Small‐Angle X‐Ray Scattering and Near‐Infrared Vibrational Spectroscopy of Water Confined in Aerosol‐OT Reverse Micelles

Abstract

The state of water confined in Aerosol-OT-hydrocarbon-water reverse micelles with cyclohexane, n-pentane, n-octane, and n-dodecane as apolar solvents is investigated by small-angle X-ray scattering and near-infrared vibrational spectroscopy of the first overtone of the OH stretching mode of water. The experiments focus on water/AOT molecular ratios W(0)=2-20, where water is strongly affected by the confinement and surface-water interactions. The pair-distance distribution functions derived from the small-angle scattering patterns allows a detailed characterization of the topology of these systems, and they indicate deviations from monodisperse, spherical water pools for some of these hydrocarbon systems. In contrast to a common assumption, the pool size does not scale linearly with W(0) in going from dry reverse micelles (W(0)-->0) to essentially bulk-like water (W(0)>20). The first overtone of the OH-stretching vibration exhibits highly structured spectra, which reveal significant changes in the hydrogen bonding environment upon confinement. The spectra are rationalized by a core/shell model developed by Fayer and co-workers. This model subdivides water into core water in the interior of the micelle and shell water close to the interface. Core water is modelled by the properties of bulk water, while the properties of shell water are taken to be those of water at W(0)=2. The model allows the representation of the spectra at any hydration level as a linear combination of the spectra of core and shell water. Different approaches are critically reviewed and discussed as well.

Tracing nucleation pathways in protein aggregation by using small angle scattering methods

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Nanoconfinement Raises the Energy Barrier to Hydrogen Atom Exchange between Water and Glucose

“…Inexpensive and nontoxic, AOT is the most widely used surfactant when preparing reverse micelles and is the amphiphile used in this study. Characterization by small-angle X-ray scattering, small-angle neutron scattering, and dynamic light scattering has given scientists a good understanding of AOT reverse micelles. , The encapsulated water molecules that form the core of the reverse micelle can interact with other molecules dissolved in the reverse micelle, for example, osmolytes, proteins, and probe molecules. − …”

Water loading driven size, shape, and composition of cetyltrimethylammonium/hexanol/pentane reverse micelles

“…Protein encapsulation within RMs have also found use in protein studies including purification [7][8][9], as nanoscale reaction vessels for biocatalysis [7,10], as a confinement based stabilizer of protein structure [11][12][13], and in a variety of protein NMR applications [14]. Perhaps the most well understood RM systems are those comprised of bis(2-ethylhexyl)sulfosuccinate (AOT), with a variety of methods being used to characterize the physical parameters of these systems [15][16][17][18][19][20][21][22][23]. Despite a large avenue of research of RM encapsulated proteins within AOT RMs, it has become apparent through protein NMR studies that nearly all proteins are unfolded when encapsulated within the core of AOT reverse micelles [24].…”