The critical behavior of {water+AOT+decane} microemulsion with various molar ratios of water to AOT

Effects of water content and chain length of n-alkane on the interaction enthalpy between the droplets in water/sodium bis(2-ethylhexyl)-sulfosuccinate/n-alkane microemulsions

“…It indicates that the interaction entropy increases with u 0 and dominates the interaction free energy. These phenomena are consistent with what we observed in investigations of liquid-liquid phase equilibrium of water/AOT/n-decane microemulsion systems near the critical region, where the low critical temperature of the water/AOT/ndecane microemulsion decreased linearly with the increase of u 0 , 17,18,[36][37][38] which suggests that the interaction entropy should dominate the instability of the microemulsion systems we studied.…”

“…[7][8][9] The microemulsion systems of water/sodium bis(2-ethylhexyl)-sulfosuccinate (AOT)/n-alkane mixtures have been most extensively studied. The droplet size of the water/AOT/n-alkane microemulsion was controlled by the water content, which was usually expressed in terms of the water-to-surfactant molar ratio 4,5,10 The water content in the water/AOT/nalkane microemulsion can affect many physicochemical properties of the microemulsion, such as the aggregation number of AOT, the area per AOT molecule and the conformation of AOT at the interface, the state of water in the microemulsion, the bending modulus of the AOT monolayer, the interfacial exibility (or rigidity), the rate constant for exchange of materials between droplets, the threshold temperature and volume fraction of the percolation, [10][11][12][13][14][15][16] and the critical temperature and concentration of the microemulsion system, 14,17,18 which was mainly attributed to the variations of the droplet-droplet interactions. 10,11,14,15,[19][20][21][22][23] The nature of the non-polar phase, especially the chain length of n-alkane molecules, can also affect the droplet-droplet interactions and hence the characters of microemulsions.…”

“…It was found that the potential was negative and the strength of the attractive interaction increases with u 0 29, 31-33 and the chain length n of n-alkane. 29,30 The liquid-liquid phase equilibrium studies showed that the water/AOT/n-alkane microemulsion systems had lower critical solution temperatures (LCST), 17,18,[34][35][36][37][38] and these critical temperatures diminished with the increase of u 0 and n, which were also reasonably attributed to the fact that the droplet-droplet attractive interaction increased with u 0 and n. However this attractive interaction possibly has a free-energy character. 39 Some percolation transition measurements 10,13,15,16 and the studies on the temperature dependence of the clustering 40 and the second virial coefficients of the droplets 27,28 implied that there existed the repulsive enthalpy interaction between the droplets.…”

Liquid–Liquid Phase Equilibria and Interactions between Droplets in Water-in-Oil Microemulsions

“…[Water/sodium bis­(2-ethylhexyl)­sulfosuccinate (AOT)/ n -alkane] microemulsions are the most extensively investigated ones among those widely applied microemulsions. − The liquid–liquid phase equilibria of these microemulsions with different droplet concentrations in the two liquid phases commonly show lower critical solution temperatures (LCSTs), and their phase behaviors vary with the molar ratio of water to surfactants ( w 0 ) and the chain type and the chain length ( n ) of the alkane. ,− This was mainly attributed to the variations in the droplet–droplet interactions dependent on many factors; the nature of which has been a matter of long-standing question. ,− …”

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The liquid−liquid phase equilibria of [water/sodium bis(2ethylhexyl)sulfosuccinate (AOT)/n-decane] with the molar ratio w 0 of water to AOT being 37.9 and [water/ 5,8,/n-decane] with w 0 = 37.9 and the mole fraction α of Dynol-604 in the total surfactants being 0.158 were measured in this study. From the data collected in the critical region, the critical exponent β corresponding to the width of the coexistence curve was determined, which showed good agreement with the 3D-Ising value.

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