Effects of Tetramethyl- and Tetraethylammonium Chloride on H2O: Calorimetric and Near-Infrared Spectroscopic Study

The effect of tetraethylammonium chloride (TEAC) on H2O was investigated by the 1-propanol (1P) probing thermodynamic methodology developed by us earlier. It was found that TEAC is an amphiphile with a small hydrophobic and a dominant hydrophilic contribution. An earlier application of the same 1P-probing methodology to tetramethylammonium chloride (TMAC) indicated that the latter is as hydrophilic as urea without any hydrophobic contribution. The hydrophilic effect of TEAC was found to be about twice stronger than that of TMAC. To investigate further these surprising findings, we applied a new analysis method using the concept of the excess partial molar absorptivity of the solute on the nu(2) + nu(3) combination band of H2O in the near-infrared (NW) range of their aqueous solutions. The results confirmed that both salts are indeed strongly hydrophilic toward H2O which manifests itself in the 5123 cm(-1) chromophore of the NIR band of H2O. Furthermore, we suggest from the behavior of the 5263 cm(-1) band that both solutes might form small aggregates in the H2O-rich region of the respective aqueous solutions.