Investigation on the trioctylphosphine oxide-based super-concentrated HCl system

• We have investigated a reverse-micelles-based super-concentrated acid system.
• The surfactant plays an important role in obtaining higher nHCl/nH2O ratio.
• The system provides a unique environment where coordination state of Cu2+ changes.

We report a new super-concentrated HCl system prepared by using trioctylphosphine oxide (TOPO)-based reverse micelles. The observed molar ratio between acid and water (nHCl/nH2O) in the super-concentrated HCl are much higher than that in the saturated aqueous HCl solution (0.28). Moreover, FT-IR spectroscopic results reveal that a significant amount of HCl remains in the molecular form rather than being ionized into H+ and Cl−. As a result, two HCl stretching bands can be observed in the FT-IR spectrum. The super-concentrated HCl provides a unique chemical environment in which many chemical substances occur in unusual states. For example, the color of super-concentrated HCl solution containing copper ion was found to be reddish brown rather than green as in conventional state. UV–Vis–NIR spectral results indicate that both d–d transition band of Cu2+ and charge transfer band of ClCu in super-concentrated HCl underwent significant variation. Additionally, copper ions bring about remarkable variation on the hydrogen bond network among HCl in the super-concentrated HCl solution as demonstrated by FT-IR spectra. According to the EXAFS results, we suggest that copper ion may occur as HCuCl3 in the super-concentrated HCl.

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