Emergence of Electric Fields at the Water–C12E6 Surfactant Interface

Abstract

We study the properties of the interface of water and the surfactant hexaethylene glycol monododecyl ether (C12E6) with a combination of heterodyne-detected vibrational sum frequency generation (HD-VSFG), Kelvin-probe measurements, and molecular dynamics (MD) simulations. We observe that the addition of the hydrogen-bonding surfactant C12E6, close to the critical micelle concentration (CMC), induces a drastic enhancement in the hydrogen bond strength of the water molecules close to the interface, as well as a flip in their net orientation. The mutual orientation of the water and C12E6 molecules leads to the emergence of a broad (∼3 nm) interface with a large electric field of ∼1 V/nm, as evidenced by the Kelvin-probe measurements and MD simulations. Our findings may open the door for the design of novel electric-field-tuned catalytic and light-harvesting systems anchored at the water–surfactant–air interface.

Observation of Strong Synergy in the Interfacial Water Response of Binary Ionic and Nonionic Surfactant Mixtures

“…Both SDS and C 12 E 6 are widely used surfactant systems and have been studied extensively before. The interfacial structure of aqueous solutions of C 12 E 6 has recently been studied using HDVSFG, Kelvin-probe measurements, and molecular dynamics (MD) simulations 29 . In this study, it was found that C 12 E 6 at CMC (70 µm) generates a strong electric eld of 1 V/nm arising from the orientational structure of both the hydrophobic and hydrophilic parts of the surfactant and water molecules.…”

“…The apparent change in the amplitudes of the C–H vibrations with SDS concentration is due to the rise of the low-frequency wing of the response of the water O–H vibrations with increasing SDS concentration. The positive water response of C 12 E 6 just by itself is explained in detail elsewhere . To briefly mention, the positive water signal between 3000 and 3400 cm –1 for solutions of C 12 E 6 results from the strong hydrogen bonding of water molecules to the ether groups of the headgroup of C 12 E 6 .…”

Water-Induced Restructuring of the Surface of a Deep Eutectic Solvent

“…The narrowing of the broad unstructured region into two sharp narrow features in region I indicates that the surface structure of Ch + becomes more uniform and ordered when the water content is increased. This type of structural change is commonly observed for surfactants at water surfaces when the concentration of polar species is increased. ,− With increasing water content, the Ch + ions acquire a more ordered and uniform alignment at the surface, thus removing the gauche conformation, as evident from the decrease of the signal at ∼2940 cm –1 . The spectral features in region II also change upon an increase of the water concentration, where the contribution to the vibrational spectrum from urea decreases (see Figure and Figure S2).…”

“…Details of the experimental setup have been discussed in previous publications and are presented the Supporting Information. ,,− Briefly, VSFG is performed by focusing two laser pulses and overlapping them on the sample spatially and temporally. One of the laser pulses, ω IR , is from a mid-infrared (mid-IR) source and is resonant with the vibrational frequencies of the molecules present at the surface.…”

Statistical physics of inhomogeneous transport: Unification of diffusion laws and inference from first-passage statistics

“…The asymmetry of soft-matter interfaces with aqueous solutions generate electric fields that could be harnessed for chemical reactions [85][86][87] relevant in pre-biotic chemistry [88] as well as artificial photosynthesis. Both the statics and dynamics of water at the interface play a crucial role in controlling the magnitude and fluctuations of the electric field.…”