(750g) Impact of Different Factors On Phase Behavior of Microemulsion System

Winsor type II and III microemulsion can solubilize more organics and are more effective for solubilizing organics than Winsor type I microemulsion, and can treat organic contaminants under room temperature so as to reduce the energy consumption of the recovery. Organic contaminants solubilization in Winsor type II and III microemulsion depends on many factors, including surfactant, the type of electrolyte, acidity and alkalinity, the size of molecular of the organics, water/oil ratio and so on. In this work the influence of different factors, including the types of surfactant, electrolyte, organics and water/organics ratio, on the microemulsion system will be investigated, in order to realize the maximum solubilization of different types of organics.

For the sodium dodecyl sulfate (SDS)- cetyltrimethyl ammonium bromide (CTAB)/ n-butanol/ kerosene/ water microemulsion system, with the increase of the content of CTAB, the Winsor III type microemulsion began to appear at lower content of n-butanol during the alcohol scan process. The HLB of the system changed when CTAB was added. Because of the strong electrostatic attraction between the cations and anions, the association of the two kinds of ions was greatly promoted, leading easier forming of micelle on the interface. On the other side, CTAB also played the role of electrolyte (to compress the double electrical layer and promote the oil-solubility). With the increase of the mass ratio between CTAB and SDS, the HLB of the mixed surfactants decreased, more organics and less water can be solubilized into the Winsor type III microemulsion.

For the salinity scan of the SDS/ n-butanol/ kerosene/ water microemulsion system, each type of salt had its typical length of salinity concentration for forming Winsor type III microemulsions. The high surface charge density of Ca²⁺ makes it more effective than K⁺ and Na⁺ in decreasing the HLB of surfactant system, and Winsor type III and II microemulsion can be formed under much less salt concentration. The microemulsion phase behavior when using KCl (or NaCl) was similar with using KOH (or NaOH). The results show that for the same cation (K⁺ or Na⁺), the type of monovalent anion (Cl^- or OH^-) has little influence on the SDS/ n-butanol/ kerosene/ water microemulsion state, even though KOH and NaOH are alkalis. It was observed that the effect of K⁺ is stronger than Na⁺. As to the effect of NaCl and Na₂CO₃, it can be found that NaCl is more effective than Na₂CO_3, althought they have the same monovalent cation, Na⁺. The reason is that the bivalent CO₃^2- leads to a lower activity of Na⁺ than the monovalent Cl^-. CaCl₂ was used to prepare Winsor type III microemulsion, in which the volume fraction of solubilized kerosene is 48%, and then Na₂CO₃ was added into the system, causing the Ca²⁺ precipitate into CaCO₃, and the Ca²⁺ in the system was substituted by Na⁺. When proper quantity of Na₂CO₃ was added, the Winsor type III microemulsion disappeared and type I formed. The volume fraction of kerosene solubilized in the Winsor Type I microemulsion is only 17%. Thus, after exchanging Ca²⁺ with Na⁺ through precipitation, 50% of the organics was released from the Winsor type III microemulsion. With the further addition of Na₂CO₃, it canbe seen that the microemulsion system changes from Winsor type I→III→II, and the volume fraction of kerosene solubilized in the microemulsion gradually changes from 17% to 67%.

Thus, the phase behavior can repeatedly convert between Winsor type III (or II) and I with the precipitation and re-dissolution of Ca²⁺. After the replacement of Ca²⁺ by Na⁺ in aqueous solution, the Winsor type III (or II) goes back to type I microemulsion, whose organics solubilization capacity is relatively small, resulting in the release of most of the initially solubilized organics. And with the addition of Na⁺, the Winsor type I microemulsion converts to type III (or II) again and more organics can be solubilized into the microemulsion gradually. These results indicate that it is promising to use microemulsion phase behavior in the recovery of organic contaminants and the reuse of surfactant (cosurfactant).

Cyclohexane, n-heptane, gasoline and kerosene are used to investigate the influence of organics on the formation of SDS/ n-butanol/ organics/ water microemulsion system (n-butanol scan and salinity scan). The molecular weight of n-heptane is greater than cyclohexane, which makes the resistance much greater for the n-heptane molecules to pass through the interfacial membrane into the micelles. More n-butanol is needed for n-heptane to reduce the interfacial tension and increase the interfacial flexibility and mechanical strength of the composite membrane formed by surfactant and co-surfactant. Gasoline is made up of hydrocarbons from C₄ to C₁₀, containing more small-molecule hydrocarbons, which makes it be solubilized more easily and less n-butanol is needed than n-heptane. Kerosene is made up of many bigger molecule hydrocarbons, so more n-butanol is needed to form microemulsion. The results show that the smaller the molecular weight of organics is, the more easily they are solubilized and the larger the volume of Winsor type III microemulsion is formed. Similarly, for salinity scan under the same concentration of n-butanol, the length of salinity for middle phase microemulsion is cyclohexane <gasoline <n-heptane <kerosene and the Winsor type III microemulsion volume is cyclohexane >gasoline >n-heptane >kerosene.

The influence of water/ratio was also investigated. Winsor type I microemulsion was formed innitially. Water was then added gradually into the system so as to change the water/oil ratio in the system. What was attractive is that the change from microemulsion Winsor type II→III→I was observed and more and more oil was released gradually. When the volume of water increased to 18ml, 9ml or 90% (v) oil was released from the microemulsion. These phenomena mean that when organic contaminants were microemulsified into Winsor type II or III microemulsion for the purpose of organic contaminants disposal, water may be added to release the organics from the microemulsion and the surfactants can be reused.