Hydration state of nonionic surfactant monolayers at the liquid/vapor interface: structure determination by vibrational sum frequency spectroscopy

The OH stretching region of water molecules in the vicinity of nonionic surfactant monolayers has been investigated using vibrational sum frequency spectroscopy (VSFS) under the polarization combinations ssp, ppp, and sps. The surface sensitivity of the VSFS technique has allowed targeting the few water molecules present at the surface with a net orientation and, in particular, the hydration shell around alcohol, sugar, and poly(ethylene oxide) headgroups. Dramatic differences in the hydration shell of the uncharged headgroups were observed, both in comparison to each another and in comparison to the pure water surface. The water molecules around the rigid glucoside and maltoside sugar rings were found to form strong hydrogen bonds, similar to those observed in tetrahedrally coordinated water in ice. In the case of the poly(ethylene oxide) surfactant monolayer a significant ordering of both strongly and weakly hydrogen bonded water was observed. Moreover, a band common to all the surfactants studied, clearly detected at relatively high frequencies in the polarization combinations ppp and sps, was assigned to water species located in proximity to the surfactant hydrocarbon tail phase, with both hydrogen atoms free from hydrogen bonds. An orientational analysis provided additional information on the water species responsible for this band.     

Study of the adsorption of sodium dodecyl sulfate (SDS) at the air/water interface: targeting the sulfate headgroup using vibrational sum frequency spectroscopy

The surface sensitive technique vibrational sum frequency spectroscopy (VSFS), has been used to study the adsorption behaviour of SDS to the liquid/vapour interface of aqueous solutions, specifically targeting the sulfate headgroup stretches. In the spectral region extending from 980 to 1850 cm(-1), only the vibrations due to the SO(3) group were detectable. The fitted amplitudes for the symmetric SO(3) stretch observed at 1070 cm(-1) for the polarization combinations ssp and ppp, were seen to follow the adsorption isotherm calculated from surface tension measurements. The orientation of the sulfate headgroup in the concentration range spanning from 1.0 mM to above the critical micellar concentration (c.m.c.) was observed to remain constant within experimental error, with the pseudo-C(3) axis close to the surface normal. Furthermore, the effect of increasing amounts of sodium chloride at SDS concentrations above c.m.c. was also studied, showing an increase of approximately 12% in the fitted amplitude for the symmetric SO(3) stretch when increasing the ionic strength from 0 to 300 mM NaCl. Interestingly, the orientation of the SDS headgroup was also observed to remain constant within this concentration range and identical to the case without NaCl.     

A vibrational sum frequency spectroscopy study of the liquid-gas interface of acetic acid-water mixtures: 1. Surface speciation

Aqueous acetic acid solutions have been studied by vibrational sum frequency spectroscopy (VSFS) in order to acquire molecular information about the liquid-gas interface. The concentration range 0-100% acetic acid has been studied in the CH/OH and the C-O/C=O regions, and in order to clarify peak assignments, experiments with deuterated acetic acid and water have also been performed. Throughout the whole concentration range, the acetic acid is proven to be protonated. It is explicitly shown that the structure of a water surface becomes disrupted even at small additions of acetic acid. Furthermore, the spectral evolution upon increasing the concentration of acetic acid is explained in terms of the different complexes of acetic acid molecules, such as the hydrated monomer, linear dimer, and cyclic dimer. In the C=O region, the hydrated monomer is concluded to give rise to the sum frequency (SF) signal, and in the CH region, the cyclic dimer contributes to the signal as well. The combination of results from the CH/OH and the C-O/C=O regions allows a thorough characterization of the behavior of the acetic acid molecules at the interface to be obtained.     

Inorganic chiral 3-D photonic crystals with bicontinuous gyroid structure replicated from butterfly wing scales

Three dimensional silica photonic crystals with the gyroid minimal surface structure have been synthesized. The butterfly Callophrys rubi was used as a biotemplate. This material represents a significant addition to the small family of synthetic bicontinuous photonic crystals.     

