Insight into Water Structure at the Surfactant Surfaces and in Microemulsion Confinement

Abstract

Interactions with surfactant molecules can significantly alter the structure of interfacial water. We present a comparative study of water-surfactant interactions using two different spectroscopic approaches: water at planar surfactant monolayers by sum frequency generation (SFG) spectroscopy and interfacial water confined in reverse micelles formed by the same surfactants using IR absorption spectroscopy. We report spectral features in the OH-stretching region (3200-3700 cm) that are observed in both IR and SFG spectra, albeit with different relative amplitudes, for ionic surfactant sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) and nonionic surfactant polyoxyethylene(4)lauryl ether (Brij L-4) reverse micelles in hexane and the corresponding monolayers at the air/water interface. A prominent feature in the SFG spectra of the OH stretch at 3560 cm is attributed to water molecules that have a weak donor hydrogen bond to the surfactant headgroup. The same feature is observed in the IR spectra of reverse micelles after deconvoluting the interfacial versus bulk spectral contributions. We performed an orientational analysis of these water molecules utilizing the polarization-dependent SFG spectra, which shows an average tilt angle of the OH stretch of surfactant-bound water molecules of ∼155° with respect to the surface normal.

Breakdown of the Stokes–Einstein Equation for Solutions of Water in Oil Reverse Micelles

“…The self-diffusion coefficients of all three componentscyclohexane, C 10 E 6 , and watershow Arrhenius behavior for the investigated temperature range. Average aggregate radii and aggregation numbers obtained from the Stokes–Einstein equation are unreasonably small, while aggregate radii and aggregation numbers obtained from the ratio of self-diffusion coefficients are more in line with data on a similar system reported in the literature. ,,,− …”

“…3 Likewise, average reverse micelle radii obtained from dynamic light scattering (DLS) for the related surfactant Brij L-4 in hexane were reported to be between 20 and 50 Å for water loads between 1 and 6, in good agreement with the radii from the self-diffusion coefficients in Figure 5. 66 The group of Aramaki extensively studied with DLS reverse micelles of glycerol monolaurate-based nonionic surfactants in various nonpolar solvents 19,67 including cyclohexane. 68−70 From these reports, it is evident that the nature of the nonpolar solvent significantly affects the micelle size in these systems and that the shape of the reverse micelle becomes increasingly nonspherical with increased size.…”

Insight into the Mechanisms Driving the Self-Assembly of Functional Interfaces: Moving from Lipids to Charged Amphiphilic Oligomers

“…[28][29][30]32 The broad signal extending from ω > 3100 cm -1 is due to the -OH stretch of water and indicates the presence of ordered water at the charged L/L interface. 17,[33][34][35] To confirm that the vSFG spectral features observed in Fig. 3 are truly from the ODMS-MIM (+) ionic oligomer at the hexadecane/aqueous interface, we performed three separate control measurements: one probed the aqueous/air interface, another probed the aqueous/oil interface, and the third probed the polymer-in-oil/air interface.…”

“…This indicates, as proposed elsewhere, that the counteranions can co-adsorb at the L/L interface at high bulk electrolyte concentrations. The very broad features near 2855 cm –1 are likely due to unresolved contributions from methylene stretches (linker from imidazolium to ODMS), combination bands, and/or the terminal CH 3 - ss on the ODMS-tail. − , The broad signal extending from ω > 3100 cm –1 is due to the −OH stretch of water and indicates the presence of ordered water at the charged L/L interface. ,− …”

Field-Dependent Orientation and Free Energy of D2O at an Electrode Surface Observed via SFG Spectroscopy

“…The behavior of water at charged interfaces plays an important role in a variety of chemical and biological process ranging from electrocatalysis to biomembranes. − Through the use of surfactants, the orientation of water at charged interfaces has been studied extensively using vibrational sum frequency generation (SFG) spectroscopy. − SFG is a second-order nonlinear optical process based on the annihilation of two input photons at angular frequencies ω 1 and ω 2 while, simultaneously, one photon at frequency ω 3 is generated . As with any second-order χ (2) phenomenon in nonlinear optics, this can only occur under conditions where the light is interacting with matter that is asymmetric (e.g., surfaces and interfaces) and the light has a very high intensity (i.e., pulsed laser), thus providing highly surface selective spectroscopy.…”