Oct., 1955
JI‘ETTING PROPEIETIES O F O R G A N I C
LIQUIDSON HIGHE N E R G Y
WETTING PROPERTIES OF ORGANIC LIQUIDS SURFACES
os
sURF.XCES
1007
HIGH ENERGY
BY H. W. Fox, E. F. HAREA N D W. A. ZISMAN Surface Chemistry Branch, Chemistry Division,Naval Research Laboratory, Vashington, D . C. Received June 2, 1966
An investiyatiou has been made of the relation between the constitution of organic liquids and their ability to wet a n i spread on solids of high melting point (high free surface energy). Wettability was measured by the contact angle a t 20 ol each liquid 011 various metals, fused silica ?nd a-alumina (sapp1iii.e). Large differences were found in the wettability of t>lievarious metals and the silica and ~-alumina. These were caused by differences in the adsorption forces and by the existence of a firmlv bound layer of w:bter of hydration on the surfaces of the non-metallic crystals. A large variation in rvett:tbility could be caused i n the Intter by the partial dehydration resulting from baking treatments. For these reasons, a greater variety of liquids wet metals than noli-metals. Few liquids exhibited non-spreading on all high energy surfaces. It, was found that many carboyvlic estei,s Iiy(1rolyze a t ordinary temperatures upon being adsorbed on silica or cy-alumina. \\:lien the products released bj- l~yclrol~~sis in situ formed adsorbed films having a low enough critical surface tension, the parent ester was nonsprending. I t is concluded that the wettability of smooth high energy surfaces by organic liquids can be predicted approximutely from knowledge of the orientation and packing of the molecules adsorbed from the liquid phase. The nature of the high energy solid surface is important only insofar as it affects the adsorptivity of the liquid or as it causes chemical changes during adsorption which change the nature of the most adsorbable moleculw species presant. Pure organic liquids kipread completely on all high energy surfaces unless the liquid is ( a ) “autophobic” 01’ ( b ) able to decompose on adsorption to release a more adsorbable product which converts the solid surface into one of low swfnce energy. The qualitat,ive effects on the vettability of vai,yiiig the t,einperature, humidity, and roughness of solid are discussed.
Introduction High melting point solids such as diamond, sapphire, silica :tnd most metals ha1-e free surface energies at ordinary temperatures which are Iielieved to range from severnl thousand to se\.eral hundred ei,gs/cm.2.1 Lorn melting solids such as organic polymers, Tvaxes and coideiit conipoiinds in general haire free surface energies rmging from 100 to as loiv as 25 ergs/cm.3. I t is con\~enientJ t80 descrille the former group of solids as 11:tving “high surfa,ce eneigy” and the latter as lm\.ing “low surface energy,” Nearly all liquids other than liquid metals ha1.e free surface energies ( ~ L V O of ) less than 75 ergs/cm.? nt ordinary temperatures. Each such liquid would he expected to spread spontaneously on any clean “high energy surface” because there would thereby be expected to result a decrease in the free surface energy ( F ) of the system. The precise condition for the spreading of the liquid on the dean dry solid is that (bF/bA) the free energy change per unit area wetted should be negative. If XLVO/SQjs the initial spreading coefficient of Harkins, the free energy change tha,t takes place when a liquid spreads on a solid is - --dF ;,A =
SLVO/SO
=
”Y8O
- YSL
-’
YLVO
Here use is made of the convenient nomenclature of Boyd and L i v i n g ~ t o n yso , ~ being the free energy per unit area of the solid/vacuum interface and y s ~ the free energy of the solid/liquid interface. Evidently, the necessary condition for spreading is that SLVO/SO be positive; this will occur only when yso > y s ~ $- y ~ v o . Many years ago Hiirkins and Feldmc~n,~ extrapolating from measurements of the spreading coefficientJs of liquids on water and merc~ry.~ concluded .~ that practically all liquids (1) W. D. Hawkins i n Alexander’s “Colloid (;hein.,” Vol. VI, Reinliold Publ. Corp., New Y o r k , N. Y.,1946, p . 4. (2) H. W.Box a n d W. .?L. Ziswnn, J . Colloid Sri., 6 , 514 (1950). (3) G. E. Boycl a n d H. K. J,iaiiig.?toii. J . A m . Chern. ,Yor., 64, 2383
(1942).
(4) W. D. Harkins a n d A. Feldinan, ibid., 4 4 , 20G5 (1922). ( 5 ) W. D. Hnrkinn nncl E. H. C m f t o n . ibid., 43, 2.534 (1920). (GI W. L). Harkins a n d \V. W. Ewing, i b i d . , 42, 2.534 (1920).
should spread 011 clean metals and other inorganic high melting solids. Our research during the past ten years has shown that if the liquid iii contact with a high energy surface is made up in whole or in part of polar-non-polar molecules of certain types, there will be produced through adsorption a t the solid/ liquid interface a low energy surface on which the liquid will not spread. When the adsorbed film comprises long-chain, unbranched, polar molecules, which are able to form a close-packed array with terminal -CH3, -CF,H or -CF, group^,^-'^ the resulting surfaces permit spreading only by liquids having loiv surface tensions. When the adsorbed molecules are branched or cyclic struct u r e ~ the , ~ ~resulting surfaces permit spreading by all liquids excepting those having high surface tensions. Recently it has been shoivn14 that many classes of pure liquids including the branched and unbranched diphattic alcohols and acids are L‘aiitophobic,’li.e., each liquid is unable to spread on its own adsorbed film. When the solid is coated with a non-polar adsorbed film such as an n-alkane or perfluoroalkane, non-spreading occurs if the liquid has a higher surface tension than the critical surface tension ( y o ) of the adsorbed layer.13 We have defined y o of a solid surface as that value of the liquid surface tension above which liquids show finite contact angles (&) on the given surf ace.a Provided that information is available on the over-all configuration and packing of the adsorbed molecules of liquid, it should be possible t,o explain (7) W. C. Bigelow, D. L. Pickett a n d W. A . Zisman. J . Colloid Sci.. 1, 513 (1940). (8) W. C. Bigelow, E. Glass a n d (1947).
