:
- Identified and analyzed CH(2) vibrational modes of crown ethers, showing splitting due to ether oxygen interactions.
- Provided evidence for a protonated crown complex moiety at the air-water interface.
- Observed strongly ordered interfacial water species influenced by the charged interface and proton tunneling.
Conclusions:
- The study elucidates the interfacial behavior of NB15C5 and B15C5 using VSFS.
- Protonation of crown ethers at the interface leads to significant ordering of water molecules.
- Interfacial water exhibits unique properties, including Zundel polarizability, due to proton tunneling within the crown ring.
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