The Young-Dupre equation for the work of adhesion of a liquid drop to a solid surface, where the solid surface is in equilibrium with the vapor of the liquid, is given as W = yi/1 + cos ), where yi is the surface tension of the liquid and the contact angle. This work (W) has generally been identified with the free energy of adhesion. It is shown here that it constitutes the total work of adhesion only under the artificial condition that the sessile drop retains its shape after detaching from the solid surface. Under "real" conditions, W represents only one component of the total free-energy change taking place when a drop is separated from, or attached to, a vapor-equilibrated smooth solid surface. In the present work, a Net Free Energy of Adhesion, AFn, is derived which gives the total free energy necessary to separate a sessile drop from a smooth solid surface to form a free sphere (its negative, of course, is the free energy of attachment of the sphere). It is given by AFn = 2 yi[(2a/sin ß)2'3 -a], where r is the radius of the solid-liquid interface and a, called the "effective area", is [2/(1 + cos )\ -cos . The Net Free Energy of Adhesion and Young-Dupre work of adhesion are compared as functions of the contact angle. This is done for systems of constant solid-liquid interfacial area and for systems of constant drop volume.

molar entropy for Hg, s0*^u sed In the calculation was that for the gaseous metal (174.8 J mol"1), the appropriate value being that for the liquid metal (76.0 J K~1 mot1). As a result, the value of dAtf>o/d7 reported In ref 7 Is high by 1.02 mV K"1 mol"1, the correct value being -0.40 mV K~1 mot"1. A similar error was made in ref 8 where the gas-phase molar entropy of Ag (182.9 J K"1 mof1) was used instead of that for the solid state (42.55 J K~1 mol"1). The corrected estimate of dA0 o/dTfor formamide on the basis of the data discussed Is 1.0 mV K'1 mol"1.

The contact angle of water on the (0001) surface of oriented graphite evacuated to the 10-1°Torr decade is found to be 35 ± 4°. The value is considerably lower and more reproducible than those in the range of 50-80°found for the surface prior to evacuation. The higher values found before evacuation are apparently due to hydrophobic organic contamination. Ion bombardment of the evacuated surface reduces the contact angle to 0°. All vacuum contact angle measurements are made in situ with surface cleanliness monitored by Auger electron spectroscopy. The decrease in contact angle from the value of approximately 35°after evacuation to that of 0°after ion bombardment is due either to removal of chemisorbed hydrogen or hydrocarbon (not detectable by AES), or to disarrangement of the hexagonal structure of a clean (0001) surface to yield high-energy surface sites. It is proposed that the wettability of graphite ( 0001) is related to its conducting properties.

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