N’. A . Ziiiman, ibid., 2, 503
(9) E. G. Shafrin a n d W.A . Zininrtn. ibid.. 7 , 106 (1952). ( l o ) F. Scfirilriian a n d 1V. .4.Zisfiirtn, i b i d . , 7, 405 (1952). (11) A. H. Ellison, I f . W.Fox and W. A . Zisnian. T H I R .JOURNAL, 67, 622 (195’3). (12) E. F. Hnre, E. G . Slinfrin a n d \V. A . Zisninn, ibid., 58, 236 (1954). (13) H. W. F o x , E. F. Hnrr a n d W. A . Zisman, .I. Colloid Sci., 8 , 194 (1953). ( 1 4 ) E. F. Hare xiid W. A . Zisnlnn. Ttris .JOLTNAL, 6 9 , 335 (14.55). (15) H. W. Fox niid N‘. A . Zisirlun, J . Colloid S c i . , 7, 428 (1952).
1098
17~1.
T,F)
TABLE I CONTACT AKGLESOF ALIPHATICCARBOXYLIC ESTERS A N D ETHERS (hieasureinents at 20” and 50% . _ R.H.) NO.
9 10 11 12 13 14 15 1G 17 18 19
Aliphatic diesters Dibutyl citraconate Dibutyl pyrotartrate Bis-( 2-ethyl hexyl 1 adipnte Dibutyl P-methyladipate Bis-(2-ethylhexyl) P-methyladipnte Bis-(2-ethylhexyl) sebacate Bis-( 1-methylheptyl) sebacate Dioctyl sebacate Bis-(3,5,5-trimethylhexyl)sebacate Bis-(2-ethylhexyl) glutarate Bis-(3,5,5-trimethylhexyl)glutarate Bis-(2-ethylhexyl) P,B’-thiodipropionate Bis-( 2-(2-ethylbutoxy)-ethyl) nzelate Diethylene glycol dicaproate Dipropylene glycol dicaproate Triethylene glycol bis-(2-ethylhexanoate) Polyethylene glycol bis-( 2-ethylhesanoate) 1,g-Hexanediol bis-(2-ethylhexanoate) 1,lO-Decanediol bis-(2-ethylhexanoate)
20 21 22 23 24 25 26
Aliphatic monoesters Decyl acetate Amyl caproate Decyl caproate Undecyl 2-ethylhexnnonte Methyl laurate Amyl laurate Decyl laurate
1 2 3
4 5 G 7
8
(I
Coinpound ohsd.
Surface tension, O n metals, degree Brass Souroe5 dynes/cin. a t Z O O 18/8 Steel
a
30.4 29.3 30.2 30.4 30.1 31.1 31.0 32.2 29.9 29.4 28.4 31.3 34.3 30.8 29.5 30.3 28.2 30.2 31.4
a a
a a a a
a R
.. a a a a a
z a a
n f g f a g f f
28.3 27.0 28.8 27.5 28.3 28.2
..
Aliphatic ethers 27 1 Ucon (DLB-50-B) Ucon (DLB-100-B) 1 28 For the meaning of the code letters, see Acknowledgments.
the essential features of the wetting behavior of each organic liquid on high energy surfaces. In det,ermining the probable orientation of the adsorbed molecules, it is necessary, as deBoer and coworkers have pointed out,I6 t o recognize the essentially additive nature of the highly localized nolipolar “dispersion forces” between the individual nt’oms of organic molecules and the solid sur.fac,e. Materials and Experimental Procedures High energy surEaces investigated included small flat discs of platinum, brass (2 copper:l zinc), 18-8 stainless steel, fused silica and colorless synthetic sapphire (a-AI203). The platinum nnd sapphire were polished on lead laps with diamond powder (e Bis-(:?-etlipllies!.I) tereiih thnlnte Dibutyl pinnte Diheptyl pinnte
D
L
a L b
I L 111 111
5 h
Cyclic monoesters 40 Butyl pheii3'luiidecnnonte 41 Benzyl pheii!.lundec:Ltic,nte 42 l - ~ I e l ; h ~ ~ l - ~ - c t I i ~hjdi.oci l o c t ~ ti~n:uiint,e ~l 43 2-Etliyllies~~l p-( 3-phenylpi*opyI niei.c:i~)t~n)-~)rnpioti:i tc
2 '1
2
a
djrlca/oiii. :Lt " 0
O n metals, degree UU9S 18/8 Steel
39.4 41,3 3 5 .S 40.1 : 3 x , (i :11.3 30.7 30. S 32.0 30.5 31.0
8
10
n
15 9 17 0 2 0 0 0 0 0
:< 1.4
0 S 0 0 0 0 0 0 9
32.9 35.0 31.i 34,:
n n
0 I1 0 0
O n lion-iiietnl?, Silicadegree Sai)pliire
11 S
4 7
1; !I 11
8 4 11
28 19
27 17
1:) 10
11
c
0 0
23
8
0
0
18
21 1 (i
(i
1!I 11
Cyclic ethers 3 12 14 3 44 '1 :3c,, 5 Benzyl pheiiylunclecyl ether 0 2:i 0 39.2 '1 18 45 I,9-Bis-(~~Iienyl1iietllosy)-noiia1ie 0 0 0 20 1, :3s.i 40 lJ5-Bis-(3 - i ~ I i e n ~ ~ l p r o ~ ~ o x ~ ) - ~ , e l i t a i i e 0 0 I1 7. 3 i ,5 5 47 l,4-Bis-(3-pheri~Iproposy)-~l-lnet~li~~l~ut1~1ie 0 5 10 z :B.n I7 48 1,G-Bis-(2-plienylet~hoxy)-3-niet~h~lliexnne r 0 0 7 49 2 :is, 5 l,ll-Diphenyl-G,l0-dimeth~~l-1,5,8, I 1-tmtoxnundecniie 5 12 G 43.2 12 50 a 1 , 5 - B i s - ( 3 - p t i e 1 i ~ ~ l ~ ~ r o p o sto)-peiitmniie ~~ine1~c~~~ 4 10 0 51 k :13.3 2 Plieiioxyl,lieiiylcet:~ne 4 5 52 39.3 7 a-Naphthyl ethyl ether 10 g For the lneniiing of the code letters, see Acltiiowledginents. D These nintei,inIs tlisciissetl bv C. h f . 1Ttirphjr. J. G. O'Rear and W.A. Zisrntiii. Znd. Eng. C h e m , 45, 119 (1053). was zero, the surface tension was inensured by the i,ing method (precision of f 0 . 