Structure of water-in-oil microemulsions of AOT by infrared spectroscopy

Abstract

(ricevuto il 28 Ottobre 1994)Summary. --The structure of water in water/AOT/n-heptane reverse micelles has been studied as a function of the [H20]/[AOT] ratio (W) by using the absorption IR due to O--H stretching modes in the 3800-3000 cm-1 range. The results show that the IR spectra can be expressed as a sum of contributions from bound-and bulk-like water. The fraction of water in the two ~,regions-within the water pool was evaluated as a function of W. The ,(bound), water region seems to hold 3.5 water molecules (corresponding to 7 0 --H oscillators) per AOT molecule and its formation is nearly complete at W > 6. PACS 31.25 -Electron correlation calculations for atoms and molecules. PACS 01.30.Cc -Conference proceedings. -I n t r o d u c t i o n .The surfactant sodium bis(2-ethyl-hexyl)sulfosuccinate (AOT) is known to form spherical, nanometer sized, molecular aggregates in a variety of non-polar organic solvents. These are often referred to as reverse micelles. The hydrocarbon tails of the AOT molecules are oriented toward the exterior of the aggregate and the sulphonate head groups, with the sodium counterions being localized in the interior of the aggregate. Of particular interest is the ability of AOT reverse micelles to solubilize relatively large amounts of water in this polar core forming a so-called ,,water-pooN. This offers a unique opportunity to study the properties of water aggregates close to the ionic centres (head polar groups and the corresponding counterions of the surfactant) without the interference due to large quantities of bulk water.Although most of the properties of water in reverse AOT micelles have been closely investigated [1][2][3], little attention has been paid to their spectra of vibrational bands [4][5][6][7][8][9][10]. In previous papers the use of infrared spectroscopy as a sensitive tool for the study of water into AOT reverse micelles has been described [6][7][8]. Infrared

Spectroscopic Studies of Water‐Soluble Porphyrins with Protein Encapsulated in Bis(2‐ethylhexyl)sulfosuccinate (AOT) Reverse Micelles: Aggregation versus Complexation

“…TSPP H‐ and J‐type aggregation in RM : These results suggest the existence of different aggregates of TSPP in AOT reverse micelles which depend on the amount of solubilized water and local pH. As mentioned before18c, 22c, 33 the water in these RM has peculiar solvent properties which are quite different from those of free water (higher viscosity, lower dielectric constant) and therefore the water activity must be such that the usual concept of pH is no longer valid. Moreover, the size of the water‐pools (or even the shape for some RM)18c, 34 depends on ω 0 .…”

The Influence of Water on the Photophysical and Photochemical Properties of Piroxicam in AOT/iso-octane/Water Reversed Micelles

“…FTIR results (50) revealed that only the first few water molecules bind to the and Na ϩ ions. So, additional wa-Ϫ SO 3 ter molecules would, then, localize in the core forming water pools.…”

Direct Observation of Terahertz Surface Modes in Nanometer-Sized Liquid Water Pools

“…These systems, nanometer-sized water droplets stabilized in heptane by the surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT), have been characterized extensively with respect to their size, shape, size dispersion [19,20], and the nature of the water pool at the core of the micelle [21,22]. For these measurements, AOT inverse micelles were prepared according to standard methods, using a surfactant purified as in Ref.…”

“…This could be because of changes in the nature of the water pool as its size decreases. In the smallest micelles, a substantial fraction of the internal water is bound rather than free water [22]. The calculated surface tension relies on parameters, such as the interior liquid density and its static dielectric constant, which could be affected by the increasingly strained hydrogen bond network.…”