Observation of Molecular Ordering at the Liquid-Liquid Interface by Resonant Sum Frequency Generation

Abstract

Amphiphilic molecules are known to spontaneously order at liquid-solid' and liquid-air interfaces,2 forming well-ordered layers, but little is known about the structure of these molecules at the liquid-liquid interface due to experimental inaccessibility. A number of linear and nonlinear spectroscopic studies of surfactants at this interface have been p e r f~r m e d ,~-~ but these studies could not probe alkyl chain ordering. The goal of our work has been to measure the vibrational spectrum of a simple surfactant at an interface between two immiscible liquids in order to understand how such surfactants orient and assemble at this interface. In this communication, we report the first measurement of this type where the vibrational spectrum of sodium dodecyl sulfate (SDS) at the D20-CCb interface is obtained. The results provide spectroscopic evidence for interfacial ordering. The success of these studies has come from performing sum frequency generation (SFG) measurements in a total intemal reflection geometry (TIR).Surface SFG is gaining in use as a probe of surfaces and buried interfaces and is thoroughly described in the literatu1e.7-I~ SFG is more powerful than conventional spectroscopies such as FTIR because of its inherent surface selectivity, and for this reason, SFG is an excellent probe of interfaces. In TIR-SFG, selective probing of the interface between the two phases is obtained with much greater sensitivity than normal SFG much in the way that attenuated total reflectance FTIR gains its sensitivity. Using resonant TIR-SFG, the vibrational spectrum of the surfactant SDS at the D2O-CCL interface has been obtained, shown in Figure 1, This spectrum of a full monolayer at the interface was obtained using a bulk concentration of 10 mM SDS.To ascertain the order of SDS as a function of surface concentration, the intensities of the methyl and methylene symmetric stretch peaks in the sum frequency spectra are examined at 10 mM SDS and at several successive lower concentrations. The data in Figure 2 show the symmetric stretch region for methyl (2866 cm-') and methylene (2840 cm-I) groups as the concentration of SDS is changed from 0.1 to 5mM.The surface coverage of SDS is determined from the corre-(1) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Interfie (2) Pockels, A. Nature 1891, 43, 437. ( 3 ) Higgins, D. A.; Com, R. M.

Total-internal-reflection Broad-bandwidth Sum Frequency Generation Spectroscopy of Hexadecanethiol Adsorbed on Thin Gold Film Deposited on CaF, 2

“…[22][23][24][25] There are two advantages when one uses the TIR condition. First, both incident light to the interface and reflected light or generated light from the interface travel in only one of the two phases adjacent to the interface, which circumvents bulk absorption by the other phase.…”

Confocal Fluorescence Microscope Studies of the Adsorptive Behavior of Dioctadecyl-rhodamine B Molecules at a Cyclohexane-Water Interface

“…During the past two decades, much effort has been made to study this unique area mainly using surface-specific spectroscopic techniques. [1][2][3][4][5][6][7][8] The advantage of employing these techniques to monitor a buried interface between two bulk phases is that there are no mechanical perturbation problems during the measurement. However, due to the limited sensitivity of these spectroscopic techniques, target molecules were usually studied in a relatively high surface-density region.…”

Adsorption Behavior of Laurie Acid at Heptane/Water Interface as Studied by Second Harmonic Generation Spectroscopy and Interfacial Tensiometry

“…The chain conformation of interfacial surfactant molecules can be analyzed by a vibrational spectrum utilizing sum frequency generation (SFG) spectroscopy. As reported in previous SFG studies on surfactant molecules at the water/tetrachloromethane interface, [24][25][26][27] there is a possibility that the alkyl chain of LA at the heptane/water interface possesses gauche defects, and the fraction of the gauche defects depends on the interfacial concentration of LA, even though the carboxylate group of LA is well ordered. A further study using SFG spectroscopy will provide detailed information on the conformation of LA adsorbed at the heptane/water interface.…”