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Previously we have investigated the structure of adsorbed CnE4 layers at the EAN−air interface using neutron reflectivity and vibrational sum frequency spectroscopy as function of alkyl chain length (n = 12, 14, 16), concentration, and temperature. The tetra(ethylene glycol) head group layer was found to be thin, compact (incorporating only ∼30% EAN by volume), and disordered; i.e., there is no preferential orientation of the ethylene glycol groups. Similarly, the alkyl tail groups have a significant number of gauche defects indicating a high degree of conformational disorder. The thickness of this nonpolar layer is much less than the fully extended chain length, but it does increase with alkyl chain length. Lowering the concentration of C12E4 from 1 to 0.1 wt % decreased the surface excess, and the head group layer became thinner and less solvated, but C14E4 and C16E4 adsorbed layers were unaffected over the same concentration range, indicating that their cmc’s are lower than 0.1 wt %. There are no previous reports of surfactant structure adsorbed at the surface of mixed ionic liquids. However, nonionic surfactant aggregation and cloud point behavior in the bulk have been studied using C14E5 in mixtures of 1-ethyl-3- methylimidazolium tetrafluoroborate (emimBF4) and 1-hexyl- 3-methylimidazolium tetrafluoroborate (hmimBF4). C14E5 is insoluble in emimBF4 at room temperature but is completely miscible in hmimBF4 with no evidence of micelle formation or a cloud point up to at least 100 °C. When the two ionic liquids were mixed, a cloud point was noted, which increased with increasing hmimBF4 content. Likewise, micelle formation was observed in the ionic liquid mixtures, and cmc’s varied as a function of ionic liquid composition. As the concentration of emimBF4 increased, the critical micelle concentration decreased, and the micelle size increased, owing to reduced solubility. This shows that surfactant properties can be controlled by varying the ionic liquid composition. In this work the adsorption of the nonionic surfactant C14E4 at the air−liquid interface of ionic liquid mixtures and ionic liquid/water solutions is investigated using surface tensiometry and neutron reflectivity. The behavior of C14E4 has previously been extensively characterized at the EAN−air surface, in micelles and microemulsions in EAN, in other protic ionic liquids, and in aqueous systems. The selected solvents (H2O, EtAN, EAN/H2O, EtAN/H2O, and EAN/EtAN) aim to explore the effect of solvophobicity and solvophilicity on solvent−surfactant and solvent−solvent interactions. Results are compared to those for aqueous and other ionic liquid systems. Materials. Ethylammonium nitrate (EAN) and ethanolammonium nitrate (EtAN) were prepared by mixing equimolar amounts of base, ethylamine (Aldrich) or ethanolamine (Aldrich), with nitric acid (Aldrich) in aqueous solution at 10 °C. Water was removed from the ionic liquid solutions by rotary evaporation at 45 °C, followed by nitrogen purging and heating at 110 °C. This led to water contents undetectable by Karl Fischer titration. Partially deuterated EAN and EtAN were prepared by the same method; however, after the heating step the ionic liquids were mixed with D2O (Aldrich). D2O:EAN was mixed in a 3:1 molar ratio which substitutes the ammonium hydrogens with deuterium, and D2O:EtAN was mixed in a 4:1 molar ratio to also replace the exchangeable hydroxyl hydrogen. The solutions were redried via rotary evaporation, nitrogen purging, and heating. Hydrogenous nonionic surfactant tetradecyl tetraethylene glycol ether (C14E4) was purchased from Nikkol, Japan, and was used as received. The tail deuterated tetradecyl tetraethylene glycol ether surfactant (d-C14-h-E4) was purchased from R. K. Thomas, University of Oxford, U.K. The surfactant was dried in a vacuum oven prior to use. Surface Tension Measurements. The air−liquid interfacial tension measurements were performed on a Dataphysics OCA20 optical tensiometer using the pendant drop method. A volume of liquid is suspended from a capillary, and an image of the drop is captured. The Young−Laplace equation is fit to the drop image using SCA20 software to calculate the surface tension. Measurements were conducted at 20 °C. The capillary and sample were placed within a transparent container, and a Petri dish containing the liquid was positioned below the capillary to minimize water adsorption by the drop. Drops were monitored for 30 min, and all samples had reached an equilibrium surface tension by this time. Neutron Reflectivity. Measurements were conducted on the INTER reflectometer at ISIS, Rutherford Appleton Laboratories, Didcot, U.K. Samples were placed into Teflon troughs inside a chamber purged with nitrogen. A neutron beam (wavelength = 1.5−16 A) was directed at the liquid surface at an incident angle θ = 0.8°. Measurements were performed at 20 °C. Solutions of tail deuterated d-C14-h-E4 in a series of hydrogenous and partially deuterated EAN/H2O, EtAN/H2O, and EAN/EtAN mixtures were prepared at (1.1−1.3)cmc, as determined by the surface tension measurements. This resulted in varied absolute bulk surfactant concentrations ranging from 8.0 × 10−4 to 2.2 × 10−2 wt %. In the technique of neutron reflectivity, a beam of neutrons is directed at the surface at an incidence angle, θ, and the specularly reflected neutrons are measured as a function of the neutron’s momentum transfer, Q, given by Neutron reflectivity is sensitive to changes in scattering length density and, therefore, provides information perpendicular to the surface about the structure of the adsorbed surfactant, provided there is contrast between the scattering length densities of the adsorbed species and the solvent. In this work, which aims to elucidate information on the surfactant hydrocarbon tail and head group regions, contrast is achieved by selectively deuterating the tail group only. This gives a large contrast between the tail and head groups Additionally, two solvent contrasts are used. The hydrogenous solvents have a scattering length density similar to that of the surfactant head group, and in these systems the reflectivity is primarily a consequence of the surfactant tails. The deuterated solvents’ scattering length density is different from those of both the surfactant tail and head groups, and hence the reflectivity arises from the entire surfactant molecule. The neutron reflectivity data were modeled using the Motofit reflectivity analysis package. A scattering length density model is generated from which a theoretical reflectivity curve is calculated and compared to the measured data. For each system, four parameters were fixed during data fitting: the instrumental scale factor (adjusts reflectivity below the critical edge to 1), the solvent scattering length density (Table 1), the gas phase scattering length density (set to 0), and the sample background. The data were fit using a slab method which models the interface as a series of layers of uniform scattering length density and constrained by known molecular dimensions (see Table 1). The hydrogenous solvent systems, which provide information on the surfactant hydrocarbon tail, are modeled as a single layer. During the fitting process, the layer thickness was upper bounded by the length of the extended hydrocarbon chain, and the scattering length density was allowed to vary between 0 (the scattering length density value for air) and the value for the deuterated tail .The deuterated solvent systems, which provide information on the entire surfactant molecule, were fit using two layers. The thickness and scattering length density of the uppermost layer, which represents the surfactant tail group oriented toward the gas phase, was set to the same parameters as the hydrogenous system. Roughness was included between the tail layer and the gas phase and the tail layer and the bulk liquid, and was constrained to remain below 5A a common value for surfactant systems. The second layer, which describes the surfactant head groups facing the bulk liquid, was limited by the length of a fully extended tetra(ethoxy) group, and the scattering length density was allowed to vary between the values for the head group and those for the partially deuterated solvent. Again, roughness was included between interfaces of adjacent slabs. The data sets from the both the hydrogenous and deuterated solvents were fit simultaneously, giving confidence in the final model. [ Last edited by zhudaiwozou on 2013-3-26 at 14:39 ] |
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