298
OWE BERG, GAUKLER, AND SQUIER
OH on the surface for Eq. 8 to be valid. This is well in keeping with the
role of hydrogen in the wetting of steel as just discussed. By the same
token, the HNO, should wet the oxide on the copper surface whether it
contains OH or not. This appears reasonable since the oxide reacts
rapidly with acids.
This reasoning should be equally applicable to all metals, including
magnesium. Accordingly, the data for magnesium shouldindicate a sur-
face layer of hydrated oxide, i.e., Mg(OH),, on the metal. It is noteworthy that the average value of tV for the two polarities is almost
exactly equal to the value of tV = 7.5 for a pair of drops.
It is a remarkable fact that the coalescence with a saturated NaCl
solution is slower than the wetting of a dry NaCl crystal. It should be
borne in mind, however, that the coalescence requires the reorientation of hydrogen bonds in both drops, whereas the wetting requires that
this process occur on one drop only. In the saturated NaCl solution,
the sodium and chlorine are surrounded by water molecules,'* and the
NaCl solution should therefore not differ appreciably from pure water.
This has been found to apply to HCl solutions.! The average value of
the product tV for the two polarities does not differ appreciably from
that for pure water. In the dry NaCl crystal, the sodium and chlorine
atoms should be directly accessible to the water in the drop. However,
such atoms exposed to the ambient air, even at very low relative humidities, may be expected to react with the water in the air and to form
effectively NaOH and HCl at the surface. In particular there should be
a surface coverage of OH. Hence, just as explained in the case of hydrogen-bearing carbide in steel, the rate-determining step in the wetting of dry NaCl crystals should be the reorientation of bonds in the
water drop in the direction of the applied field. The value of tV for the
negative crystal is approximately one-half that for two drops. This
indicates that the reorientation of the hydrogen bond is such that H
connects with OH at the surface. The low value of tV for the positive
NaCl crystal should then indicate some accelerating effect of the sur-
face upon the formation of bonds with OH in the water. The nature of
this effect is not revealed by these experiments.
In the case of metallic magnesium, there is a similar polarity ef-
fect. The average value of tV for the two polarities is almost exactly
equal to that for a pair of drops. Again, the polarity effect cannot be
explained.
CONCLUSIONS
1. Wetting of a solid consists in the formation of hydrogen bonds
between H,O in the water and chemically bound OH on the solid. Physically adsorbed H,O on the solid does not cause wetting.