When Is Water Not Water? Exploring Water Confined in Large Reverse Micelles Using a Highly Charged Inorganic Molecular Probe (original) (raw)
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Increased fraction of weakened hydrogen bonds of water in aerosol OT reverse micelles
The Journal of Chemical Physics, 2009
X-ray absorption spectroscopy (XAS) and small angle x-ray scattering (SAXS) were utilized to study the effect of fluoride (F − ) anion in aqueous solutions. XAS spectra show that F − increases the number of strong H-bonds, likely between F − and water in the first hydration shell. SAXS data show a low-Q scattering intensity increase similar to the effect of a temperature decrease, suggesting an enhanced anomalous scattering behavior in F − solutions. Quantitative analysis revealed that fluoride solutions have larger correlation lengths than chloride solutions with the same cations but shorter compared to pure water. This is interpreted as an increased fraction of tetrahedral low-density structures in the solutions due to the presence of the F − ions, which act as nucleation centers replacing water in the H-bonding network and forming stronger H-bonds, but the presence of the cations restricts the extension of strong H-bonds.
Novel Structural Model of Reversed Micelles: The Open Water-Channel Model
Langmuir, 1998
According to the classical model of reversed micelles, any water molecules present are solubilized in the micellar core. Here we report unique findings on the nanostructure of reversed micellar aggregates obtained by proton magnetic resonance (1 H NMR) spectroscopy and molecular simulation. 1 H NMR measurements of the chemical shift of water protons indicate that solubilized water can exist in different environments in reversed micellar systems. The molecular modeling shows that water molecules can be localized in channels within the surface of some rodlike micellar aggregates, thereby confirming the "open waterchannel model" of reversed micelles. This finding has significant implications regarding the physicochemical properties and technological applications of reversed micelles.
The Journal of Physical Chemistry, 1996
We have used the fluorescence probe Prodan to characterize the structure of reverse micelles formed in the ternary system of surfactant Aerosol OT, sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/heptane/water. Our results demonstrate that Prodan is a novel and powerful probe of the features of reverse micellar systems as a consequence of its solubility and measurable fluorescence intensity in a wide range of solvents of varying polarity. These characteristics govern the distribution of the probe into the microregions of the reverse micelle and yield fluorescence properties simultaneously indicative of multiple locations. We observe four principal microenvironments for Prodan, including an inner "free" water pool, a bound water region, the AOT interface, and the surrounding hydrocarbon solvent phase. As the parameters of surfactant concentration and the molar ratio of water to surfactant are varied, we attribute the observed emission characteristics of Prodan to specific micellar structural features including heterogeneity of the water pool, the variable polarities of the bound and free water regions, the hydrophobicity and permeability of the surfactant interface, and the hydration of Na + counterions in the bound water region.
The Journal of Physical Chemistry B, 2004
This paper reports results from four nanosecond constant pressure and temperature simulations of sodium di-2-ethylhexylsulfoccinate (AOT) and water reverse micelles (RMs) in an apolar solvent, isooctane. The concentration of our simulated micelles was chosen to fall in a range which in nature corresponds to the L 2 phase of the ternary system. To our knowledge, this is the first study to develop a full molecular model for AOT micelles in an apolar solvent. We address here the problems of the shape of the RM and of its hydrophilic inner core. For the AOT-water system, we obtain nonspherical aggregates of elliptical shape with ratios between major axis, a, and minor axis, c, between 1.24 and 1.41. The hydrophilic inner core is also ellipsoidal with larger a/c ratios. Although experiments indicate that the L 2 of the AOT-water-oil system is likely to be polydisperse, we can only simulate monodisperse RMs. Nonetheless, our simulations are capable of reproducing well the dimensions of the water pool and their dependence on W 0 , as determined in some smallangle neutron and X-ray scattering experiments. Stimulated by recent experiments showing anomalous behavior of the confined water for AOT-water RMs, we have also investigated the static and dynamic properties of the RM's water inner core. From smaller micelles to larger, we find that the properties of confined water tend to near those of bulk water. In particular, we find that the solvation of the counterions is more effective in larger micelles and that diffusion of water is retarded with respect to bulk more in smaller RMs than in larger.
