Transport of salicylic acid through supported liquid membrane based on ionic liquids

Abstract

Transport of salicylic acid (SA) through flat-sheet supported liquid membrane (SLM) was investigated using as liquid membrane the ionic liquids 1-hexyl-3-methylimidazolium hexafluorophosphate ([C 6mim][PF 6]) or ethylammonium nitrate (EAN). Using [C 6mim][PF 6], it has been observed that the transport efficiency decreases with increasing pH, indicating that the un-dissociated form of SA is mainly extracted. On the other hand, the ionic dissociated form of salicylic acid is mostly extracted via the anion exchange mechanism between nitrate and salicylate anions when EAN is used as liquid membrane. Parameters such as nature and concentration of the strippant in the receiving phase and concentration of the SA in the feed phase were studied. By comparing the SLM transport efficiency of SA (initial flux) of the two used ionic liquids, EAN appears to be slight efficient than [C 6mim][PF 6]. Despite the use of different stripping solutions (NaCl, NaOH and Na 2CO 3) and even with pH maintenance around initial values, uphill transport driven by pH difference was not observed using both ionic liquids. The absence of uphill transport has been attributed to the formation, along the course of the experiment, of water microenvironments (aggregates) inside the ionic liquid. SA transport through these water microenvironments inside the liquid membrane becomes the main mechanism. The main feature of SLMs based on ionic liquids is their higher stability compared to classical SLMs. In fact, our SLM system retained its stability and initial performance during the 9 days long experiment.