Critical Behavior of {Water + AOT + Decane} Microemulsion with Small Molar Ratio 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.…”

“…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

“…It also has been confirmed by different independent experiments for similar microemulsion systems. [11][12][13][14]33,35 Moreover, in a wider temperature range of |T − T c | < 6 K, we fitted the experimental data of Z cxc to eq 5 with the values of β and Δ being fixed at theoretical values of 0.326 and 0.50, respectively, to obtain the values of B ̂0Z and B ̂1Z , which are listed in columns 4 and 5 of Table 1. From Table 1, it can be seen that the differences between the values of B ̂0Z from the two fits described above and the correction-to-scaling term are not significant, which implies the critical singularity being applicable for the coexistence curves in the temperature range of about 6 K from the critical point.…”

“…It has been found that the droplet–droplet interaction in (water/AOT/ n -alkane) microemulsions was attractive, and its strength increased with w 0 and n of n -alkane. ,,− This interaction was interpreted by an attractive square-well energy potential between two droplets, which seemed to be supported by the fact that the (water/AOT/ n -alkane) microemulsions had LCST and it diminished with the increase in w 0 and n because the depth of the energy potential should increase well with temperature, w 0, and n . ,− However, some studies showed the existence of repulsive enthalpy (or energy) interaction between the droplets, ,,− which was unfavorable to droplet aggregation and phase separation. Therefore, drawing the droplets too close together and inducing the phase separation in AOT-based microemulsions are driven by the positive entropy change due to the release of the solvent molecules confined in the surfactant tails of the droplets; thus, the “energy potential” should have the Gibbs free energy character. , Our recent isothermal titration microcalorimetry (ITC) studies on the (water/AOT/oil) microemulsions confirmed the importance of the entropy effect on the stability of droplets in these microemulsions and found that the droplet interaction enthalpy varied with the nature of the oil phase and the molar ratio of water to surfactants, in some cases, from positive to negative. , However, as a key feature of microemulsion systems, interactions between droplets remain a confusing and challenging issue …”

“…[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. ,− …”