0 3 tlyws/crii .); and where tlie contact angle mas not zero, eithei, the t1iFFeienti:tl mnxiniumbubble-pressure method of Sugden20 or the pentlnut, drop method of Aiirhas, Ifnuser and Tucltei,?1 ivns used u,it,h a pi'ecision of &0.0:3 dviie/cm. By exercising sufficient c:tre the polished and cleniied speciineiis could be stored for four tlnys i i i the oliservntioii cells without loss of swfnce cleaiiliuess. E:Lc'IIcell i v n ~:I, rectnngulnr pnr:tllelepipc~d of ceiiietited plate glnss ( 3 3 / 4 X 11/, X 2 inches) eovei,ed by n Ant plate of glnss. Between t)he tiiiir of clenning niitl use, the speoiinriis \j:ci'e stored in covered, acid-cleniied, P:wr weigliiiig dishes. Of cou'se, one liquid n t n time \vns studicd i n ench cell to prevent coiitainiii:Lt)ioii1 ) viipor-ph:ise ~ trntisfer, and the c l f w spcciineiis were :~lnnysI~nutlletl\rit,hgreiise-free tongs. Ench liquid \vas usunlly nppliecl to tlip sui.fnce with a "t'riple dropper" made of t,lil,ee par,allrl staiiilew steel piiis (each 1 mni. i i i dinmeter) hcld rigidly togcthei, P O that t h e tips foimetl the :bpeses of a11 equil:ttei,nl t ri:tngle hnviiig sides 7 inin. long. These piiis \\.ere clc:tiietl 11y successive riiisings in A.C.;3. gi~ntle Iienzeiie. 1 iiiech:inicnl device WBR used to ~iiitlti: tlie three piiis gently n i i t l simultniieously t,ouch the surfarc of the liquid. This resulted i n the trailsFei, of the t,est, liquid to t81iepins as :3 iiiwly eclual peiitlniit cli,oplets which tlirn coultl lie t,oriched to the tJest suiface without touching t,lie ends of the piiis to tlie leveled solid surf:tce. Eypei,iniriits showed tlint, tlie liquid linnging froin each pin and just, touching the solid siii,f:Lce foriiied : L I ~hourglass shaped diol). Liquid cohesionnl forres pi,eventetl the adhei,ing di,op fmin spi~f~:ttling to the di:rnietcr it would assume :It equilil~riuni. Rlo\vly witlidixwiiig the piiis cnused
~
-
_
(30) S. Siigclcn, "The Pnraclior w r 1 Y n l m c y , " K n o p f , New Yolk, N. Y . , 1930. ( 3 1 ) J. .4ndrcns. IC. Hiiiiser n n d \V. Tuvker, T i m J O U R N A 4L2,, 100 (1938).
each drop to sepnrnte from the pin, advance slowly over the clean sui,fnce, niid assuine a new equililiiimn coiifigui,atjioii. The result, n a s th:it three iie:u.Iy rqunl drops were left) on the solid which eshi1)ited esseiitially eqirnl nc1v:inritig contact' angles. When t,he di,ops had lieeii plnced on the surfncra, the cell ivns covered, n i i d the s j m \vas nllo\retl t,o attain eqriilihi,iuiii i n n constant, tenipemtui.r room a t 20" and 5Os1relative humidity. Coiitnct angles of the liquid drops were ineasuiwl \vit h n lo\r-po\reretl goniometrr microscope descrilml elwvherc.' ,111 coiit:Let angles produced iii this way \vert indepeiidriit of the drop tliniixetei,. Ench corit:wt angle reported rep,eseiits tlie :tver'nge of mensui'ements oii nt least sis diffewiit tlro;)s on n Kivrii surfncc, the pi,ecisioii of the nieasureinents heiiig +2O :it contart angles of 10" and *lo at 40". \Ir:tsui,erneiits nii each drop n w e made after nppnrent attniiiiiiriit8 of equilil)i,iuin (usrinllv iri ahout 15 minutes) niid :igniii nliorit 24 hours Int,ei,. N o appreciable change in the cout:rct, niigles of thew liquids occuiwd after the first 34 llOUl,S.
Experimental Results In Tnl>les I thimigh
will be found the iiiensuretl contact the vnrious high enei'gy surfaces stju(lietl. The liquids are nrrnngerl in each tnlile in groups according to honiology, 01' to some sinii1:irit)y of Inoleculnr structure, n,nd w e coiisecut,ivcly iiuml)ered for reference. The sowre of enrli cornpoutid is given i n the second column nntl the liquid sui,fnc,e k i i a i o i i nt 20" is given in the third column. 111 contrast to the result,s olitniiietl by us previously on various low enei'gy sui,f:xes,g-lj the liquid surface t*ensioii nloiie did not, determine n.lietlier sprentliiig ivoultl occur on t>hesesurfaces; ho\revei, with the escept.ion of one liquid out of the 97 studied, the surface tensions of all the liquids which were non-spreading on :dl foul, t,ypey of solid sui,fnce?s examined were grenter t h a n 33.5 t l ~ " ~ / c i i i . This \vas iiot. angles for 97 liqiiitli:
\r 011
1100
H. W. FOX,E. F. HAREA N D W. h. ZISM.4N
Yol. 59
TABLE 111 CONTACT ANGLESOF SATURATED HYDROPARBONA (Measurement's a t 20" and 50y0 R.H.) NO.
53 54 55 5G
Compound obsd.
