## H+ and OH− in H2O

The effects of H^{+} and OH^{−} on liquid H_{2}O have attracted much attention by spectroscopic and computer simulation studies. In this work, we offer an alternative approach; from a differential solution thermodynamic. Namely, we use higher order derivative quantity of *G* with respect to independent variables (*p*, *T*, *n*_{i}) or a combination thereof, where *n*_{i} is the molar amount of the i-th component. We believe that the higher order derivative quantity contains finer and more detailed information than the original *G*. In Figure 1 we show the mole fraction (*x*_{i} = *n*_{i}/∑*n*_{i}) dependence pattern of a 3^{rd} derivative quantity, , we call i-i enthalpic interaction function.

Until we discovered the results shown in Figure 1, the definition of hydrophobic/amphiphilic/hydrophilic solutes remained primitive. Amphiphiles have hydrophobic and hydrophilic moieties within the same molecules. Figure 1 shows that their effects on work additively by hydrophobic and hydrophilic moieties. This finding and others led us to what we call the 1-propanol (1P) probing methodology. We first dissolve a test sample, **S**, whose characterization of its effect on H_{2}O is to be sought to its mole fraction , more dilute than point X in Figure 1. We let **S** works on H_{2}O. We then characterize the situation of the resulting aqueous solution by determining by using 1P as a probe. 1P is a typical hydrophobe and the pattern takes a peak type anomaly as shown in Figure 1. Since **S** has already modified the bulk H_{2}O somewhat the pattern will be modified a little. If **S** is a hydrophobe the peak top shifts to the west (to a smaller value of *x*_{1P}). While a hydrophilic **S** moves the peak top to the south. An amphiphile, on the other hand, shifts the peak top to the south-west direction, the westward component of which is its hydrophobic contribution and the southward one its hydrophilic component. All these movements are linear to. Their slopes we define as hydrophobicity and hydrophilicity indices respectively.

We applied this methodology to **S** = NaCl salt. Since is small ( < 0.035), NaCl dissociates completely to Na^{+} and Cl^{−} in aqueous solution. The results suggest that both Na^{+} and Cl^{−} ions form hydration shells around Na^{+} with 5.2 H_{2}O molecules and 2.3 for Cl^{−}. More importantly, however, they do not perturb bulk H_{2}O away from hydration shells at all. This finding opens a new route to characterize in terms of hydrophobicity and hydrophilicity indices. For a given test ion **S**^{+} and **S**^{−} we use Cl^{−} and Na^{+} as the counter ions respectively and treat a neutral salt. From the results, the pertinent indices for Cl^{−} and Na^{+} can be subtracted. Thus, we applied the methodology to characterize the effects of H^{+} and OH^{−} on H_{2}O. The results are mapped in Figure 2 together with other normal ions. It turned out that H^{+} is an amphiphile and its equilibrium structure takes the Zundel type H^{+}(H_{2}O)_{2}. OH^{−}, on the other hand, acts like a hydrophobe-like hydration center which is hydrated by about 12 H_{2}O molecules, but the bulk H_{2}O away from hydration shells remain unperturbed at the outset. However, as the probing 1P increases and as the available H_{2}O decreases, OH^{−} exerts such an effect beyond the hydration shells that the hydrogen bond probability of bulk H_{2}O away from hydration shells is reduced gradually just as hydrophobes do to H_{2}O.

*Yoshikata Koga, Kumiko Miki, Keiko Nishikawa*

*Department of Chemistry, The University of British Columbia, Vancouver, BC, V6T 1Z1, Canada*

*and Suitekijuku, Vancouver, BC V6T 2J4, Canada*

### Publication

Effects of H+ and OH- on H2O as probed by the 1-propanol probing methodology: differential thermodynamic approach.

Koga Y, Miki K, Nishikawa K

*Phys Chem Chem Phys. 2017 Oct 18*

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