Influence of the pH of Water on its Electron-Accepticity and Donicity

ABSTRACT

By means of contact angle measurements on dry layers of electrostatically neutral dextran with pure water (pH 6.1), water acidified with HCl (to pH 1.94) and water made alkaline with NaOH (to pH 12.8), it could be shown that there was essentially no change as a function of pH in the ratio of γ⁺/γ⁻ of water as compared with the aqueous acid and alkaline solutions. (Here γ⁺ is the Lewis acid parameter of the polar surface tension component of water and γ⁻ is its Lewis base parameter). In contrast, with contact angles measured with the same liquids on negatively charged clean glass, a significant decrease in contact angle was observed with water at pH 12.8, which was caused by the fact that at this alkaline pH an increase in surface hydrophilicity took place. This is because surfaces that have a given surface electrical potential at neutral pH generally acquire an even higher surface potential under more alkaline conditions which, concomitantly, also gives rise to an increase in surface hydrophilicity, and thus to lower contact angles with water. Finally, contact angles with acid water, pure water, and alkaline water, deposited on hydrophobic Parafilm surfaces, were exactly the same.

Keywords:

• pH of water
• Contact angles of water drops
• Surface tension of water
• Lewis acid parameter of surface tension
• Lewis base parameter of surface tension

Acknowledgments

This paper is one of a collection of articles honoring Manoj Chaudhury, the recipient in February 2005 of The Adhesion Society Award for Excellence in Adhesion Science, Sponsored by 3M.

We gratefully acknowledge the role played by Mr. Arno Hefers, Research Assistant, Texas Transportation Institute, Texas A & M University, College Station, Texas 77843, who queried by e-mail on 29 April 2004: “Do you know if anyone has determined surface energy components (LW, Acid and Base) for water at different pH levels?” As we did not believe that anyone had done that so far, we tried it and hope that the present paper furnishes an answer.

Notes

^a Precleaned glass microscope slides, Fisher

^b Dextran (Mw 100,000–200,000; ICN Pharmaceuticals, Cleveland, OH, USA), deposited on glass microscope slides, air-dried and kept in a vacuum desiccator

^c Reverse osmosis purified water, as is

^d P = 0.0002: This signifies that there is one chance out of 5,000 that the difference between a water contact angle of 9.7° (at pH = 1.94) obtained on glass, and a water contact angle of 6.8° (at pH = 12.8) could have occurred by chance.

^a RO water, as is; ^b For the definition of P, see note d of Table 1. All three P values shown in Table 2 indicate that the (very slight) differences between any of these three (averaged) contact angles are insignificant.