Water/surfactant molar ratio

Arriagada FJ, Osseo-Asare K (1999) Synthesis of nanosize silica in a nonionic water-in-oil microemulsion Effects of the water/surfactant molar ratio and ammonia concentration. J Colloid Interface Sci 211 210-220... 

Experimental Information. The review by Ekwall — offers a whole series of phase diagrams which all show similar behavior. In order to dissolve an anionic surfactant with a sodium counter ion in an alcohol a minimum water/surfactant molar ratio of about six is needed to achieve solubility. The corresponding ratio for the potassium ion is three. 

T3.8.2.3 Obtained in Microemulsion Ammonium hydroxide was added to a solution containing Igepal Co520 (surfactant) and cyclohexane, and the mixture was shaken to obtain a microemulsion. Solutions of zirconium n-propoxide in acetylacetone and n-butanol (l 0.5 3) and of tetraethyl orthosilicate in water and ethanol (1 1 4) were mixed for 3 h, and the mixture was added to the microemulsion with stirring, and hydrolyzed for 3 d. The final concentrations of alkoxides were 0.0075 M, the water surfactant molar ratio was 0.8, and the water alkoxide molar ratio was 10. The powder was washed with acetone and heated at 900°C for 2 h. 

FIG. 7 Dependence on the water/surfactant molar ratio of (a) the maximal reaction rate normalized to the enzyme concentration, V/E, of a-chymotrypsin-catalyzed hydrolysis of N-benzoyl-L-tyrosine p-nitroanilide, and (b) the rotational frequency, v, of the spin label in the active site of the enzyme in the system AOT-water/glycerol-octane. Water/glycerol volume ratios 1—100 0 2—80 20, 3—50 50, 4—20 80, 5—6 94. Dashed lines show V/Eq and v values in aqueous solution. (From Ref. 42.)... 

Such behavior appears to be quite typical regardless of the type of surfactant present. For instance, in the system sodium bis(2-ethylhexyl)phosphate-w-heptane-water, the transition from a W/O to a bicontinuous and then to an O/W microemulsion can be induced by increasing the water/surfactant molar ratio. Again in the transition region higher viscosities are observed where this increase is significantly more pronounced at the O/W to bicontinuous transition [90]. 

Figure 9 Effect of the water/surfactant molar ratio (/ ) on the mean diameter of SiOi particles prepared with different ammonia concentrations. (From Ref 78.)...

Guizard et al. [84,85] studied the acid-catalyzed hydrolysis and condensation of titanium alkoxides [i.e., titanium isopropoxide (TIPO) and titanium tetrabutoxide (TTBO)] in polyoxyethylene- )-octylphenyl ether/decane/water microemulsions. The nonionic surfactants used were Triton X-15 (TX-15), TX-35, and TX-45. Dynamic light scattering [85] showed that the hydrodynamic radii of droplets in the TX-35/decane/water system increased from about 2,2 nm in the absence of water to about 4 nm at water/surfactant molar ratio R = 2. The radius remained approximately constant up to about R = 4, above which phase separation occurred. The constant droplet size observed above R = 2 suggests that 2 mol HiO per mole of surfactant is needed for the hydration of the polar groups. 

The gelation time (tg) was found to decrease exponentially with the water/surfactant molar ratio [84,85] ... 

Hirai et al. [97] used the AOT/isooctane/water system and titanium tetrabutoxide (TTBO). Particle diameters on the order of 3 nm were obtained by dynamic light scattering, a dimension that is smaller than the diameters of the reverse micelles (9-19.3 nm). Particle formation was strongly influenced by the water/surfactant molar ratio (R) and by the alkoxide concentration. The reaction kinetics was followed with UV-Vis absorption spectrophotometry. The absorbance of the reaction system increased monotonically with time for R = 9, whereas it went through a peak (after 400 min) for R = 30. These results were rationalized in terms of differences in availability of reactant water molecules. For... 

The term microemulsion indicates an optically transparent, thermodynamically stable, isotropic dispersion of nanometric-sized droplets of one liquid in another (immiscible) liquid, stabilized by interfacial layers of surfactant molecules. Note that there is no generally agreed extent of solubilization when a micellar solution can be said to have transformed into a microemulsion [99]. However, in case of water solubilization, Pileni [100] indicates that when the [water]/[surfactant] molar ratio (= w) exceeds a value of 15, we have a microemulsion, below which the term reverse micelle is preferred. In this text, this differentiation has not been strictly adhered to instead, a general term reverse microemulsion has often been used as a matter of convenience. 

