Structure and Lateral Interaction in Mixed Monolayers of Dioctadecyldimethylammonium Chloride (DOAC) and Stearyl Alcohol

Abstract

π-A isotherms, atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy are employed to investigate the molecular structure and lateral interactions in mixed monolayers of dioctadecyldimethylammonium chloride (DOAC) and stearyl alcohol (SA) at air/water and air/solid interfaces. To avoid possible interference between the two molecules in the SFG spectroscopic measurements, perprotonated DOAC and perdeuterated SA (dSA) were used. The thermodynamic analyses for the π-A isotherms show that DOAC is miscible with dSA. SFG observations reveal that DOAC molecules become conformationally ordered as dSA molecules are introduced into the monolayer. AFM observations demonstrate coexistence of DOAC-rich and dSA-rich domains in the mixed monolayer with ratios different from their initial composition in the subphase. The present study suggests that DOAC molecules in the mixed monolayer are condensed by mixing with dSA in which the repulsive interactions between positively charged head groups of the DOAC molecules are largely reduced along with an increase of van der Waals interactions with dSA.

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.…”

Unveiling the Membrane-Binding Properties of N-Terminal and C-Terminal Regions of G Protein-Coupled Receptor Kinase 5 by Combined Optical Spectroscopies

“…These were also observed for GRK5 2–31 but were not as significant. These two peaks could be attributed to amino acid side chains, 51 disruption of the lipid bilayer, 52 or both. The SFG amide I spectra of GRK5 546–565 , however, is very different from those of the N-terminal peptides.…”

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.…”