Open-chain compounds Hexadecane V-120 polyethylene 85-903 polyethj~lene 58-006 polyethylene
Sourcea
I, i I,
Surface tension dynes/ch. at, 20'
27.G 27.8 30.4 30.7
On metals, degrees On non-metals, degrees Bras3 Silica Sal)l~iiire
18/8 Steel
0 0
n 0
0 0 0 0
0 0 0 4
0 0 0 0
Cyclic compounds 57 9-( cy-( cis-0~3~3-Bi-cyclooctyl)-metliyl)-hept~adecnne I> 31.2 0 0 0 0 58 1,7-Dicyclopentyl-4-(3-cyclopent~lprop~l)-2ie~~t~niie 11 34.G 2 0 8 0 1 59 3,3'-Dicyclopentyldicyclopentane jc 0 0 0 0 GO 1,3-Dicyclopentylcyclopentane i 34.G 0 0 0 0 61 l-Cyclohexyl-2-( cycloliesyl niethyI)-peii t n , t l ~ cne n 1) 32,7 0 0 0 0 02 4-Cyclohesyleicosnne I> 31.0 0 3 0 0 63 0-n-~odecyIperIiydrophenniitlirr~ie 1, 34.2 n 0 D 2 64 1-cr-Dec;tlylhendecane I) 32.7 0 0 0 0 r 65 1,l-Di-( cu-decnlyl)-lientlec:mie 4 h 35.1 3 3 66 l-cy-Decaly1-2-c~~clohex~~let~liniie d :32.0 0 0 32 8 67 1,l-Dicyclohexylet,hnne d 33.0 0 0 5 0 GS 2,6-Dimethy1-4-( a-clecslyl met hyl)-lieptnne tl 35.0 0 0 0 0 69 2,G-Dimeth~l-4-c~cloliex~lmethyllie~~t:t1ie d 27.0 0 0 0 0 70 Isoamyl-3,3,5-triinet~hylr.yclolies:i.1ic d 25.3 0 0 0 0 71 Decalin g 30.5 0 0 0 0 72 2-Ethyldecnlin e 30.6 0' Oe oe O* For the meaning of t,he code letters, see Acknowledgments. These mnterials discussed hv C. M. Murphv nnd C. E. Saunders in "German Synthetic Polyetlivlene Oils." PelrokwL ReJirLer, Mav, 1947. Unable to be satjisfactoi,ily cleaned. Not enough material for surf:ice tpnsion clcterminntioii. I t should be slightly higher than f59 in surface tension. Evaporated. surprising since many liquids will adsorb on such solids to produce surfaces with free surface energies (and critical surface tensions) more charactei,istic of the absorhntes than tJhe adsorbents.13 TABLE
Iv
CONTACT ANGLESO F AROMATIC HYDROCARBONS (hleaaurrments a t 20' and 50% R.H.) Surface tension, dynes/ cm.
No.
at
Compound obsd. Sourcea 20' 73 8-p-Tolylnonadecene b 30.7 74 p-Di-sec-ainylbenzene c 28.1 76 p-Octadecyltoluene c 31.R 76 p-Dodeoyltoluene c 29.9 77 1,l-Diplienylethane lib 38.0 78 Aniyldiplienyl g 34.0 79 ~-RIetliylnaplitlialene gh 3F.4 80 t-Butylnaphthalene i 33.7 81 Monoainylnaphtiirtlene ib 34.3 82 Nonylnaphthalene ib 32.6 83 Dinonylnaphthalene ib 31.5
On metals, degrees
On nonmetals, degrees 18/8 SilSapSteel Bmss ioa pllire 0 0 0 0 0 0 0 0 0 0 8 0 0 0 0 0 0 0 0 0 0 0 0 0 Oc 3' Oc 4' 4 3' 4c 3' 4 3' 4' G 0 0 10 2 0 18 12
a For the meaning of the code letters, see Acknowledgments. Unable t,o he satisfactorily cleaned. c Ev:ipnrated.
A. Open-Chain Aliphatic Esters.-Table I contains the results for 26 liquid esters containing only aliphatic structures. Every aliphatic ester exhibited a zero cont,act angle on each of the metal surfaces studied, but twenty-one of the twenty-six esters were non-spreading on silica while 15 were non-spreading on sapphire. I t will he noted that the contact angle of each non-spreading liquid was always larger on silica than on sapphire. Of the 7 monoesters studied, only decyl acetate (compound no. 20) and methyl laurate (no. 24) could conceivablv adsorb as the unhydrolyzed ester to form oleophobic monol a y e r ~ . ~However, that is most unlikely since T\v (the critical temperature of wetting oleophobic films of tliefie esters) must be below 20". Evidence for this conclusion
is the fact that T , for the ester must be much less than the vnlue of 50" observed for an alcohol of the same number of carbon atoms.* The non-spreading property observed with most of these monoesters could arise from hydrolysis of each ester in situ, i.a., immediately after adsorptive contact with the silica or sapphire surface. If j t were due to hydrolysis products existing before contact with the surfaces, these liquids could not have exhibited zero contact angles on clean metals. The differences i n spreading observed cannot be caused by the variation of the surface tension among these esters, since Y L V O lies between 27.0 to 28.8 dynes/cm.; therefore, they must be due to differences in either the rates of hydrolysis or the nature of the hydrolysis products. The three lauric acid esters (no. 24, 25 and 26) will each release lauric acid for which T , is much above 20°.* The ester will be unable to spread upon the resulting adsorbed film of lauric acid, because yo for a close packed surface of methyl groups is between 22 and 25 dynes/cm.,O and the surface tension of each ester is about 28 dynes/cin. The observed cont,act angles range from 5 to 20'; these valuefi are in rensonnhle accord with this theory, since the slope of the graph of cos eE us. ~ L V Oin reference 9 is such that for liquids with surface tension only slightly greater than 24 dynes/cm., BE should be under 20'. If a greater degree of contact hydrolysis of the esters on silica than on sapphire is assumed, i t nccounts for the larger contact angles observed on the former. This assumption is reasonable, because the surfme of silica is more hydrated and more difficult to dehydrate than that of sapphire.14 A lesser degree of hydrolysis on the sapphire surface would lead to a less close,ly packed film of adsorbed lauric acid. This means ye will be newer 28 dynes/cm. than 25, and hence the contact angle will accordingly be smaller than on silica. The zero contact angle on sapphire observed for amyl laurate (no. 25) may have been caused by the accidental dehydration of the particulrm sapphire surface uscd,'4 because the necessary precautions against intensive dehydration adopted i n refcrence'4 had not been rccogiiized a t that time. Undecyl 2-el~hylhexanoate(no. 23) exhibited zero contact angles on bot,h silica and sapphire. We have recently tlemonstrnted~'~ that, y o for a solid coated with n monohyer
1101
CONT.tC'T
TABLE V ANGLE';OI" a4RUAlhTIC CHLORINE A N D / O R
PHOSI'FIATE
COhlPOUNDS
(Rleasrireinents a t 20' and 50% R.H.) Saillihirc, NU.
