ROBERT A. ERB
1806
Wettability of Metals under Continuous Condensing Conditions
by Robert A. Erb Chemistry Division, The Franklin Institute Laboratories, Philadelphia, Pennsylvania (Received October g2, 1964)
1910s
The wettability of a number of metals has been measured under continuous condensing conditions in pure steam for extended periods of time. Water exhibits high contact angles on the noble metal samples studied; the average advancing angles are Au, 55-85”; Ag, 68-89” ; Rh, 65-82’ ; Pd, 74”; and Pt, 50”. Water condensed in a filinwise inanner on Ni, Cd, Ti, Cr, Type 316 stainless steel, copper-nickel lo%, and quartz samples, indicating zero or low contact angles on these surfaces. Experimental evidence relating the wettability of clean metal surfaces to surface oxide present is considered and the conclusion is established that a clean “high-energy surface” of metal substantially free of oxygen is not wettable by water.
Introduction The statement has often been made in surface chemistry that a clean “high-energy surface” is by nature wettable. That this is not a true general statement has been seen recently in the experimental work of Whitell in which he showed that water has a fairly high contact angle on clean gold and that water will spread on gold only if a gross oxide film is present. Fowkesz has provided a theoretical basis for the nonwettability of oxide-free metal surfaces, relating to the dispersion-force nature of the interaction between water and the oxide-free metal surface. This approach predicts accurately the high interfacial tension between water and mercury (approximately 426 ergs/cni.2) and confirins the experimentally observed nonspreading of water on mercury. The principal experiniental problem in studying the wettability of solid metals (for example, in the forms of electroplated or mechanically polished specimens) is the requirement that organic surface contamination must be reinoved without subjecting the surface to powerful oxidizing treatments which could result in oxide formation. This problem has been overcome in our program by the use of a system in which the samples to bg studied serve as vertical condensing surfaces in a pure steam atmosphere in a closed refluxing system where they may be washed continuously by the freshly condensed water for inany weeks a t a time. Under these conditions, physically adsorbed organic surface contamination is generally reinoved within a few hours and The Journal of Physical Chemistry
chemisorbed contamination within a few days; this statement is based not only on our experience, but also on the extensive literature on the behavior of organic promotors of dropwise condensation on metal condensing surface^.^
Experimental Two different apparatus were used in this study, each with outer vessels constructed of Type 316 stainless steel, with boiling water in the bottom part of the vessel and a condenser in the upper part. I n the first apparatus a hollow, water-cooled copper core served as support for 2.5 X 7.6 cni. metal flats attached to its surface. In the second, eight vertical tubes, 1.3 cin. diameter X 13 cin. in length and closed a t the bottom end, were internally water-cooled. Electrically heated Pyrex windows were provided in each apparatus for examination of the condensing surfaces. Great care was taken to avoid organic contamination. The following steps were taken in this direction. (1) Grease-free fittings were used throughout, with Teflon or Viton seals for windows and top plate. ( 2 ) The stainless steel and Pyrex vessels were cleaned rigorously (1) M.L. White, J . Phys. Chem., 68,3083 (1964). (2) F. M. Fowkes, ibid., 67, 2538 (1963); also in “Contact Angle, Wettability, and Adhesion,” Advances in Chemistry Series, No. 43, American Chemical Society, Washington, D. C., 1964,pp. 99-111. (3) R. A. Erb, “Dropwise Condensation: A Bibliography of the Literature on Dropwise Condensation from 1930 to 1964,”to be published a s an Office of Saline Water Research and Development Progress Report.
WETTABILITY O F AIETALS UXDER CONTINUOUS C O N D E N S I N G CONDITIONS
before being put into service, including refluxing with trichloroethylene, then with isopropyl alcohol, then with water, followed by a final wash with sodium dichromate-sulfuric acid cleaning solution and a rinse with pure water. (3) The water to be refluxed (about 5 1. in a charge) was redistilled in an all-Pyrex apparatus from alkaline permanganate solution in order to destroy any organic contamination. (4) During the condensation runs, pressure in the vessels was maintained above atmospheric to prevent laboratory air from entering the vessel. ( 5 ) The refluxing water was completely drained and replaced with organicfree water several times in each condensation run; this is an additional aid in eliminating residual organic contamination. (6) Steam was bled off many times during each run; this assisted in removal of noncondensable gases, as did the procedure used at the outset involving displacenient of air with argon followed by evacuation by water aspirator. For the first apparatus, the sample flats whose wettability is discussed later were prepared in the following ways. The solid silver sample had a rolled finish and was heated in a tube furnace to 600' in a nitrogen atmosphere. The other silver sample was electroplated from a cyanide bath on a Type 316 stainless steel base. The gold sample had a commercial rolled finish. The rhodium samples were electroplated to a thickness of 35 p by Sel-Rex Corp. The palladium, platinum, and titanium samples were polished mechanically using aluminum oxide abrasives. The nickel and cadmium samples were electroplated with a matte finish on a copper-nickel 10% base. The fused quartz sample was given a final cleaning in sodium dichroniatesulfuric acid cleaning solution, with a distilled water rinse. The metal samples were rubbed lightly with Bon-Ami and water, with copious rinsing before mounting. The tube samples for the second apparatus were prepared without mechanical polishing of the final surfaces. The Type 316 stainless steel tube and copper-nickel 10% tube were used as received, having a bright finish. The gold-, silver-, and rhodium-surfaced tubes were bright-electroplated by Sel-Rex Corp. The gold coating is 99.5% gold and 0.5% silver. The chromium-surfaced tube was bright-electroplated by Philadelphia Rustproof Co. The only treatment given these tubes before insertion in the apparatus was a quick wipe with ethanol on a cellulose wiping tissue. Contact angles were measured 011 drop profile images using a Gaertner telemicroscope with protractor eyepiece and right-angle cross hairs. The flat plate apparatus has a slow condensation rate per unit area (because of a high ratio of condensing area to power input),
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and measurements could be made on drops by direct observation through the telemicroscope. The tube apparatus had for this experimental run a condensation rate of water of about 4 X g./cm.2-sec,,which is one order of magnitude higher than that with the flat plate system. For convenience in measuring the contact angles of the rapidly growing, merging, and sweeping drops, the images and cross hairs were projected onto a ground-glass screen. For each sample, both advancing and receding contact angles were measured. The advancing angles were measured a t the leading edge of drops about to slide under gravitational influence. The receding angles were measured similarly a t the trailing edge of drops a t the start of sliding. Five advancing and five receding contact angles were measured for each subst rate.