Soluble monolayers of n-decyl glucopyranoside and n-decyl maltopyranoside. Phase changes in the gaseous to the liquid-expanded range

To examine the transition from the gaseous to the liquid-expanded monolayer state, surface tension data were recorded for n-decyl beta-d-glucopyranoside (Glu) and n-decyl beta-d-maltopyranoside (Mal) solutions at low concentrations and at different temperatures. Comparisons were also made with n-decyl beta-d-thiomaltopyranoside (S-Mal) solutions at room temperature. The transitions observed occur at very low concentrations and surface pressures, about 0.5% of the critical micelle concentration (cmc) and between 0.8 and 1 mN/m for Glu and Mal at 22 degrees C. For S-Mal the transition is recorded for a concentration of 0.5% of the cmc as well, but the surface pressure is lower, about 0.4 mN/m. The gradual change in molecular area about the transition is from about 500 to 200 A(2) and 400 to 150 A(2) for Mal and Glu, respectively, and from about 800 to 250 A(2) for S-Mal. The comparatively large molecular areas after the transitions are incompatible with the notion that a coherent hydrocarbon film would cover the entire surface already at this stage. Standard surface thermodynamics was applied to elucidate the nature of these transitions in combination with two model concepts: The formation of an infinite network of surfactant molecules and, second, the formation of surface micelles. Hard-disk simulation results were employed to quantify the additional surface pressure after the transition attributed to the formation of surface micelles. In conclusion the formation of surface micelles is plausible as the hard-disk model is capable of accounting for the additional surface pressure increase with acceptable accuracy. Further, vibrational sum frequency spectroscopy was used to investigate the transition for Mal. Using the distinct feature of the non-hydrogen-bonded OH ("free OH") at 3700 cm(-)(1) for probing the surface water state, it could be determined that the surface holds a sizable fraction of unperturbed surface water even after the transition from the Henry range. The decrease in the free OH signal was found to correlate with the increase in surface density of surface micelles.     

Preferential adsorption of amino-terminated silane in a binary mixed self-assembled monolayer

The composition and structure of a binary mixed self-assembled monolayer (SAM) of 3-aminopropyltriethoxysilane (APS, NH(2)(CH(2))(3)Si(OCH(2)CH(3))(3)) and octadecyltrimethoxysilane (ODS, CH(3)(CH(2))(17)Si(OCH(3))(3)) on a silicon oxide surface have been characterized by water contact-angle measurements, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy. XPS demonstrated that APS in the mixed SAM is significantly enriched in comparison to that in solution, indicating the preferential adsorption of APS during the SAM formation. AFM observations showed that the mixed SAM becomes rougher. SFG revealed that the coadsorption of APS induced a conformation disordering in the ODS molecules present in the mixed SAM. The surface enrichment of APS has been explained in terms of differences in the surface adsorption rates of the two components as well as in the self-congregation states of APS molecules in the bulk solution. Furthermore, the structure of the water molecules on the mixed SAM surface in contact with the aqueous solutions at different pH's has also been studied. The results indicate that the mixed-SAM modified surface is positively charged at pH < 5 and negatively charged at pH > 7.     

Simultaneous conductivity and viscosity measurements as a technique to track emulsion inversion by the phase-inversion-temperature method

Two kinds of transitions can occur when an emulsified water-oil-ethoxylated nonionic surfactant system is cooled under constant stirring. At a water-oil ratio close to unity, a transitional inversion takes place from a water-in-oil (W/O) to an oil-in-water (O/W) morphology according to the so-called phase-inversion-temperature method. At a high water content, a multiple w/O/W emulsion changes to a simple O/W emulsion. The continuous monitoring of both the emulsion conductivity and viscosity allows the identification of several phenomena that take place during the temperature decrease. In all cases, a viscosity maximum is found on each side of the three-phase behavior temperature interval and correlates with the attainment of extremely fine emulsions, where the best compromise between a low-tension and a not-too-unstable emulsion is reached. The studied system contains Polysorbate 85, a light alkane cut oil, and a sodium chloride brine. All transitions are interpreted in the framework of the formulation-composition bidimensional map.