84 85 SG 87 88 89 90
!)I !)2 '39 94 95 9B
97 a
~ O l l l ~ J O l l lUl iactaiigle of 11" The ol)sc:iw~lcontact angles of each liquid is rcasonal)le if on both si1ic:t and sapphire appears reasonable. 711 > ~ L V O > yi. For example, the data of Table I1 \vi11 As regards their spi*eadingbehavior 011 inet,:tls, tlic i.yclic lie c*onsistriit wit,li these conclusions if in each case yi is less tli:in 37.6 dynes/cni. and y11 exceeds 39.2 dynes/cin. This esters can be divided int,ot w o g r o u p : (i) compounds like no. 2'3, 30, 31, 32 and 41, nrhich have dumb-bell shaped molr~- expinnation of the wetting behavior is quitmeconsistent with cules containing two rings; and ( i i ) compounds like no. 3 : 3 , t.he contart :tngle d a h of Tnble 11, for it is seen that t,he 30, 42 and 4:3, \vhic,h contain only oue ring. Compounds i n :iveixge eE foi. the two metals is greater the larger the escees group (i) are all non-spreading on niet:~ls; c>ompouiitlPof 01' y ~ 1 . 0over the 37.5 dynes/cin. It is concluded that a group (ii), with the escept.ion of the two o-plit,htili~acid di- satis1':iator.v esplnnation of the wettiiig behavior of these et hew c:in be given Iiy using reasonable assumptions a l ~ o u t esters no. 33 and 34, spread on metnls. All of t h e esters of group (i) :ire able t,o adsorb unhydrolyzed on metitls to form the orieiit:ttion of the adsorbed films of surh dumb-bell oriented f i l m 011 which t,he estei, cannot, spi'end I)ecause shaped niolecules. E. Saturated Hydrocarbons.-Each of the four highAs i n tlir ~ L V O > y o ; i.c., t8tieyare all aut,opliol)ic liquids. Iioiliiig :ilipliatic hydrocarbons studied exhibited zero C O I I argument presented for the open-chain estei,s in section A , the est,ers of group (ii) which spread on metals did not hy- tact, nngles on t,he high energy surfaces studied (see Tnlile dt,olyze upon adsorbing on tJhe inrtal surfaces, :tiid 7 1 . v ~ 111). The liquid surface tensions ranged from 27.6 for n< yc. o-Phth:~latediesters 110. 33 and 34 are believed the hesadecane to 30.7 dynes/cm. for the liquid polyethylenes. Ea,ch Iiydi~ocarlion niolecule would be expected to adsorb exceptions in group (ii) liecause t>heyare known to exist i n :t t,csonntice-st:tbiliz~~lstate having a planar double-riny- with as many methylene groups as close to the solid surface *haped ronfig~rat~ion.Such a molecular st,ructure pei,niits as steric comiderations permit,, and hence the resulting low adsorption on the metal suimface with the plane of the ring energy surface should have essent,ially the same value of yc ~ L Sthat of the solid polyethylene-air ii~t~erface, i.e., 31 dynes/ i t i the sui,fnce. If these ideas are correct, it would he expected t,hat the ether analogs and aromntic hydi,ocnrlioti cm.15 Evidently, these aliphatic hydrocarbons eshiliit.ed ati:ilogs of these esters would also behave as autophobic zero rontact itngles because t>heirsui*facetensions are always smaller t,lian the critical surface tension of the solid coated liquids (compare compounds no. 41 and 44). D. Cyclic Ethers.-Nine ethers or thioethera containing mit,li the adsorbed liquid film. Results obtained ivith sixteen cyclic snturated Iiydrocarcyclic groups have been studied (see Tnble 11). Six of these et hers (110. 44 through 40) have molecular configurat,ions bons are also given in Table III. Although the liquid surlike adumb-bell, and each liquid eshibited zero contact anglea face tensions varied from 25.3 to 35.6 dynes/cni., only comon both silica and sapphire (110. 44 atid 48 being es- pound no. 70 had a surface tension below 27 dynes/cm. :mtl reptioils). Compounds 110. 50, 51 and 52 are sufficiently only coinpounds no. 65 and no. 68 had values over 34.6 different to need special consideration. All nine cyclic dynes/cm. Twelve of these compounds spread freely 011 all ethers \vei'e non-spreading on metals like the analogous es- of the surfaces studied. The four exceptions (no. 58, 63, ters of section C. 65 and 6G), which have surface tensions of 32 dynes/cni. Because of the abseiice of strong dipole forces, each et,Iier or more, exhibited non-spreading 011 both silica and sapcli:iin and each terminal aromatic group must adsorb \vit,h phire. Only no. 58 and no. 65 were non-spreuding 011 metiil surfaces. Evidently, non-spreading is observed with this its many cov:tlent atoms in cont,act with the surface ae possible. The critical surface tensions of t.he resulting low class of naplithenic compounds only when ~ L V O exceeds 32 to energy sut,fnc:es will be greater than that of a polyetmhylene 34 dynes/cin.; also, BE is usually greater the larger the exs y f a c e and less than that of a surface of coplanar aromatic cess of over that value. Although it is not certain rings. Hence, yo must considerably esceed 31 dynes/cm. how the molecular coiistitution of the liquid affects y o , it is .4s liquids no. 44 through no. 49 have surface tensions of 36.5 to be expected t.hat yc would not he very different from thr to 30.2 dyner;/cin., and as all exhibited 2 ~ 1 ' 0contact angles, value of 31 dynes/cm. of a surface of fairly closed-packed it follows t'hat yo > 39.2 dynes/cm. -CH1-groups. Certainly, yo would be greater the looser thc Conipound 110. 50, which contains two thioether atoms, packing in t,he adsorbed film, and the surface packing of has the high surface tension of 43.2 dynes/cin. In order -CHT rtnd -CH, groups should be loose for compounds no. to explain ivliy this liquid is non-spreading on silica and 57, 58, 59, 60, 61, 64, 68 nud 69. The observation that, snpphire it is assumed t h a t ~ L V O> y c . Coin ound 110. 51 ~ L V Omust exceed 32 to 34 dyncs/cm. suggests that yo also contains nn oxygen atom connecting two pfienyl groups ranges from 32 to 34, depending on the packing; this is :t both of which cannot adsorb in the sanie plane. Hence, y o reasonable conclusion because of the coustitution of the adsho111cI be lower than the value for the dumb-bell shaped sorlied film. cthcrs. The fact that 110. 51 is non-spreading on silica and F. Aromatic Hydrocarbons.