Results and Discussion Table I lists the average advancing and receding contact angles and their standard root-mean-square deviations for sample flats in the first apparatus after 3650 hr. of continuous condensation. The contact angle of water appears to be zero on the quartz sample, which is one indication of freedom of the system from organic contamination. The contact angles of water on the nonnoble metals, cadmium, nickel, and titanium, appear to be near zero, with any break in the condensed film of water occurring near the bottom of the sample, where the accumulated washing by condensed water is the greatest and where the oxide film might be partly removed by the washing action.
Table I : Wettability of LMetal Surfaces after 3650 Hr. of Continuous Condensation with Pure Water --Average contact angle, deg.Advancing Receding
Sample
Silver (99.9%), heated in Hz to 600" Silver, plated on Type 316 stainless steel Gold (99.9%) Rhodium, plated on silver Rhodium, plated on copper-nickel 10% Palladium Platinum Titanium Nickel Cadmium Quartz
85 i 5
74 f 2
89 f 3 85 f 3 65 f 3
55 f 11 46 f 2 51 f 7
82 f 2 73 f 1 74 f 3 51 f 7 50 f 6 30 f 2 90% filmwise condensation; 5 % mixed 70% filmwise condensation; 20y0 mixed 100% filmwise condensation 100% filmwise condensation
Volume 69,Number 4
A p r i l 1965
ROBERT A. ERB
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Water has a fairly high advancing and receding contact angle on each of the noble metal surfaces studied here. Platinuni is the most wettable of these, with a 50" advancing angle and a 30' receding angle. Table I1 lists the advancing and receding contact angles of water on the tube condenser surfaces in an atmosphere of water vapor a t 115'. Neither the condensation conditions nor the sample preparations were the same in the two apparatus, and the contact angles on the bright electroplated noble metal surfaces were somewhat lower than for the sample flats listed in Table I. The gold advancing angles averaged about 61' as compared to 85', the silver was about 69" compared with 87', and the rhodium about 65" compared with 73'. The essentially hydrophobic nature of the noble metals under continuous condensing conditions with high purity steam is confirmed, however. It is of interest to note that water condenses in a filmwise fashion on the chromium, stainless steel, and copper-nickel 10% surfaces. This wettability, we believe, is due to the oxide films on the surfaces. Our
Table I1 : Wettability of Metal Tube Surfaces after 1500 Hr. of Continuous Condensation with Pure Water Sample description
-Average contact angle, de&Advancing Receding
7.6 p Au over 5.1 p Xi over copper66 f 3 55 f 3 nickel 10% base 1.3 p Au ovcr 7.6 p Xi w e r copper55 f 3 47 f 2 nickel 1OY0 base 13 p Ag over 13 p Xi over copper68 f 2 60 f 1 nickel 10% base 13 p Ag over Type 316 stainless 70 & 2 50 f 2 steel base 0.25 p Rh over 2.5 p Au over cop65 & 2 55 f 1 per-nickel 10% base 13 p Cr over 25 p Ni over copper- (Filmwise condensation over 95% of the area) nickel 10% base ( Filmwise condensation over Copper-nickel 10% 95% of the area) ( Filmwise condensation over Type 316 stainless steel 1007, of the area)
experimental evidence for this could be seen when, after about 1900 hr. total condensation time, the dark oxide film on a small area near the bottom of the copper alloy tube had washed off, leaving a bright copper-colored area; on this area the water condensed in a dropwise fashion, with a contact angle in the order of 60-70'. Because the steam at this time was substantially oxygen-free, the bright area continued to exist without darkening. Further evidence of the role of oxide in the wettaThe Journal of Physical Chemietry
bility of the metals was provided in an experiment at 2000 hr. in which oxygen was introduced into the chamber in an aniount equal to 0.3% of the chamber volume. Several effects could be seen. (1) The copper-nickel and the chromium samples, which had showed dropwise or mixed condensation in the areas most subject to washing, reverted to 100% filmwise condensation ; this was accompanied by a darkening of the coppernickel sample. (2) The contact angle on the silver surfaces was lowered temporarily (by perhaps 20'). (3) The gold and rhodium samples appeared to be unchanged as to wettability. The final wettability in contact with oxygen at this small partial pressure and the changes caused by the addition of the oxygen are closely related to the "nobility" of the metals under consideration. Two factors appear to affect the degree of coverage of the metal surfaces by oxide under the continuous condensing conditions. One factor is the fraction of surface covered by oxygen (if less than a monolayer) or the thickness of oxide layer formed as a function of time and partial pressure of oxygen. For the noble metals, Rao, Damjanovic, and Bockris4 have provided values for fractional surface coverage with adsorbed oxygen a t a temperature of 20': Au
