Nature of Water in Reverse Microemulsions

Numerous investigations in nanoparticle formation through W/O microemulsions have used extensive ranges of relative water content (indicated by the ratio w = [water]/[surfactant]) not only to obtain optimized conditions of synthesis, but also to examine the effect of relative water content on the size of the aqueous droplets and the particles synthesized therefrom. Such control parameters are not linked with merely the water content perse. It has been shown (as discussed below) that the structural arrangements and chemical bondings of the water molecules in the droplets of W/O microemulsion can vary with water content. 

Furthermore, it has also been shown that these different solubilized states of water in reverse microemulsions can in addition affect the particle synthesis process. Thus, Arriagada and Osseo-Asare [137] showed that with low [water]/[surfactant] values (less than 4) in Aerosol OT/decane reverse microemulsions, all water was bound to the surfactant head-groups and could not be used for reactions related to particle synthesis (see Section 5.3.1). 

It is therefore interesting and important to have a look at the results of investigations made in this respect using various techniques, e.g. nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy and electron paramagnetic resonance spectroscopy. The major and overall findings are discussed below. 

In an early work, Kotlarchyk et al. [67] studied AOT reverse micelles where purified AOT still had a small quantity (w = 0.7 0.2) of water. These authors showed that the polar core contained water bound to the sulfonate head-groups. Jain et al. [138] suggested at least three different solubilizate environments and hence, three types of molecular water in AOT/isooctane/water reverse microemulsions. They are (i) the interface water, (ii) the bound water layer and (iii) the water pool. Interfacially trapped water molecules were indicated to be located in between polar head-groups of AOT as monomers or dimers. Bound water molecules were bonded to the head-groups of AOT through the counterions (Na ) present at the interface. Bulk water deeper in the pool had greater mobility. Water molecules, of course, also hydrated the counterions. Another possibility was presence of water, albeit in negligible amounts, in the oil phase. 

Somewhat more specific definitions of the above description came from other authors. Thus, Hauser et al. [139] found that in reverse micelles in the system AOT/isooctane/water, thirteen molecules of HjO, divided among the three types of water with differing attractions for AOT-Na, were affected by one AOT molecule two of them were more strongly bonded, while the rest eleven were weakly associated with the surfactant. Kon-No and colleagues [130, 140, 134] studied reverse microemulsions in systems AOT / (heptane / cyclohexane / isooctane /dodecane)/water and BDDAB (butyldodecyldimethylammonium bromide)/ chlorobenzene/water for examining the state of water molecules. They observed that, as discussed above, there were up to three types of water as w was increased  

---