-The four alkyl-substituted Rapphire may be considered evidence that 31 < y c < 33.3 benzene compounds studied (see Table I V ) , exhibited zero dynes/cm. Compound no. 52 can be espected to adsorb contact angles on all the high energy surfaces with one eson tlie silica and sapphire surfaces with the plane of the ception: p-octadecyltoluene ( ~ L V O = 31.5 dynes/cm.) did iwphthalene group in the plane of the surface. The sniall not spread on silica. However, p-dodecylt,oluene ( ~ L V O = proportion of paraffinic to aromatic hydrocarbon atoms in 29.9 dynes/cm.) did spread. Each of these four hydrocarthis molecule causes yo to be essentially t,hat of the naph- lions will adsorb with as many aroinat,ic and aliphatic hythalene portion. Since ~ L V Ois 39.3 dynes/cm. and t,he drogen atoms located contact.ing the adsorbing surfaces as liquid is observed t o be non-spreading, it ia concluded t>hat possible, and hence yr should be determined by the mlativc for such i i low energy surface yo < 30.3 dynes/cni. How- proportion of aliphatic and aromntic atoms. As p-oct,:iever, y: must exceed that of I~enzeneor napht~haleiiegt'oups decyltoluene atid 8-p-t,olylnooadecane have the reatest, proad~oi~l)ed with tjhe plane of the i,ing a t ri::ht angles to the portion of aliphatic at,oms and p-di-scc-ainyl~eiizcrle t,hc solid xurfarp. Hence 35 < yc < 30.3 dyiies/cni. for com- least,, y o should be greater for the Iirt,ter and smaller for t , l i c b ~ ~ o u nno. d 52 :tiid similar structures. former compound. Xow, t,he relative surface tensions of The greater teudency of these cyclic duiiii)-bell sliitped these liquids is in the inverse order, i.e., p-di-scc.-:tniyIl,eiiethers to s1irc:ad on non-metals than on inetals can be ox- zene hae the lowest value of ~ L V O . Therefore, it, should 110 pl:iined if ivc: start with several reasonalile assunipt,ioiis. most, liltely to spread on all surfaces; similarly, p-octatlecylFirst, the :wonintic portion or t8hemolecule is SO much loss toluene should b e tlie least, likely. The differeitcw i i i strongly tid~nrbedby the metals than by the silica and sap- spreading of t)he tlvo long-chain substituted t o l w i i w i t i r l i phiw aurf:tres that, they need not, lie coplnnar with the ad- rtrtes that for p-octadecyltolueiic. 30.i < y o < 3 1 .A tlyrir~s'[:in. rodiing sui,f:ivi:. Sc?cond, when ndsorlied on i i metal, thc This ia reasonable from tho relat,ive :ireas covercd by t i l i niolcculnr dipoks :il\va,ys rcm:iin i n contact with the met:tl phatic r,h:tins and tuomatic rings. surf:tcc. Fixlier-Hii~sc~liErl~~er atmomniodels how that e:ich Compounds no. i 7 and 110. 78 were thr oiily sul)s~itjul,(~il niolrctilc ci~ii:trlRoi,l) with the planes of it,s t i r o :troni:Ltic diphenyls studied which sprrad coniplrt,ely 0 1 1 :ill t.hc high i,ing:s o!iet!ted l)"i,l)eiitlicrilarlsr to t,he surface. Also, the enei'gy suifaces. This is ren~tii~ltal~lo since their surfwe (PII:woni:itic i'ingii of adjacent adsorbed niolecules can cohere i n sions were as high as 34.6 and 38.0 tlynos/cm. Amyldipairs through the action of the London "dispersion" forces. phenyl is assumed to adsorb with both :troinatic ring8 i n t'li(> Ttierdore, i t is assumed thnt such mutual cohesional forces plane of the solid surface. The zero contact angle observed S P ~ Tto st:il)ilizv this :~tlsoi~I)etl film on met:tls. Such a film on all suifaces means t,liat, y o > R4.G dyncs/cm.; this is :I wou111 1i:ivi. :I sni:tllot, vi,itii.:d sitt,f:w> tcrrsioti (yl) t,linii t , l i c s i~e:~~on:~I)le iiiterrnrdinte v:tlue I ) e t \ r w t i t I i r s low v:rluc 01' y,, v:iIiir f y I li of filnis 01' J I I W i~lrw-pi~,l 38 dyies/cm. All five alkvl-substituted nmhthalenes were difficult to purify and maintain pure. Measurements of .!?E with the naphthalenes of lower molecular weight were complicated by rapid evaporation of the liquid around the periphery of the drop; thus, drops of compounds no. 79, 80, 81 evaporated completely during the 24-hour test. Probably, the observed values of BE or 3 to 4” would become zero in the absence of evaporation. These compounds should adsorb on high energy surfaces with the planar naphthalene ring in the surface; and hence on closest packing yo > 35 dynes/cm. Since their surface tensions are less than 36.4 dynes/cm. these liquids should have BE = 0. The two nonylnaphthalenes studied (no. 82 and no. 83) exhibited small but reproducible contact angles on metals which may be caused by impurities. When pure, these liquids would be expected to behave the same on all four surfaces studied because the orientation of such adsorbed non-polar hydrocarbons would not be expected to vary among these high energy surfaces. G . Chlorinated Aromatic Hydrocarbons and Ethers.All five chlorinated aromatic hydrocarbons and ethers of Table V are mixtures of isomers which are difficult to separate. The average composition of the chlorinated diphenyls (Aroclors 1242, 1248 and 1254) corresponds to three, four and five atoms of chlorine per molecule, respectively, but in each liquid there are present molecules containing more or less chlorine. For this reason, Aroclor 1248 was distilled through a Vigreux column into four fractions, which are characterized by the boiling point, refractive index and contact angles given in Table V. The large dipole moment of the carbon-chlorine bond and the relatively strong London “dispersion” or adsorption forces which must exist between covalent chlorine atoms and metal or other high energy surfa~es23>*~ will cause as many as possible of the chlorine atoms in each molecule to adhere to the adsorbent. Thus, in the low energy surface formed by the adsorption of the first monolayer of chlorinated diphenyl, the orientation will be such that the minimum possible number of chlorine atoms will be in the monolayer/ air interface. From inspection of Stuart-Briegleb molecular models,26 which accurately represent such aromatic structures, and from the fact that the preferred position for chlorine substitution in biphenyl is ortho, it will be seen that the two aromatic rings will not be coplanar. A surface of close packed covalent chlorine atoms has a critical surface tension of 43 dynes/cm.,*3 while a surface of benzene rings has a value of 35 dynes/cm. when oriented edgewiseI3 and a greater value when oriented coplanar. Hence, the adsorbed film of chlorinated diphenyl will have a critical surface tension between 35 and 43 dynes/cm. which will be closer to the upper limit. Of course, in working with mixtures of the chlorinated diphenyls, some homolog or isomer will be preferentially adsorbed. Increasing the extent of chlorine substitution in diphenyl raises YLVO (see Table V). This factor alone would cauRe the BE to increase with the chlorine content, and Table V shows this is true for each surface studied. Generally, the results obtained on silica and on platinum agreed reasonably well. The lower values of BE observed with sapphire may lie evidence of a slightly looser packing of the adsorbed molecules. Inspertion of the molecular models of the two chlorinated diphenyl ethers (no. 89 and 90) reveals that such ethers are somewhat less sterically hindered than the biphenyls, and so are able to adsorb with one aromatic ring coplanar with the adsorbing surface. This may cause the value of yc an adsorbed film of such an ether to be higher than that of the chlorinated diphenyl, and it would explain why lower contact angles are observed with compounds no. 89 and 90 on silica and a-alumina than with no. 91 through 93.
