Ionic liquid-in-oil microemulsions

Eastoe J, Gold S, Roger SE, Paul A, Welton T, Heenan RK, Grillo I (2005) Ionic liquid-in-oil microemulsions. J Am Chem Soc 127 7302-7303... 

A good example was brought by Moniruzzaman et al. which have used ionic liquid-in-oil (IL/o) microemulsions to enhance the topical and transdermal delivery of acyclovir (ACV). The microemulsion was composed by a blend of nonionic surfactants, namely polyoxyethylene sorbitan monooleate (Tween-80) and sorbitan laurate (Span-20), isopropyl myristate (IPM) as an oil phase, and IL [Cimim](CH30)2P02 (dimethylimidazolium dimethylphosphate) as a pseudophase. The solubility of ACV on the microemulsion system significatively increased in the presence of IL, which act as a drug reservoir during the process of delivery. Moreover the transdermal delivery was only achieved when the IL was present in the microemulsion mixture. 

The reaction yields obtained greater in microemulsions than in pure IL highlight a strong effect of confinement. Moreover, a direct correlation between the quantity of IL and the reaction yield was observed. IL in oil microemulsions confined the reaction and offered the advantage of using a smaller quantity of ionic liquid compared to bidk phase. 

Our experiments demonstrated that micelle formation is possible in ILs and that size and aggregation number of the micelles can be timed by changing the exact nature of the IL. Moreover, as observed in water the formation of lamellar structures of DPPC in different ILs was demonstrated. Lastly three micro-regions of the microemulsion - ionic hquid in oil, bicontinuous and oil in ionic liquid - were identified in the ternary system... 

An interesting application of ionic liquids (ILs) concerns their use in combination with classical surfactants [1,2]. Indeed, they can suitably replace each of the microemulsion components (aqueous phase, apolar phase, and surfactants) conferring peculiar features to self-assembled systems. Indeed, ILs are salts and as such have affinity for water, but they also typically possess a lipophilic moiety, and this means affinity for oils. Depending on their chemical structure, ILs can act as cosolvent either for water or for oil. In addition, when their hydrophilic and hydrophobic nature are both strong enough, a fraction of ILs will reside preferentially at the interface formed by the surfactant, and this can impact dramatically the interfacial physics, drastically changing the microemulsion structure and dynamics. 

The behavior of liquid primary alcohols in various surfactant systems is of course not universally the same. It can depend on the type of surfactant and/or of the other components in the microemulsion system [7,8,18,19,106-132] the latter fact justifies and increases the importance of the previous sections, which reveal the properties of pure alcohols and alcohol/water systems. The effects of alcohols also strongly depend on their partition between the water phase, oil phase, and interface surfactant film. Alcohol partition behavior in ionic surfactant systems was thoroughly reviewed by Zana [7]. In the following we focus more on their different roles and the variety of effects on the structure and intermolecular interactions in microemulsion systems. These phenomena will be presented for the example of a ternary system composed of the nonionic surfactant Brij 35 at moderate concentrations, water and one of the simple alcohols from ethanol to 1-decanol, and will be supported with an additional literature review [7,109-119]. 

Moreover, we could demonstrate that biodiesel can act as oil phase in high temperature microemulsions highlighting a way towards the formulation of biocompatible microemulsions (Zech et al., 2010 c). These model systems can be extended to other ILs, with [bmim][BF4] instead of EAN as polar phase, where a remarkable thermal stability can be achieved as well (Zech 2010). The different microemulsions containing protic ionic liquids, the methods used to characterize them and the corresponding references are summarized in Table 2. 

Zech, O. Bauduin, P. Palatzky, P. Touraud, D. Kunz, W. (2010 c). Biodiesel, a sustainable oil, in high temperature stable microemulsions containing a room temperature ionic liquid as polar phase. Energy Environmental Science, 3(6), 846-851. 

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