Effects Of The Dispersed Droplet Sizes On The Critical Behavior Of Aqueous And Nonaqueous Microemulsions

The coexistence curves (T, n) (T, n are temperature and refractive index, respectively) of water/l,4-di(2-ethyl-l-hexyl)sulphosuccinate(AOT)/decane and dimethylacetamide (DMA)/AOT/octane with different droplet sizes were determined by measurements of refractive indexes of coexisting phases at constant pressure for various temperatures in the critical regions. The coexistence curves of (T, n) were converted to the coexistence curves of (T,φ) and (T,Ψ), (φandΨare volume fraction and effective volume fraction, respectively) by the standard curves. The critical exponentsβ, the critical amplitudes B, the amplitude of Wegner correction terms B₁ and the diameters of coexistence curvesρ_d were deduced from the coexistence curves. The effects of the dispersed droplet sizes on the critical behavior of aqueous and nonaqueous microemulsions were discussed. The experimental results are summarized as follows:(1) All the aqueous microemulsions (AM) have the lower critical temperatures which decrease as the disperse droplet sizes increase; on the contrary, the upper critical temperatures increases with the dispersed droplet sizes for all the nonaqueous microemulsions (NM). The phase transition for the former is controlled by the change of enthalpy, but the latter is controlled by the change of entropy.(2) The critical volume fractions little decrease with the increase of the dispersed droplet sizes for the AM systems and NM systems.(3) The values ofβapproach the 3D-Ising value of 0.3265 for all the AM systems in the regions sufficiently close to the critical temperatures and the Fisher's Renormalization value of 0.365 for all the NM systems. Two types of systems belong to the different universal classes that may be attributed to the solubilization of DMA in the octane, which induces the critical exponentβto be renormalized. The values of critical exponebtsβfor both AM and NM are not affected by the change of the dispersed droplet sizes.(4) The system-dependence amplitudes B of both AM and NM almost linearly increase with the dispersed droplet sizes which are ascribed to the increase of interaction between the droplets.