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 ν₂ + ν₃ combination band of H₂O in the near-infrared (NIR) 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

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 ν₂ + ν₃ combination band of H₂O in the near-infrared (NIR) 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

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 ν₂ + ν₃ combination band of H₂O in the near-infrared (NIR) 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

Effects of Ethanol and Dimethyl Sulfoxide on the Molecular Organization of H₂O as Probed by 1-Propanol

We characterized the effects of ethanol (ET) and dimethyl sulfoxide (DMSO) on H₂O within a limited H₂O-rich region by the 1-propanol (1P)-probing methodology developed by us earlier. The results are displayed on a two-dimensional map with twin coordinates: one pertaining to hydrophobicity and the other to hydrophilicity. The locus of ET on this map was at a point in between methanol (ME) and 2-propanol (2P) as expected from our earlier findings by thermodynamic studies. That for DMSO, however, was surprisingly more hydrophilic than ME. Similar to N-methyl groups discussed recently (<i>J. Phys. Chem. B</i> <b>2011</b>, <i>115</i>, 2995), it was argued that the methyl groups attached to the S atom are made susceptible for direct hydrogen bonding to the surrounding H₂O molecules due possibly to the electronegativity of the S atom. In view of these findings, we suggest caution to be exercised for the conventional general trend of taking any methyl groups to be “hydrophobic.

How Much Weaker Are the Effects of Cations than Those of Anions? The Effects of K⁺ and Cs⁺ on the Molecular Organization of Liquid H₂O

We characterized the effects of K⁺ and Cs⁺ ions on the molecular organization of H₂O by the 1-propanol probing methodology, previously developed by us (<i>Phys. Chem. Chem. Phys.</i> <b>2013</b>, <i>15</i>, 14548). The results indicated that both ions belong to the class of “hydration center”, which is hydrated by 4.6 ± 0.8 and 1.1 ± 0.5 H₂O molecules, respectively, and leave the bulk H₂O away from hydration shells unperturbed. Together with our previous results for the total of 7 cations and 11 anions, we display resulting characterization on a 2-D map and show a quantitative difference in their strength of the effects on H₂O between anions and cations

Effects of Carboxylate Anions on the Molecular Organization of H₂O as Probed by 1-Propanol

We characterized the effects of carboxylate anions, formate (OFm^–), acetate (OAc^–), and propionate (OPr^–), on the molecular organization of liquid H₂O by the 1-propanol (1P) probing methodology. The latter thermodynamic methodology provides two indices: one pertaining to the hydration number, <i>n</i>_H, and the other being related to the net increase/decrease of the entropy–volume cross fluctuation of the system. The results indicated that OFm^– is a hydration center with <i>n</i>_H = 1.2 ± 0.5 and leaves the bulk H₂O away from the hydration shell unperturbed. We suggest that this single H₂O hydrates preferentially one of the O’s in the COO^– group, showing the hydration center character. The values of <i>n</i>_H for OAc^– and OPr^– were found to be 3.7 ± 0.8 and 9 ± 2, respectively, out of which one H₂O molecule is used for hydrating the COO^– side and the remaining 2.7 and 8 H₂O molecules hydrate the respective alkyl group. Hence, OPr^– is more hydrophobic than OAc^– in terms of the hydration number. However, both alkyl moieties seem to equally retard the hydrogen bond probability of bulk H₂O away from hydration shells around nonpolar sites, as much as the probing 1P does