Sangjun Seok

Imaging of collapsed fatty acid films at air-water interfaces.

In situ imaging ellipsometry is employed to monitor the morphology of collapsed films of fatty acid Langmuir monolayers on pure water and on CaCl2 solution. The ellipsometry images reveal the existence of multilayer domains in the collapsed region, and analysis of the images yields the thicknesses of these domains. The multilayer films formed on water are mainly trilayers, while those on CaCl2 solution are mainly bilayers. The structure of the collapsed films also changes sensitively depending on the history of compression of the molecular layer.

Side chain assisted nanotubular self-assembly of cyclic peptides at the air–water interface

Langmuir monolayers of two artificial cyclic peptides with an alternating sequence of L-glutamic acid and 3-aminobenzoic acid subunits, a cyclohexapeptide (C6G) and a cyclooctapeptide (C8G), were investigated using a variety of techniques, including π–A isotherms, in situ surface sum-frequency generation (SFG) and Brewster angle microscopy. The monolayers were also transferred onto a solid substrate by the Langmuir–Blodgett technique and characterized by grazing incident X-ray diffraction (GIXD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The investigations indicated that C6G forms 2D crystallite structures at the air–water interface, whereas no such structures were observed for C8G. Being amphiphilic, both peptides attain a horizontal orientation on the water surface after spreading. Surface compression causes the molecules to flip to a perpendicular state, thus minimizing the molecular area. The measurements also indicate that, in the perpendicular state, self-assembly of C6G leads to a tubular arrangement of the peptide rings. According to GIXD and TEM data, pairs of tubes arrange in a well defined and oriented order producing 2D crystals. Surface vibrational spectroscopic methods (sum-frequency generation and polarization modulation IR reflection–absorption) combined with molecular modeling gave insight into the arrangement of individual C6G molecules in the tubes. Overall, our results indicated that the tubular assembly of C6G is most likely due to intermolecular H-bonding between the CO group in the glutamic acid side chains and peptide NH group of a neighboring peptide ring.

Structural change of interfacial water by adsorption of counterions on Langmuir monolayer

Langmuir monolayer of lipid molecules on LaCl<inf>3</inf> solutions was investigated by sum-frequency vibrational sepctroscopy. Adsorption of La³⁺ cation on lipid headgroups increased with higher LaCl<inf>3</inf> concentration, finally overcompensating negative charges of the lipid headgroups.

Sum-frequency spectra of Langmuir monolayers of lipid molecules having different headgroups

In infrared-visible sum-frequency spectra, interference between nearby bands is important in determining the spectral shape. Using Langmuir monolayers consisting of lipids having oppositely charged headgroups, we studied the influence of sum-frequency signal from interfacial water on the spectral shape in the CHx region.

Determination of absolute polar orientation of dyes incorporated into the channels of silicalite-1 films

Two different hemicyanine dyes having C18-alkyl chain on opposite ends of the dipolar NLO chromophore were adsorbed into the vertically oriented channels of silicalite-1 films supported on glass plate. Second-harmonic phase measurement was used to measure the absolute polar orientations of these films, and found the dyes enter zeolite channels with the tail part first.

Charge Inversion at the Surface by Adsorption of Trivalent Cations on Langmuir Monolayer

Infrared-visible sum-frequency generation spectroscopy was used to study cation adsorption and charge inversion behavior at the lipid/water interface of dimyristoylphosphatidic acid (DMPA) monolayer. The alignment of interfacial water molecules was inferred by electric field produced from the adsorbed surface charges at the lipid/water interface, which suggested charge inversion behavior.