H. Phosphates.-Because of the autophobic properties observed in carefully purified tricresyl phosphate, a study was made of five aromatic esters which are homologs of triphenyl phosphate. On the assumption that the strongly polar P=O group of the ester will physically adsorb on the high energy surface with its axis oriented normally to the surface, the most probable configuration of the adsorbed molecule of triphenyl phosphate is with the planes of the three aromatic rings all oriented as nearly normal as possible to the surface. This permits close packing of the molecules with the planar aromatic rings of adjacent molecules in contact. The critical surface tension of such a filmcovered surface will approach that of an aniline monolayer (35 dynes/cm.) with certain difference peculiar to such phosphates. In the first place, such a large, branched molecule cannot adsorb with the aromatic rings packed as closely as those of aniline. This will decrease the population density of the aronintic-CH- groups i n the outermost surface of the film and so will raise yc. Ball models of such phosphates all show that the ether oxygen atoms may be partially exposed to the liquid resting on the adsorbed layer, and that will also increase yo. Methyl substituents on the phenyl groups of triphenyl phosphate will change yo by influencing the orientation of the aromatic groups, tending to make their planes more vertical to the adsorbing surface or less so depending on the position of substitution. Also, this will introduce methyl groups in the plane of the resulting low energy surface and thereby reduce yo. The methyl substituents will orient away from the adsorbing solid but will not completely shield the ether oxygeti in the ineta or para positions. When the methyl group is ortho to the ether oxygen, the oxygen atoms will be nearly completely shielded. The meta and para isomers of tricresyl phosphate are contained in commercial preparations in a ratio of approximately 80% para and 20y0 mela. It is assumed that the surface tension of tricresyl phosphate (40.9 dynes/cm.) is roughly equal to yo for the “parent” compound, tri )henyl phosphate, since the contact angles are nearly zero /or this mixture on each of the three solid surfaces listed in Table V. Tri-o-cresyl phosphate should have a lower value of y c because of (i) the shielding of the ether oxygen atoms by methyl groups and (ii) their presence in the outermost poition of the adsorbed monolayer. This surmise is confirmed by the experimental data of Table V for the contact angles of tri-o-cresyl phosphate on platinum, silica and sapphire. In adsorbed films of diphenyl mono-( o-xenyl) phosphate, the xenyl (biphenyl) group would be expected to shield the ether oxygens somewhat. In addition, since the surface tension of this ester is considerably greater than that of trio-cresyl phosphate, this should cause the contact angles to be somewhat great,er than those of tri-o-cresyl phosphate. The experimental results of Table V are in accord wit,h this conclusion. Chlorine substituents, such as in tri-o-chlorophenyl phosphate, will orient the phosphate molecule with the chlorine atoms next to the adsorbent because of the large electric dipole created by the carlion-chlorine bond. As B result of the much larger size of the chlorine than the hydrogen atoms, this substituent will tilt the benzene ring toward the phosphorus atom and thus will partly shield the adjacent ether oxygen. The surface tension of the liquid increases with the chlorine content so that the contact angles of the chlorinated esters should be larger than those of tricresyl phosphate. In particular, tri-o-chlorophenyl phosphate should have larger contact angles than td-o-cresyl phosphate, and this is the case. The larger phosphat,e contact angles on silica and aalumina than on platinum are caused by the diffe1,encefiin the nature of the forces of attraction to the adsorbing surfaces. The molecular at,traction to platinum is most probably the electrostatic attraction of the dipoles with their electrical images in the conducting surface; that of these esters to d i c a and a-alumina include dipole-ion attractions as well as hydrogen bonding to the adsorbed layer of water found on these oxides.14 Molecules adsorbed by such hydrogen bonds arc probably more mobile than molecules adsorbed on a metal lattice and so closer packing is likely to occur.
(23) A. H. Ellison a n d W. A. Zisrnan, THISJOURNAL, 6 8 , 2 6 0 (1954). (24) R. C. Bowers, W.C. Clinton and W.A . Zisinan, .lfodej I I Plast i c s , 31, 131 (Feb. 1954); J . Applied P k y s . , 24, 19G6 (1953). ( 2 5 ) Anon., Cheni. Eng. News, Sa, 2534 (1954).