Probing the Structure and Dynamics of Confined Water in AOT Reverse Micelles
The Journal of Physical Chemistry B, 2013
Reverse micelles are attractive nanoscale systems used for the confinement of molecules in studies of structure and chemical reactions, including protein folding, and aggregation. The simulation of reverse micelles, in which a water "pool" is separated from a nonpolar bulk phase by a surfactant layer, poses significant challenges to empirical force fields due to the diversity of interactions between nonpolar, polar, and charged groups. We have explored the dependence of system density, reverse micelle structure, and water configurational relaxation times as a function of reverse micelle composition, including water:surfactant ratio, absolute number of water molecules, and force field using molecular dynamics simulations. The resulting structures and dynamics are found to depend more on the force field used than on varying interpretations of the water:surfactant ratio in terms of absolute size of the reverse micelle. Substantial deviations from spherical reverse micelle geometries are observed in all unrestrained simulations. Rotational anisotropy decay times and water residence times show a strong dependence on force field and water model used, but power-law relaxation in time is observed independent of the force field. Our results suggest the need for further experimental study of reverse micelles that can provide insight into the distribution and dynamics of shape fluctuations in these complex systems.
Proton NMR study on the structure of water in the Stern layer of negatively charged micelles
The Journal of Physical Chemistry, 1987
The proton chemical shift of water was studied as a function of the concentration of three negatively charged surfactants (sodium dodecyl sulfate, sodium dodecylbenzenesulfonate, and sodium perfluorooctanoate) and the deuterium content of the solvent. The effect of the simple presence of ionic head groups of the surfactants (Le., in the absence of the micellar interface) on the structure of water was determined by studying the behavior of the model short-chain compounds: sodium butyl sulfate, sodium p-toluenesulfonate, and sodium perfluorobutyrate. Graphs of the chemical shifts vs. the mole fractions of the solubilizates were linear in all cases and the slopes were used to calculate the so-called "fractionation factor, cp" (cp = 1 for the bulk solvent) of the water of hydration. Little structure perturbation was produced by the solubilization of the short-chain compounds, the sulfate and the sulfonate anions being slight structure breakers (cp = 0.97 f 0.04) whereas the perfluorobutyrate was found to marginally enhance the structuring (cp = 1.02 f 0.03). The water in the Stern layer of the micelles was found, however, to be more organized than bulk water (cp = 1.07 f 0.04), in agreement with other measurements of NMR chemical shifts, relaxation times, and self-diffusion coefficients. The utility as well as some possible limitations of this new approach is discussed.
Simple Oxovanadates as Multiparameter Probes of Reverse Micelles
Langmuir, 2007
Using a wide range of different methods, researchers have found that the environment inside reverse micelles differs from bulk aqueous solution in many ways. Here, we present a new tool, a series of aqueous oxovanadium(V) reactions, to probe pH, viscosity, and ionic strength in the aqueous interior of reverse micelles. In addition to their potential as anionic probe analogues to phosphates, simple oxovanadium(V) compounds have equilibrium characteristics in aqueous media exquisitely sensitive to their environment. Therefore, the speciation of vanadate equilibria can be used as a parameter to characterize the intramicellar medium. Vanadate speciation is monitored through 51 V NMR spectroscopy, which also yields information through chemical shifts and linewidths of spectral features. The speciation observed suggests that the relative acidity of a basic vanadate stock solution is slightly reduced in large, w 0 g 12, reverse micelles, but that for smaller reverse micelles, speciation reflects the strong interaction of these negatively charged oxometalates with the reverse micelle and suggest an increased solution viscosity in the reverse micelles. This interpretation is obtained through different responses closely linked to the reverse micellar size and the specific conditions in the stock solutions used to form reverse micelles.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2016
The microenvironment of water droplets of nonionic polyoxyethylene (4) lauryl ether reverse micelles was investigated using infrared, ultraviolet-visible and nuclear magnetic resonance spectroscopy, also by dynamic light scattering. The different influence of kosmotropic and chaotropic ionic additives on the microstructure of reverse micelles was revealed. The values of chemical shift in the presence of structure-breaking perchlorate ions are higher than their values in case of structure-making acetate ions according to measurements by proton magnetic resonance spectroscopy. Deconvolution of the O-H stretching vibrational absorption spectra in the region of 3000-3800 cm-1 into three subpeaks with a Monte Carlo method showed that perchlorate ions promote an increasing of free water fraction in the water pools of the reverse micelles, whereas acetate ions support formation of bound water. Influence of salt additives results in slight different sizes of water droplets, measured by dynamic light scattering method. Different influence of kosmotropic and chaotropic anions on binding constant K b of optical probe to ethylene oxide groups was revealed.