Volumetric Properties of Water/AOT/Isooctane Microemulsions

Abstract

The densities of AOT/isooctane micelles and water/AOT/isooctane microemulsions with the molar ratios R of water to AOT being 2, 8, 10, 12, 16, 18, 20, 25, 30, and 40 were measured at 303.15 K. The apparent specific volumes of AOT and the quasi-component water/AOT at various concentrations were calculated and used to estimate the volumetric properties of AOT and water in the droplets and in the continuous oil phase, to discuss the interaction between the droplets, and to determine the critical micelle concentration and the critical microemulsion concentrations. A thermodynamic model was proposed to analysis the stability boundary of the microemulsion droplets, which confirms the maximum value of R being about 65 for the stable AOT/water/isooctane microemulsion droplets.

The Thermodynamic Properties of Dynol-604/AOT Mixed Reverse Micelles in Isooctane

“…The experimental points in the higher surfactant concentration range in Fig. 2 show that v / 2 linearly decreases with increasing W 2 , indicating negative values of a, which is in accord with what we observed previously for AOT/isooctane reverse micelles in aqueous solutions [14]. We fitted the values of v / 2 by using Eq.…”

“…C m1 was determined to be (4.62 ± 0.58) 9 10 -4 [14]. C m2 cannot be experimentally determined due to the low Dynol-604 solubility in isooctane.…”

“…5 to obtain the intercept v 0 drop and the slope a, which are given in Table 2 When a micelle solution is continuously diluted, the apparent specific volume of the solute shows a sharp decrease with W 2 as shown in Fig. 2, which may be attributed to the non-negligible dissolution of monomeric AOT and Dynol-604 in the isooctane solvent, since the apparent specific volume of the solute in the micelle state is larger than that in the continuous phase due to the strong repulsion between the negative charged head groups of AOT [11,14]. This behavior may also be interpreted by considering the tails of the surfactants being able to rotate freely in the continuous phase.…”

“…However, only a few studies of the volumetric properties of reverse micelles [14], and no volumetric studies of mixed reverse micelles, have been reported. Bis(2-ethylhexyl) sodium sulfosuccinate (AOT) is a well known surfactant that has been widely used to prepare reverse micelles and microemulsions in oil and water/oil systems, among which the AOT/isooctane reverse micelles and AOT/isooctane/water microemulsions have been most extensively studied.…”

The Size of AOT Reverse Micelles

“…Given W 0 = 7.5, n AOT = 62 (from this study), and ρ = 1.1 g/cm 3 (estimated from 2014 Du et al), 52 equation 7-a yields V core 18,414 Å 2 . Equation 7-b yields v̄ = 33 Å 3 per molecule, and r core = 16.4 Å.…”