Molar ratio of water to surfactant

The amount of water solubilized in a reverse micelle solution is commonly referred to as W, the molar ratio of water to surfactant, and this is also a good qualitative indicator of micelle size. This is an extremely important parameter since it will determine the number of surfactant molecules per micelle and is the main factor affecting micelle size. For an (AOT)/iso-octane/H20 system, the maximum Wq is around 60 [16], and above this value the transparent reverse micelle solution becomes a turbid emulsion, and phase separation may occur. The effect of salt type and concentration on water solubilization is important. Cations with a smaller hydration size, but the same ionic charge, result in less solubilization than cations with a large hydration size [17,18]. Micelle size depends on the salt type and concentration, solvent, surfactant type and concentration, and also temperature. 

Water activity can be nsed to quantify water in all non-conventional media and is used to an increasing extent. However, in microemulsions containing reversed micelles, water is still often quantified as the molar ratio of water to surfactant (abbreviated as Wq or R). This parameter is used because it is correlated with the size of the reversed micelles, which in turn is correlated with enzymatic activity. 

C. Wang, X. Guo, X. Wang, R. Wang, J. Hao, Gas-phase propylene epoxidation over Ag/TS-1 prepared in W/O microemulsion Effects of the molar ratio of water to surfactant and the reaction temperature, Catal. Lett. 96 (2004) 79. 

The structure of the AOT micellar system, as well as the state of water entrapped inside swollen micelles, have been characterized using different techniques, such as photon correlation spectroscopy (25), positron annihilation (26), NMR (27, 28), fluorescence (29-32) and more recently small angle neutron scattering (33). The existence of reversed micelles has been demonstrated in the domain of concentrations explored by protein extraction experiments. Their size (proportional to the molar ratio of water to surfactant known as wo), shape and aggregation number have been determined. Furthermore, the micelle size distribution is believed to be relatively monodisperse. 

Several groups conducted SD experiments on the reverse micelle system. They all found a slow component in the SD in the nanosecond timescale. The timescale was found to depend on the radius of the water pool rw and molar ratio of water to surfactant Wo. Sarkar and co-workers studied the SD of C480 in AOT- -heptane-water microemulsions. They observed a distinct rise in the nanosecond timescale at the red end of the emission spectra. They observed that in a small water pool (Wq = 4, = 8 A) the solvation time was 8 ns, wltile for a large water pool (Wq = 32, =... 

Also, for W/O microemulsions, the conductance usually increases with the water content of the system, which is customarily expressed as the molar ratio of water to surfactant, symbolized by u o or R. Naturally the situation is even worse for quaternary systems of any type (W/O or O/W) if the fourth component is an electrolyte. In this case, the birefringence experiment ( jump) is associated with a simultaneous temperature jump that complicates the interpretation of the data. Nevertheless, usefiil information has been obtained from such experiments. 

Table 4 Linear Correlations for the Size Dependence (tw) on the Molar Ratio of Water to Surfactant,...

W 0 is the molar ratio of water to surfactant. Self-diffusion coefficient values are for neat water =271 and for neat isooctane Z) = 235. Dpaimoii for 5% palm oil in isoctane is 60.3. Confidence interval 80%. 

In the opposite case of very small droplets, AOT-hy-drated micelles can be considered. The high-frequency dielectric response of very small AOT reverse micelles has been analyzed (118,119) at a molar ratio of water to surfactant ofW< 10. The avrage radius R- of the water core is related to Why the semi empirical relation R = (1.25 W + 2.7) + (13, 36). For almost dehydrated reverse micelles < 5 A, one can expect that nearly all the counterions are bound in the siufactant layer stractiue and immobilized. The dynamics of such a dehydrated system with a charac-... 

Titanium dioxide nanoparticles were produced by the controlled hydrolysis of titanium tetraisopropoxide (TTIP) in the presence of reverse micelles formed in CO2 with the two fluorinated surfactants [53]. Based on dynamic light scattering measurements, the amorphous Ti02 particles formed by injection of TTIP were larger than the reverse micelles, indicating surfactant reorganization. The size of the particles and the stability of dispersions in CO2 were affected by the molar ratio of water-to-surfactant headgroup, (iVq), the precursor concentration, and the injection rate. 

Ikushima et al. made a detailed study of the effects of pressure and molar ratio of water to surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) on the physicochemical properties such as the type of interior water molecules, micropolarity, and the structures of AOT reverse micellar aggregates in SCFs media by using high-pressure FTIR and UV spectroscopies (see Section 2.3.2 for their results) [20]. 

A microemulsion is a thermodynamically stable isotropic dispersion of two immiscible liquids, where the microdomain of either or both liquids is stabilized by an interfacial film of surfactant molecules. In water-in-oil microemulsions, the aqueous phase is dispersed as microdroplets (typically 1-50 nm in diameter) surrounded by a monolayer of surfactant molecules in the continuous hydrocarbon phase. The size of the reverse micelle is determined by the molar ratio of water to surfactant ... 

Compared to macro- and miniemulsions, the water uptake is lower in microemulsions. By virtue of their smaller sizes and, subsequendy, the more efficient packing of the surfactant at the interface, microemulsions are not subject to flocculation effects. In the field of microemulsions, the water uptake is defined as the molar ratio of water to surfactant, W. The corrected Wq (called taking account of the solubility of water in CO2 is defined as ... 

Figure 18.12 The appareDt second-order rate (fc ) of Diels-Alder reaction (DAR) between A-ethylmaleimide and 2,3-dimethyl-l,3-butadiene at 313.15 K (a) in the IL tnicroemulsion with various molar ratios of IL to surfactant (w) and surfactant concentration (C ot)> microemulsion with various molar ratios of water to surfactant (w) and surfactant concentration (Caot)- Reproduced from Lii et al. [52] with permission from Wiley Blackwell.

The effect of water content (expressed as the molar ratio of water to surfactant, w ) of the microemulsion system on the catalytic behavior of enzymes has been studied most extensively in recent years. For many enzymes studied in reverse micelles, bell-shaped dependence of enzyme catalytic behavior (expressed as V,n.T or k ) as a function of w, usually with an optimum activity occurring when is between 5 and 15, has been observed [8] (Figure 13,2),... 

Figure 9.12. Solvent effect on the partition coefficient of tryptophan, represented by ln(/Tx) versus the inverse molar ratio of water to surfactant. Plots of 100/ Wo for 0.2M AOT are shown for the following solvents O, cyclohexane , heptane , isooctane , decane. (Reprinted with permission from ref. (37a) copyright 1990, the American Chemical Society)...

---