Cd²⁺-Induced Interfacial Structural Changes of Langmuir−Blodgett Films of Stearic Acid on Solid Substrates:  A Sum Frequency Generation Study

Abstract

The molecular structures and their stabilities at the outmost-layer of the Langmuir-Blodgett (LB) films of stearic acid on solid substrates have been investigated by a highly surface-sensitive spectroscopic technique, sum frequency generation (SFG), in air and in aqueous solution, using the combination of both normal and deuterated stearic acid. Peaks observed in the SFG spectra are mainly attributed to the terminal methyl group at the outmost layer of the LB films. The SFG spectra in air are virtually identical and are independent of the odd-even property and thickness (1-12) of the LB films, indicating that the even-numbered LB film changes its surface structure after passing through the interface between the water subphase and air, especially when the Cd2+ cation was included in the water subphase. Furthermore, we have demonstrated for the first time using in situ SFG measurement that the interfacial molecular structure at the LB bilayer of stearic acid on the hydrophilic substrates significantly change with immersion in the water subphase containing Cd2+ cation while such a structural change has not been observed in the water subphase without Cd2+. These results clearly indicate that a reorganization process takes place on the surface of the stearic acid bilayer induced by the Cd2+ cation. The electrostatic interaction between the carboxylate headgroup of stearic acid via the Cd2+ cation seems to play an important role in the surface reorganization process both in air and in solution.

Interference effects in the sum frequency generation spectra of thin organic films. II: Applications to different thin-film systems

“…1͒ has been constructed experimentally. LB multilayers have been extensively employed as standard model systems for experiments [2][3][4] and for theoretical simulations. [5][6][7] In our experiments, dielectric films comprising Y-type multilayers of per-deuterated arachidate with various thicknesses ͑4-80 layers͒ were deposited on gold substrates followed by the deposition of a monolayer of perprotonated arachidate on the top surface, as depicted schematically in Fig.…”

“…The ppp-SFG spectrum at n = 4 is similar to that of a fatty acid monolayer reported previously. 2,3,7 The interference phenomenon in the ppp-spectra is different from that in the ssp-and sps-spectra. The interference behavior is significantly dependent on the vibrational mode.…”

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

“…29 A LB monolayer of arachidic acid (AA), CH 3 (CH 2 ) 18 COOH (AA), was constructed on the surface of water containing 0.3 mM NaHCO 3 and 0.2 mM CdCl 2 (pH = 6.6) in a LB trough (FSD-500, USI) at room temperature and then transferred to the substrate surface at a surface pressure of 30 mN/m. 30,31 A SAM of octadecyltrichlorosilane (OTS) was prepared on the TiO 2 surface following the procedures described elsewhere. [32][33][34][35][36] A broad-band SFG system was used to characterize the structure and conformational ordering of the two monolayers.…”

“…The two shoulders at 2848 and 2920 cm −1 were attributed to the C-H symmetric and asymmetric modes of the CH 2 group in the long alkyl chain of AA, respectively. 30,31 As already discussed, the vibration modes of the CH 2 group in a long alkyl chain with an all-trans conformation are SFG-inactive due to the centrosymmetric structure; thus, the SFG peaks observed for the CH 2 group cannot be used to evaluate its population but mainly used to characterize the number of the gauche defects in the long alkyl chains. 39,40 The conformational gauche defects break the all-trans structure of the alkyl chain and reflect the disordering of the monolayer.…”

“…1(a) indicates that an all-trans conformation of the AA monolayer with only a very small amount of gauche defects was constructed. 30,31 The contact angle measurements showed that the surface is very hydrophobic (>90 • ). As soon as the UV irradiation started, the SFG spectra quickly changed.…”