Recently, Moran et al. [264] made a detailed study of nanoparticle formation in reverse micelles in the system AOT/cyclohexane/water with tetrapropyl titanate or its solution in cyclohexane added into it. One of their observations, relevant for the present discussion, was that with a given water/alkoxide molar ratio the Ti02 crystallite size increased with increasing water/surfactant molar ratio (= w), while for a given value of vw the crystallite size increased with decreasing values of water /alkoxide molar ratio. 

For controlling interdroplet interactions, co-surfactants (alcohols of different chain lengths) were added into the system. The effective particle diameter of hydrated titania was found to decrease monotonically with increase in the chain length of the alcohol (butanol to octanol) irrespective of the water/surfactant molar ratio (10 or 20). It was also shown that with a systematic increase in the ratio [octanol]/[Ti(AOT)4], i.e. 0.2-1.0, the effective diameter of hydrated titania decreased monotonically. Finally, the particle size was found to increase with the concentration of Ti(AOT)4. 

Two reverse microemulsions containing AOT/isooctane/aqueous solution of zinc nitrate and AOT/isooctane/aqueous solution of sodium sulfide were mixed by Hirai etal [375] for obtaining ZnS. The water/surfactant molar ratio was varied in the range 3-10. With the ratio at 6, the particle diameter was found to increase from about 2.4 nm to about 3 nm as a function of time (up to 100s). This increase was found to be restricted by the micellar size. In a similar work by the same group [358], the particle diameter was demonstrated to increase as a function of water/surf actant molar ratio from about 3.1 nm (w — 4) to 3.7 nm (w = 7.5-10). At w > 10, excessive agglomeration was noted. 

Parameters like water/surfactant molar ratio, the specific permittivity of the oil phase and temperature of synthesis were examined vis-a-vis their effect on the particle size. 

FIG. 7 DSC-exo (top) and endo (bottom) of two samples of the system water-Na(AOT)-isooctane [volume fraction (water + surfactant)/total = 0.31 water/surfactant molar ratio = 37]. The isooctane freezes at 145 K and thus does not influence the thermal behavior of the water phase. Curve 1, To (the temperature at which the samples were kept isothermaUy at the very begiiming of the DSC analysis) = 313 K curve 2, Tq = 300 K. The small exotherm in the melting curve is due to recrystalUzation. (From Ref. 41.)... 

In system A the surfactant becomes saturated with water at Nweo = 3, where iVw/Eo is the number of (interphasal) water molecules per ethylene oxide (EO) group of the surfactant. For this water/surfactant molar ratio, (total) water content is again about 30 wt%. 

FIG. 17 Fraction of bound water (a) as a function of the water/surfactant molar ratio (W ) for system A, along the water dilution line W5. (O) DSC data [45] ( ) NMR data [14]. (Results based on chemical shift measurements were taken from Fig. 7 in Ref. 14 and reduced to scale. The decrease of a with the increase of W is slower when a is evaluated from Ti relaxation time data see Table 2 in Ref. 14.)... 

Water-in-oil microemulsions are characterized by a micellar core formed by the polar heads of the surfactant protruding into the water droplet, surrounded by a layer of alkyl chains protruding into the surrounding apolar liquid (Fig. 12). The micellar size and shape depend on the nature of the surfactant molecule, the water/surfactant molar ratio, and the presence and location of solutes within the micellar core [93]. 

An important factor (also valid for other systems) is the careful choice of the parameter w = [water]/[surfactant] molar ratio. With small values of w, the available water is often utilized in hydrating the polar head-groups of the surfactant molecules, and does not take part in the hydrolysis of die alkoxide molecules (Stathatos et al., 1997). Hirai et al. (1993) showed that wito w < 30, hydrolysis of Ti-tetrabutoxide in isooctane/NaAOT/water system did take place, but no particle was formed unless w went beyond 30. There are, however, instances where very low w values were used for synthesis of titania particles (Kluson et al., 2001). 

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