General Discussion 1\11 of the contact ttiigles presented liere Jvere olhiiiied 011 speculnrly polished solid surfaces. If
packed a s the Inct,hyl groups i i i ii siiigle, crj,st:tl of :t paraffiii. Sitice yc of Iiesat~i.iacoiitnii(~ is ashout 21 dyiies/cm., l5 the \.:due of y o for tlie silicone moiiolayer must exceed 21. Hellre, Y L V O must always be below yo mid the silicones cniiitot be autophobic. A similar argument using the critical surface tellsion of polyethylene of 31 dynes/cn1.,~5aiid the surface tensions of liquid aliphatic hydrocarbons, which are always less than 30 dynes/cm., lends 11s a t once to understand why such hydroc:wbons are not au tophobic. This study has added to the list of autophobic liquids repoked earlier14; the various c1iloih:ited diphenyls, the deriT-atives of tripheiiyl phosphate y L V 0 = c1 - C,T listed in Table Tr, certaiii a,romatic hydrocarbons But we have showii tJhat cos BE = a - b y ~ v o if , (Table 111) and certain aromatic esters and ethers the surface composition of the solid is constaiit. (Table 11). Here GI, Cs, u aiicl b are positive. Therefore We have choseii to define tJheclass of autophobic liquids as those compounds which are non-spreadCOS BE = (U - bC1) bCi2' ing by virtue of the fact that they adsorb un:uid cos BE must increase (and OE decrease) ivitli altered to form low energy films on which the bulk increasing temperature. In addition, the effect liquid will not spread. Liquid compounds, which of raising the temperature will be to cause de- release polar decomposition products able to sorption of any physically adsorbed compounds. adsorb aiid form low-eitergy surfaces, behave like Temperat'ure iiicrense will therefore decrease the pure liquids which contain polar additives. As packing of the adsorbed film. This will raise the we have pointed out earlier (7-13), the noncritical surface tension of the system and thus spreading property can be produced in nearly all cause $E to decrease with rising temperatures. pure liquids by the addition to each of a minor Wheii the temperatures become high, there may concentration of a polar compound which preferresult chemical changes in the liquids such as entially adsorbs to form a suit'able low energy hydrolysis, oxidation and pyrolysis, or there may surface. develop surface-chemical changes in the solid due The degree of hydration of the surfaces of silica to oxidation, dehydration or crystallographic re- and sapphire has a marked influence on the contact arrangement. The products of chemical reaction angles of the esters. It is reasonable to assume in the liquids may be highly adsorbable and may that the rate and extent of hydrolysis in situ incause the foi,matJionof new low energ:y surfaces on creases with the amount of adsorbed water. We \vhich the liquicls will not spread. This effect have preI.iously shown1a that increased hydration m:Ly o1,erbalance the above-mentioned normal de- favors the closer packing of adsorbed polar molecules. crease iii & with rising temperature. The osidaI n a number of instances, the value of the crititioii or dehydratioii of these inorganic solid sur- cal surface tension of the film of adsorbed polar faces may greatly alter the wetting behavior of molecules depended on the constitution of the the liquicls. But if the chemical reactivity or atl- hydrolysis fragments generated. Thus, esters havsorpti\'ity of the liquid is little changed thereby, ing the same surfnce tensions will exhibit different 110 large cliaiige i l l wett:tbilit,y is to be espected. contact angles depending on whet~lierthe adsorbed Decrertsitig; the re1atii.e liumitlity a t ortliiiary fragments are liiiear or :ire brunched-chain monoteniperatrire:: will 1tai.e lit'tle effect on OE unless acids or monohydric alcohols or are either dithe atmospliwe is so dry as to dehytlrat'e the solid carboxylic acids or dihydroxy alcohols. These surface (ns iii silica and a-nlumina). But usually coiiclusions agree with results obtained in preirious i'ery loiig esposure to dry air will be required to ~ t ~ u d i e s 'in ~ ,which '~ the critical surface tensions of sigiiificautJly change the contact angles. As the the adsorbed film were shown to depend on the 1 relative humidity approaches 1 0 0 ~ ~increased composition :uicl packing of the atomic groups condensation of water on the surfaces of both uppermost in the surface. metals and metallic osides will ini.ite the hydrolysis Our result's on the molecular mecliaiiisni of it^ situ of atlsorbed liquids, ant1 so the wettjing spreading can be generalized as follows : eiyery properties of these surfaces \\,ill become more like organic liquid spreads freely 011 specularly smooth, those of such highly hydrated surfaces a s glass, clean, high energy surfaces a t ordinary temperasilica and alurnina. tures unless the film adsorbed 1,~' the solid is so Polymethyisilosane liquicls spread 011 d l high coustitut'ed that the resulting film-coated surface energy surfnces hecause the surface teitsiotis of 19 is a low eiiergy surface h a \ i i g a critical surface tjo 20 dynes/cm.?' are always less thaii the critical surface tensions of their own adsorbed films. This tension less t1ia.n the surface tension of the liquid. follows because an adsorbed close-packed mono- This in turn is caused by the highly localized iiature layer of the silicoiie molecules has an outermost of the forces between each solid surface and the surface of methyl groups which are not as closely molecules of the organic liquid and also between the molecules of each liquid. The localization is ( 2 6 ) R. Wenzel, I t i d . E i r ~ Chem., . 23,088 (183(3). so extreme that the influelice of molecules or atonis (27) H. \V. Fox, P. \V. Taylor a n d IV. 4. Z i s i i i m , i b i d . , 39, 1401 beneath the first monolayer must be miiior. A (1947).
the siirfaces were rouglieiiecl, \Vetlael's I a \ P statcs that tthe coiitact angle will decrease, siiice it is dways less than 90" for such sj7st)erns. Rougheniiig the surfaces will also introduce differences iii the advancing and receding contact angles which will increase with the roughness. Although the observations report'ed here were all made of 20" and .50% relative humidity, the effect of variations in the temperature and humidity can be outlined. It is well established that the surface teiisions (yL1:o) of pure organic liquids decrease linearly with rising temperature until close to the criticd temperatures. Hence
+
llO(i
SOTES
inoliolayer of :~dsorliedmolecules is always fouiid suficient to gi1.e the high energy solid surface tlie s.me wettability properties with respect to all liquids as the Ion7 energy solid having the same surface constitutioii. Applicntions of the results of this investigation are too numerous for presentation here. Tlie mechanisms reported here account completely for the noii-spreading properties obseryed in various mineral, animal and vegetable oils as well as in the synthetic liquids reported by many workers including WoogIz8Bulkley and Barker, et C L Z . , ~ ~ and Bielalc and Mardles.31 Clock oils, which have been troublesome and expensive for producers and users of watches and clocks as well as of timers, fuses, and other instruments, can now he synthesized and/or controlled adequately. In the production and use of paint's, varnishes, plasticizers, cements ant1 printiiig inks, hitherto unesplainable c h i g e s i n adhesivetiess, spreatlability, or permeability can he understood :~tidcall be avoided by eliminuting compounds capable of coiiirerting the solid surfaces involved (28) P. Woog. Compt. rend., 81, 772 (10251, and "Contribution a 'etude du graissage onctuosite," Deiagraue, Paris, France, 1926. (29) R. Bulkley and J. Snyder, J . A m . Chem. Soc., 55, 194 (10331. (30) G . E. Barker, U. S. Patent 2.355,Glli (August 15, 1944); also G . E. Barlioiiof argoii (rut1 1) a t - 105" u p t u a r.elat:ivc pressure of 0.88 rt~snltetli u a typicd Type I1 isot,lierni mid showed iio 1iyst8eresis. Fui*thcr SUCcessi\.e t reatmmetit,s coiisist8iiigof out8gassingt.lie iruii a t room t,emperature for 12 hours ( i u n 2 ) , retlucing iii hydrogen for 1 110ur and outgassing for 12 hours a t -100" (run 3 ) , aiitl outgassing a t room t,empera( 2 ) ..\ iiioi'i' dptniled desrrilitioti of t l i e i m n can lie ohtnined froni tlie T l i p snitir I)t,odiirt, iktiown ns C.P. iron pnnder, is n i n t , k p t e d I)\. , I . T. B : ~ k e vC'IIPIII. PO. tiinniifiirtiiwr.
