Effect of Functional Group on the Monolayer Structures of Biodegradable Quaternary Ammonium Surfactants

Abstract

The monolayer structures and conformational ordering of cationic surfactants including the biodegradable quaternary ammonium molecules have been systematically characterized by π-A isotherm, surface potential, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and sum frequency generation (SFG) vibrational spectroscopy. It was found that the monolayer of the typical dialkyl dimethylammonium on the water surface was less densely packed along with many conformational gauche defects. The packing density and ordering of these monolayers were improved as halide ions were added to the subphase. A similar condensation effect was also observed when amide or ester groups are present in the alkyl tails of the surfactant. These results are discussed on the basis of the repulsive electrostatic interactions between the terminal ammonium moieties, the hydrogen bonding between the functional groups in the alkyl chains, as well as the flexibility of the alkyl chains in these surfactants. The present study is crucial to understanding the relationship between the interfacial structures and the functionalities of the biodegradable quaternary ammonium surfactants.

Molecular orientation of organic thin films on dielectric solid substrates: a phase-sensitive vibrational SFG study

“…The broadband SFG system with a tunable femtosecond infrared pulse and a picosecond visible pulse has been described elsewhere [39][40][41]. Briefly, a Ti:sapphire oscillator (Mai Tai, Spectra-Physics) generates a mode locked 40 MHz pulse wave with a pulse duration of about 100 fs and a wavelength at 800 nm.…”

Interfacial Structure of Soft Matter Probed by SFG Spectroscopy

“…In the present Personal Account, our recent SFG studies on the molecular structures at interfaces of functional soft matter such as polymers, self‐assembled monolayers (SAMs) and Langmuir–Blodgett (LB) ultrathin layers and lipid bilayers in various environments will be briefly reviewed. A general model dealing with the optical interference effect in thin layers of soft matter, which can dramatically affect the SFG spectral shape and intensity, will be introduced first.…”

Photocatalytic oxidation of the organic monolayers on TiO2 surface investigated by in-situ sum frequency generation spectroscopy

“…Details about the system were described elsewhere. 37,38 A broad-band femtosecond infrared pulse of 3 µJ (FWHM ∼200 cm −1 , centered at 2900 cm −1 ) and a narrow-band picosecond visible pulse of 8 µJ at 800 nm (FWHM ∼10 cm −1 ) were spatially and temporally overlapped at the sample surface with incident angles of 50 • and 65 • , respectively. An SFG signal was detected by a CCD detector attached to the spectrograph.…”

“…Thereby, we could not use the ratio between the peak amplitude of CH 2 and CH 3 groups to specify the ordering of the film as typically done by previous studies. 37,38,41,42 Instead, we could employ the number of the gauche defects, i.e., the peak amplitude of the CH 2 group, to characterize the relative ordering of the film during the photocatalytic oxidation reaction. Based on the assumption, the AA monolayer seems to maintain the same ordering during the first stage of the oxidation.…”

“…Furthermore, a very thin layer of water is known to be formed between the organic monolayer and hydrophilic substrate surface during the transfer process. 37,38 On the other hand, it is known from previous studies that the long-chain fatty acid molecules tend to aggregate together to form condensed domains where an attractive van der Waals interaction works between the AA molecules on the water surface. [26][27][28] We believe that a similar process also occurs for the AA monolayer on the hydrophilic TiO 2 surface during the UV-irradiation.…”